By Lynn Olson
Associate Editor, Positive Feedback Magazine

Now that you've built a triode amplifier, what kind of speaker do you use with it? The audiophile favorites just aren't efficient enough, and you aren't sure if you'll be satisfied with a horn system. Well, the Ariel and the ME2 might be just the ones for you; they occupy the uncharted territory between BBC mini-monitors and full-sized theatre speakers. Curious? Read on ... Next

Table of Contents
Illustrations

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The Ariel

My cabinet-making collaborator, Mike Spurlock, wanted a simple system for his beach cabin. "That's not too hard, Mike, you just build this," I said, drawing a small picture of a transmission-line using a 5.5" Vifa midbass driver and a 1" Scan-Speak soft-dome tweeter.

As I filled in the details, I commented, "These particular drivers are really easy to use, since they're quite flat on their own, sound pretty good, and only need a minimal crossover. The transmission-line ought to provide better bass and midrange than a closed box, and it also avoids the need for an expensive stand."

Mike then asked the key question: "Why don't you also draw me a D'Appolito configuration, Lynn? That'll be twice as efficient, so I can use my Magnequest 2A3 triode amps."

"Hmmm," said I, in my best I'll-deal-with-this-later System Administrator style. "Let me draw that one on my computer, so we can have accurate dimensions to work from. Let's talk about this some more ... "

As I worked on the computer drawings, it dawned on me that the new loudspeaker might meet the needs of more people than Mike. After all, I did need something to talk about for my next Positive Feedback article, and many friends had been asking me for years to design something simple and straightforward that sounded like the speakers I designed for Audionics back in the Seventies. Maybe something a bit more significant was beginning to stir ... time to do a little reflecting on what this project was really all about.

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Design Principles

Three key ideas emerged:

As a result of this last restriction, I decided to use golden ratios throughout the system to mitigate the damaging effects of diffraction in a box-type enclosure. The diffractive echoes are still there in the time domain, but they are carefully spaced so they don't create big peaks and divots in the frequency domain. This spreading-out process helps the subjective qualities of imaging and spatial perspective.

An experienced woodworker with an accurate table saw, a precise miter box, a router, and a complete set of clamps (or better yet, a vacuum bag) should be able to complete this system. Do not, repeat not, attempt this system if this is your first loudspeaker. On a scale of difficulty, it is definitely more complex than an ordinary bookshelf speaker, but is lot simpler than a horn system ... it's comparable to a typical transmission-line in complexity and requirements for accurate (1/32" tolerance across 4 feet) woodworking.

Since the inspiration for this system came out of the clear blue sky, I named it after the archangel Ariel. For me, the name Ariel captures the way it feels to have a new creation emerge from the great beyond, come to life, and go out into the world. Thanks to Positive Feedback magazine and the Internet, people all over the world can enjoy the Ariel.

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The Drivers

Since "keep it simple" was uppermost in my mind at the beginning of this project, I picked drivers that were up-to-date, easy to work with, and had first-class subjective qualities without requiring tricky equalization circuits in the crossover. (Not many drivers qualify!)

The Vifa 5.5" P13WH-00-08 midbass is refreshingly free of the muddled and confused sound of typical 7" and 8"polypropylene drivers. This is almost certainly due to the small size of the cone, which makes it more rigid, as well as careful Vifa engineering of the vented pole-piece assembly. The frequency response of this driver is also pretty remarkable, with a ruler-flat midband gently descending to a smooth 12 dB/Octave rolloff beginning at 5kHz. That means the crossover can be simple, and won't require midband equalization.

The flatness of the 5.5" Vifa driver is more unusual than it first appears. Since the ear is so sensitive to spectral imbalances in the midrange, most high-fidelity speakers have traditionally used modest equalization in the crossover to straighten out the crucial midrange. It is only in the last few years that we have started to see high-performance drivers that don't require any equalization, and the Vifa 5.5" midbass is an excellent example. (The only other one I can even think of is the Focal 6V415, and even that one has a small bump at 4.5kHz).

I first heard the 5.5" Vifa in the Lineaum LFX, and was impressed with the wonderfully transparent midrange. I was not so impressed with the thin bass. I figured if this driver had any future as a good midbass unit, a pair of them driving a 6 foot transmission-line would bring it out. Doubling up would gain 6dB of efficiency and headroom, and the transmission-line would extend the bass down to the F3 of the driver - 60Hz. By contrast, a vented box with conventional Theile/Small QB3 tuning begins a 24dB/octave rolloff at 80Hz.

The two orthodox approaches to get deeper, more powerful bass would be:

Yes, I could have done that. If I had, the system would have ended up sounding like any of thousands of perfectly ordinary 2 or 3-way systems. I wanted this speaker to have midrange and treble as good as any on the market ... and no reason why not, since building truly excellent midrange and treble is much cheaper than excellent sound combined with deep, full-power bass.

High-end speakers in the middle price points usually compromise the mids in favor of getting more of that powerful bass that many audiophiles want. Increasing the size of the midbass driver reduces the midrange quality in the case of the Vifa (and many other drivers). The 6.5" and 8" Vifa drivers need modest midrange equalization, and after you go to all that extra trouble and expense in the crossover, they still don't sound as good as their little brother (EQ is not a substitute for quality).

Going to a 3-way by adding another crossover at 200 to 300Hz is just asking for trouble in terms of large, expensive caps and inductors, as well as difficult problems with phase shift at frequencies where it is much more audible than the 3.8 kHz of the mid-tweeter crossover. Besides, most 10 or 12" paper-cone woofers in vented boxes don't have such great bass anyway. It's loud and deep all right, but these types of woofers do not have the speed and delicacy of a driver like the 5.5" Vifa, making system integration difficult.

The tradeoff in the Ariel is very high quality midrange, seamless coherency across the spectrum, and good efficiency at the expense of very deep and powerful bass. To "get it all," in the manner of more conventional speakers, would require a complete redesign of the system, and very possibly losing the focus and immediacy that are the hallmark of the Ariel. Speaker design is one realm of audio where gains in one area are almost invariably compensated by losses elsewhere ... regardless of cost or complexity.

I cannot recommend any other midbass driver as a substitute. Sorry, tweakers. If want to use other drivers, particularly Kevlar or carbon-fiber drivers with high-Q peaks in the response, you are on your own. Be aware what measures flat and what sounds flat are two different things, and the complex radiation pattern of the D'Appolito can make this pretty challenging.

The situation with tweeters is somewhat better. Good tweeters aren't as rare as good midrange drivers, and have benefited from many advances in materials sciences, acoustic holography, and computer modeling in the last decade. Soft domes in particular have made great strides, and are head and shoulders above the popular soft-domes of the Seventies.

I know of three tweeters that fit the bill of flat-as-a-pancake response and excellent subjective sound: the Vifa D25AG-35-06 aluminum dome, the Dynaudio Esotec D-260 soft-dome, and the Scan-Speak D2905 soft-dome family of tweeters.

Note: The Ariel crossover would probably be quite usable with the other two brands of tweeters if you remove the notch filter in the tweeter section.

Small variations in the series resistor (R1) and shunt resistor (R2) in the tweeter circuit are required if you use different tweeters, or simply want to tune the subjective balance. The nominal value for the series resistor is 0.5 ohms; it can be usefully adjusted over a range of zero to 1 ohm. The nominal value for the shunt resistor is 10 ohms; it can be adjusted over a range of 16 ohms (if the series resistor is zero) to 8 ohms (if the series resistor is 1 ohm). The level of the tweeter will depend on the sonics of the rest of your system; rest assured that adjustments as small as I'm describing will be easily audible!

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The Transmission Line

Since I have two midbass drivers, not one, I took the liberty of changing the shape of the usual transmission line to an asymmetric "Y," with the two midbass drivers at the two tips to the Y, and a labyrinth at the bottom. (A tip of the hat to Randy Hedgebeth, who showed me his innovative design for a hybrid labyrinth-line, with a labyrinth at the far end of the line venting to the floor. I took his design a little further by having two separate line sections, one for each driver.)

Side View I extended the golden-section philosophy used elsewhere in the Ariel design and made the lengths of the upper arms of the Y conform to the 1:1.618 ratio used elsewhere in the design. The arms of the Y join in the first section of the labyrinth, which acts as a low-pass filter for any waves that are traveling from driver-to-driver.

I lightly fill the straight portions of the line (the upper arms of the Y) with cleaned, combed, and carded long-fiber wool, while using crimped Fortrel or Acousta-Stuf in the topmost section of the folded labyrinth. (The rest of the labyrinth is intentionally left unfilled.) The wool appears to be most effective in the midband, which is why it appears to work best in the region closest to the drivers. By contrast, crimped Fortrel has little effect in the midband and selectively damps the bass. If you use wool, make double-sure it is moth-proofed; I can assure you that having moths in your house is no joke!

Another tuning system is embodied in the adjustable height feature of the enclosure. If you look closely at the cabinet diagram, you'll see that the entire upper enclosure is actually resting on four adjustable bolts that thread into the oversize bottom plate. The distance between the bottom plate and the enclosure acts as a variable-resistance vent, allowing you to tune the amount of low-bass output to the room. The dimensions of the bottom plate itself are not critical; just leave it large and heavy enough so the Ariel is stable. 3 to 4 inches larger than the outer dimensions of the Ariel is sufficient.

(Tuning hint: Play wide-band music through the system and listen carefully to what comes out of the vent. Adjust the vent height and/or density of the filling material so only deep bass emerges. If you are lucky enough to have access to test equipment, measure the frequency response at the vent opening and adjust the cabinet height and/or filling density to achieve a smooth rolloff from the vent.)

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Cabinet Design

I placed the tweeter in an off-center D'Appolito configuration, hoping to mitigate the upper-mid forwardness that many commercial D'Appolito systems exhibited. By creating a lateral asymmetry based on the golden-section ratio, it would help damp any standing-waves that might form on the front panel. There would still be a potential for a vertical standing-wave, but at least it wouldn't be doubled in magnitude by a left-right symmetry as well.

Why did I depart from the well-known symmetric D'Appolito configuration? Well, it's a long explanation ...

As I mentioned at the beginning of this article, a box is a terrible shape for a loudspeaker. After all, a loudspeaker operates at the speed of sound (not a figure of speech), and what was the last time you saw an airplane that looked like a box? Just like an aircraft, sharp edges and bumps create shockwaves at the rough spots, and these shockwaves create drag in an aircraft and secondary reflections in a loudspeaker. Since these reflections have delays comparable to the delays created by the head and shoulders of the listener, the edge reflections confuse the stereo image, flatten the depth perspective, and create listening fatigue.

The truly correct approach is to use gently sweeping aerodynamic shapes, resulting in speaker cabinets with large-radius curves at least equal to the radius of the midrange cone. Gee, you might think, nobody does this. Why? Cost and cosmetic expectations. That's all.

In aircraft, safety and performance rule, dictating the shape of the aircraft. After all, badly designed aircraft waste fuel and crash; by comparison, badly designed loudspeakers only destroy the sense of musical realism and beauty.

Front View Having thoroughly demolished the hi-fi pretensions of the box-shaped speaker, here I go telling you to go ahead and build one. Well, I heard the reactions of woodworkers in many places, and it's pretty clear that exotic (by loudspeaker standards) shapes require exotic machinery and skills. Besides, most of us are used to the sound of box speakers, defects and all. So I decided to minimize the subjective effect of the reflections by carefully spreading out their timing.

This is where the golden section ratios come in. It is the ratio called phi, and it is close to 1:1.618, one of the best ratios to choose if you are trying to discourage standing waves. All of the driver placements on the front panel are determined by a series of golden sections, with the left-to-right tweeter location being the most important ... fortunately, the midbass drivers have fewer problems with diffraction, so the layout is less critical.

So I took the standard D'Appolito configuration and offset the tweeter just a bit, following the phi ratio. In addition, the two radii on the left and right sides of the cabinet are asymmetric. This results in both the edge radius and the edge distance having a L/R 1:1.618 ratio. In effect, the left side of the tweeter sees a cabinet that is 1.618 times smaller than the right side. (The pattern is reversed on the other speaker, of course.)

The intentional asymmetry dissipates energy from standing-waves on the front panel, which provides a measurable, and audible, improvement on the frequency and impulse response compared to conventional D'Appolito systems. Although it takes a bit more time to build mirror-imaged Left and Right cabinets, it's worth the effort. (The drawings show the Right speaker system.)

(Note: The imaging will be trashed if you forget to mirror-image the left and right speaker. I found that out the hard way at the 1975 Consumer Electronics Show when the Audionics shipping department accidentally shipped us a pair of Left speakers by mistake. We had to ship a Right speaker by overnight air freight ... not cheap for a 120lb. TLM-200.)

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Cabinet Construction

Exterior dimensions: 48" high by 8" wide by 10.5" deep.

Panel dimensions: All panels are nominally 3/4" thick, with the exception of the double-thickness front panel, which is two pieces of 3/4" MDF laminated together.

Remember to accurately measure the thickness of all of your panels; we had to adjust our dimensions by 1/32" throughout the system because our MDF was just a little thicker than the expected 3/4."

All exterior panels are premium-grade MDF (we used high-density Canadian Rangerboard) and all interior panels are plywood (plywood is 3 to 4 times stronger in compression and tension than MDF composites). You could do worse than use multi-ply Baltic Birch plywood, which is the choice of the BBC. The amount of wood in the Ariel is pretty modest, so you needn't be deterred by the cost of the Baltic Birch. The Interior panels slide into grooves that are dadoed or routed a quarter-inch into the front, side, and rear exterior panels. I can't recommend screws, butt joints, or biscuit joiners, since the mechanical properties and air-seal are nowhere close to the performance of a well-made dado groove. Think about how the best furniture and musical instruments are made and you'll be heading in the right direction.

The mechanical integrity of the cabinet comes from the glued dado joints, not fasteners. Precise construction and test-fitting assures that all glued surfaces will mate accurately. If the cabinet is well constructed, it can be completely pre-assembled without gluing, which lets you check for interference and clearance problems before you commit to gluing.

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Driver Cutouts

The midbass flange diameter is 5.53"(5 17/32"), flange depth is 0.188" (3/16"), and the diameter of the inside hole is 4.56" (4 9/16"). T-nuts spaced at 45, 135, 225, and 315 degrees are recommended for driver mounting, since it facilitates removal and testing as well as providing a predictable amount of torque on each bolt. I use 10/24 by 2" hex bolts for appearance, durability, and the ease of adjusting torque evenly on the driver frame.

(I do not recommend using wood screws to mount drivers. Wood screws tear up the MDF, making driver replacement difficult, and cannot be torqued to uniform tension without further damaging the wood.)

Although it's a hassle, you'll get improved driver performance and easier mounting for the T-nuts if you make large-radius scalloped cut-aways between the T-nut mountings, leaving about a 3/4" lip for each T-nut mount. This makes connecting the hook-up wires and stuffing the interior of the enclosure much easier, as well improving the airflow around the magnet.

The tweeter flange diameter is 4.125," flange depth is 0.156" (5/32"), and the diameter of the inside hole is 3.25." T-nuts spaced at 45, 135, 225, and 315 degrees are recommended for driver mounting. As before, a scalloped opening between the T-nuts is a good idea; not for airflow, which is zero for the tweeter, but to be able to get the hook-up wires connected to the tweeter without bending the rather fragile terminals on the tweeter.

Although it's not very clear in the drawing, the tweeter chamber is acoustically isolated from the midbass TL sections. If you want to open a hole into this section, either make it barely large enough for the tweeter wire and seal it with RTV silicone, or drill a 1/2" hole in the bottom section only, then cover it with felt.

Be careful about setting up the router depth and getting a good air-seal with the gasket material. Even tiny air leaks will degrade the quality of bass significantly. (This is true for all types of cabinets, not just the Ariel.)

Access Door and Binding Posts:
Well, they have to go somewhere (like the old Vaudeville joke, everybody's gotta be someplace!) If you put them here and make the panel itself removable, you can insert the wool and/or crimped Fortrel after you've built the enclosure. For convenience, the inner cutout can be 4.56" (4 9/16") in diameter to match the driver cutouts.

The binding posts are then mounted on a 5.53" (5 17/32") diameter disk of 1/4" Masonite or transparent acrylic, which is fitted into the routed-out flange with a gasket. Don't forget about a good air seal here also, so be sure to use a gasket and 6 or 8 screws (or better yet, bolts and T-nuts) to mount the Masonite or acrylic disk.

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Konstruction Korner

Don't expect anything terribly profound, this is just what I observed while I helped Mike build the first prototypes.

WEAR EAR PROTECTORS!!! Even though I wore them during all of the sawing and routing work, my ears still rang with a 9 kHz whistle for many hours afterwards. Hearing tones like this means the nerve cells in the cochlea (inner ear) have been traumatized and that you should keep things very QUIET for a day or two afterwards. As for me, it made me aware that audiophile woodworkers should go out of their way to use extra high quality ear protection ... what I was wearing obviously wasn't good enough.

Mike and I were exasperated by a persistent series of small (1/16" to 1/32") errors in the lengths of the outside panels and in the corresponding dado locations, which required a fair amount of rework as we went along. We later traced this to an inaccurate miter box combined with not cutting all 8 outside panels at the same time. Make sure your miter box is set to exactly 90 degrees, and don't allow any domestic interruptions to a sequence of related operations.

The various dado-cut slots make it possible to test-fit the various portions of the enclosure as you assemble them. This lets you check if any tolerances have gone astray or if any panels have the wrong dimensions. This is an essential step! Make sure you do this frequently as you build up the various assemblies that go into the enclosure.

The final step before gluing is to pre-assemble the enclosure and check for tight, accurate fit in all locations. You'll find that it assembles like a big Chinese puzzle and fits quite snugly without glue. If all the pieces fit, you can disassemble, clean up all of the interior surfaces, glue the felt down in the appropriate places, get out your clamps, and go ahead with the final assembly and gluing procedure.

I've spoken to two different constructors who used a vacuum bag to laminate the front panel and also do the final enclosure clamping. This sounds like a really great idea; if you've got the facilities, it neatly sidesteps all of the troublesome warping problems that can arise from conventional clamping procedures, as well as assuring uniformly high pressures on all glued surfaces.

The decorative faceplate of the tweeters (and mids) must be mounted flush with the front panel of the enclosure. Even tiny variations create reflections in the time domain which are measurable and audible. On the subject of reflections, try and space the (optional) grill frame about 2" away from the front panel using the old Infinity-style standoffs. The major problem with grill cloth isn't the absorption, which is quite minor in sheer fabrics, but the reflection, which causes roughness in the high frequencies if it is closer than about 1" to the dome.

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Panel Damping and Sound Absorbers

Testing for panel damping is really simple; play loud orchestral or choral music through the loudspeaker and press your ear directly on the rear or side panel of the enclosure in an area close to the drivers. With many speakers, you'll hear a loud, offensive drone, which you'll then discover is still audible when you listen to the speaker in the normal seated position. One of the most important differences between mass-fi and advanced loudspeakers is the quietness of the cabinet panels; with the best speakers, the cabinet is almost totally silent.

In the Ariel and ME2, the panels have very small free areas, and full-width plywood cross-members are used to increase rigidity. Deflex pads absorb the remaining vibration in the region directly behind the driver. Use a 1/2" or 3/8" thickness of F13-density 85% wool felt to line the areas around the drivers. (The interior spaces above, below, left, and right of the driver.) The panel directly behind the driver is most critical, and must be covered with an effective damping material, preferably Deflex pads. These pads both absorb acoustic energy (like felt) and mechanical energy in the panel where it is attached (unlike felt).

My experience with foam damping pads (2" thick #2 gray polyethylene foam) has not been very successful. Using the foam at various locations within the enclosure all resulted in a substantial loss of impact and realism and the addition of a toneless "gray" coloration to the overall sound (by contrast, the measurements showed a tiny improvement). My partner, Karna, immediately objected. (Her exact words were "They sound constipated! Take that stuff out!") She made me remove all the foam blocks from the speakers, and sure enough, they sounded much better and more alive. So watch out when you use foam damping material.

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Evolution of the Crossover

I started out with a truly minimal crossover; a 2uF capacitor in series with the tweeter, and a 16 ohm resistor in parallel with the tweeter (to damp variations in tweeter impedance). In theory this should have worked just fine. It measured very well, as I expected, since the drivers are exceptionally flat and easy to drive. It just didn't sound very good. It sounded like lots of other audiophile speakers out there; overetched, unnatural, with a sort of crude and annoying "look-at-me, don't I have great treble" quality. I played around with various values for the C and R, but the annoyance-factor stayed the same. So much for the minimal crossover audio fad. Nice try, but no cigar.

After a while, I realized the source of the problem ... even the most deluxe dome tweeter doesn't like too much excursion, and that's exactly what a minimal crossover forces the tweeter to do. All direct-radiators (tweeters, mids, woofers, etc.) have diaphragm excursion that increases with decreasing frequency at a rate of 12dB/octave. (Drivers are constant-acceleration devices.) To prevent the excursion from actually increasing below crossover, you must use at least a 2nd-order (12dB/Octave) crossover ... even this rate simply keeps the excursion constant below the crossover point.

I didn't need too much additional persuasion to increase the rolloff rate on the tweeter; the one-cap crossover also required the tweeter to be connected in reverse-phase in order to match the midbass units (which have their voice coils about 1.5" behind the tweeter.) I'm not too fond of drivers wired in reverse phase ... I prefer to adjust the rolloff rates and other factors so I can keep all drivers wired in the same polarity. That way the group-delay errors are only at the crossover frequency, not spread across the entire treble region.

So I reverted to my familiar pattern of designing the crossover to carefully optimize the performance of each driver, and nuts to fashionable minimalism. So the tweeter received a low-Q 2nd-order network(C1, L1), a damping resistor in parallel with the tweeter (R2), and a standard Zobel inductance compensation (R3, C3). Now the tweeter sounded like I expected ... sweet and very natural.
Tw Crossover
It still needed work, though. The tweeter was fine, but I now could hear the typical D'Appolito forwardness from the midrange drivers. This was something I was a little worried about when I started the Ariel project. Every commercial D'Appolito system I'd heard to date had a forward and rather unpleasant midrange that was also quite directional ... the snarl didn't really go away until you walked into another room. I was now hearing it from the Ariels, and I wasn't too sure I could do anything about it. Time to find out.

Even though the Vifa midbass units measured beautifully flat with a textbook 2nd-order Bessel rolloff, something wasn't quite right. The mids were too aggressive, smooth MLSSA curves notwithstanding. With pink noise and music, it was quite obvious. So I took the path of least resistance, thought a little while, and picked a multi-tapped inductor (L5) from North Creek Music Systems so I could play around with the woofer rolloff.

I shut off the tweeters and just listened to the midbass units, adjusting the tapped inductor as I walked around the room. Much better, but not home free yet. I picked out several different capacitors and increased the rate to 2nd-order, starting with a low-Q Bessel slope. The original midrange coloration disappeared, replaced by a new one about an octave or two higher, near the corner of the crossover slope. Hmm. Adjusting the filter Q by changing the C/L ratio didn't fix the problem, so I took the direct approach and added an R in series with the C. Ah. That sounded good. The D'Appolito coloration was gone for the first time. How interesting.

Now it was time to have a little fun. I wired a L-pad to 15 feet of cable, connected it in series with the C, and walked around the room while I turned the pot. There was a critical value at about 1/3 of the rotation; below that, D'Appolito coloration, above that, all gone. Turn the pot all the way up, and the mids reverted to the L-only sound. I repeated this several times, marked the value on the pot, and measured the resistance. It turned out that 3 ohms was the critical threshold, so I wired two 6.8 ohm resistors in parallel and left it at that.
MB Crossover
(Looking at the circuit a little more closely, it is evident that R6 and C6 in the woofer crossover approach a pole-zero cancellation for the woofer inductance, allowing the main 1.0mH inductor to reshape the response without interference from the woofer's rise in inductance.)

Now it was time to match up the tweeter crossover, but this was easy, since by now the midbass units had acquired a very gentle 4th-order Gaussian slope ... the way out of the troublesome D'Appolito coloration. I wouldn't have suspected at the beginning of the Ariel project that doing very subtle phase adjustments on the woofer crossover would remove the forwardness in the midrange.

By now the acoustical rate (the sum of the electrical crossover plus the drivers) approached a low-Q 4th-order slope, so I decided to smooth out the tweeter rolloff just a bit more. Like many other dome tweeters, the Scan-Speak D2905/9000 has a small bump at 1.5kHz. I wasn't too sure if this 1.5dB bump was created by the tweeter itself or the reflection off the midbass surrounds, but there wasn't anything I could do about it, since various experiments with felt damping pads above and below the tweeter produced no useful result.

I don't like the extra complexity of notch filters, but going the extra mile to smooth the tweeter rolloff is an excellent idea, since it improves power-handling as well as ensuring a smooth phase and amplitude hand-off between the midbass and treble drivers. Interestingly, the notch filter (L4, R4, C4) is almost inaudible in terms of removing coloration, but the improvement in power-handling is quite evident.

At this point the Ariel sounded very sweet, relaxed, and natural. It measured reasonably flat, with a shallow dip from 3 to 6 kHz. Every time I brought this region up, the sound became less coherent and more ragged, so I left it as is. That's obviously what the drivers want to do, so I let them do it.

Freq Resp
The one remaining thing to add was the impedance correction network at the input of the crossover. This isn't audible per se, but some power amps don't like reactive speakers. Without the impedance-correcting network, the impedance rises to about 30 ohms around 2kHz (due to the wide frequency separation between the low and high-pass filters). With the network in place, the impedance rise is limited to 8 ohms. The impedance curve stays between 3.6 ohms and 8 ohms from 10Hz to 100kHz, a simple load for nearly any amplifier. The average value in the 200Hz to 800Hz range is 4.5 ohms, so if you have a tube amp, connect the Ariel to the 4 ohm tap.

I should add this whole process actually took about 3 months, checking and re-checking with MLSSA and LMS each time I tried a slightly different crossover variant. Many of the variations were nearly invisible on MLSSA; I had to refine my measurement technique quite a lot in order to see the half-dB differences the changes were making. 15 crossovers later, I was done, and Karna and I were very glad to settle back and simply enjoy the Ariel.

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Crossover Construction

The parts quality of the crossover components has a significant effect on the subjective balance of the speaker, and can easily outweigh re-balancing the crossover or even changing a driver. My experience with the Ariel crossover is that changing an important capacitor or resistor for one of equal value but a different construction requires at least some re-balancing to get the speaker "zeroed-in" again. For this reason, it is essential to use the specified parts (or better ones) for all sections of the crossover.

I have been using the Sprague 730P and the Hovland MusicCap polypropylene capacitors, 12 and 14 gauge air-core inductors, and Ohmite and Mills 1% 10 watt non-inductive wirewound resistors. Do not use conventional sand-cast power resistors, Mylar or electrolytic capacitors. If you do, the quality of the drivers and the cabinetry will be go for naught.

The Sprague 730P capacitors should placed so the "S" of "Sprague" is wired to the ground, or driver, side of the circuit. If you use the Hovlands, place the unmarked wire to the ground, or driver, side of the circuit I surmise this effect occurs because the capacitor tends to be slightly self-shielding when the outer wrap is on the low-potential side of the circuit.. When you buy the tweeter caps, ask for a tested and matched pair ... 1% matching would be nice if possible. (Yes, the Hovlands sound better than the Sprague 730P's, but both are significantly better than the Solen/SCR caps used in typical high-end speakers.)

If you want to have fun experimenting with even more exotic caps, start with the 4.7uF series cap (C1) in the tweeter circuit, then go after the 10uF shunt cap (C6) in the woofer circuit. You can also have fun with different exotic resistors in the tweeter circuit; rest assured that these changes will be audible.

The best way to mount the crossover is to give it a box of its own. This isolates the capacitors from vibration (which they don't like) and lets you tweak in comfort while you listen. Just keep the crossover at least 12 inches away from any large iron or steel objects, which means don't put it next to your power amp or TV set.

Keep the tweeter and midbass filters electrically isolated with independent star grounds ... also, separate the inductors by at least 8 inches and place them at 90 degree angles to each other.

Bi-wiring (independent sets of speaker wires for the tweeter and midbass) is recommended, particularly if the amplifier has a low damping factor (like many tube amps). My initial experiments indicate that single wires sound more compressed and less open than bi-wire pairs, even if the single wire is quite exotic and costly. This is another area where experimenting can make a real difference to the overall sound of the system.

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Crossover Tuning

There are three adjustable portions of the crossover: the discrete resistor L-Pad (R1, R2) for the tweeter, the tapped inductor (L5) for the midbass, and the resistor in series with the capacitor in the midbass circuit (R6). The Ariel crossover is quite sensitive to small variations in component value, so keep your adjustments small and do a lot of listening before making the next change.

For example, the recommended range of the tweeter L-Pad is zero to 1 ohm for the series resistor (R1), and 16 to 8 ohms for the shunt resistor (R2). The small change of 1.5dB in tweeter level is quite audible! The choice of 1.0 and 1.2mH for the midbass inductor lets you make a 1dB adjustment in midrange level, which makes a considerable difference in the overall balance of the speaker.

When you look at the schematic, it's pretty obvious the tweeter L-pad lifts and lowers the response of the tweeter, but the behaviour of the midbass components is a bit less obvious.

The tapped inductor (L5) raises or lowers the 800 Hz to 3 kHz region of the woofer, while the shunt resistor (R6) alters the shape of the rolloff region as well as the perceived off-axis balance of the midbass drivers. The resistor adjusts the Q of the low-pass filter, so if you decrease the resistance, the response in the crossover region rises, while if you increase the resistance, the crossover region droops. After much to-and-fro'ing, I ended up with R6 set to 3.4 ohms. (The two 6.8 ohm resistors in parallel provide improved power-handling.)

I use a combination of music, pink-noise, 2mSec MLSSA, and 6mSec MLSSA measurements to do the crossover balancing. All subjective tests have their blind spots, so cross-checking is vitally important, particularly with music. Remember, you need to listen for what the system does well (and improve it as much as possible) as well as what it doesn't do so well (and minimize that). You may have to choose between a sense of verve and directness and a tuning that is totally neutral; this is your call. The drivers in the Ariel are very flat all by themselves, which makes the subjective tuning go a lot quicker. If the drivers had large peaks like most Kevlar and carbon-fiber drivers, the tuning process becomes much more difficult, and requires a lot of experience in knowing what to equalize and what to leave alone.

Subjective pink-noise testing is a very sensitive measure of resonant coloration (more so than music, and much more repeatable), but it fails to detect notches in the spectrum unless they are very sharp and deep. So you have to be careful when you tune a crossover so you don't inadvertently create broad depressions. In addition, pink-noise testing tells you nothing about dynamic qualities, so you can end up with speakers that are smooth and inoffensive but don't sparkle and sing on real music. Still, pink-noise testing lets you quickly detect and remove resonant colorations; just alter the crossover and add or remove cabinet damping until the speaker really begins to sound like an actual waterfall.

MLSSA, FFT, LMS, or 1/3 octave measurements provide an essential cross-check to make sure that you're not subtly skewing the spectrum as your ear gradually adapts to the sound of pink-noise. (Adaptation is a serious problem with pink-noise testing. Listening and tweaking sessions should be kept under 10 minutes so your mental reference point doesn't begin to shift.)

In practice, subjective tuning results in a 3-way round of testing, using pink-noise, measurements, and music listening. This is the sequence I use:

  1. Aim for the intended acoustic target slope (4th-order Gaussian or whatever) by using computer optimization with LEAP or XOPT, or use old-fashioned cut-and-try with many repeated measurements made at 2 meters distance at 0 degrees, 15 degrees, 30 degrees, and so on.

    (WARNING: Do not make system measurements at a 1 meter distance. For all multiway speakers, not just the Ariels, the crossover radiation pattern doesn't fully "gel" until you get at least 2 meters away. If you fine-tune the system at a 1-meter distance it will almost certainly be wrong at 2 meters. Fortunately, the measurements at 2, 3, and 4 meters are very similar, unless it's a giant horn system, which aren't intended for close-up listening anyway.)

  2. Fine-tune the new crossover in half-dB steps by ear and repeated MLSSA, LMS, or IMP measurements. Keep doing this until you are satisfied with the overall technical and subjective performance.

  3. Record the measurements, crossover description and topology, and the version number.

  4. Have music-listening sessions using several amplifiers and a wide variety of sources. Instead of listening critically, ask yourself if you're enjoying what you hear. Do you feel the music? Does it move you? Focus on the emotional qualities, not just the usual audiophile sonics. In other words, what does it do well?

  5. Go back to Step 2 until you feel genuinely satisfied that the whole system has reached its full potential. For example, the Version 2.2 crossover is actually the 15th go-round using this multi-step procedure above. So don't expect perfection the first time around, and don't expect music-listening or measurement sessions to do it all. It takes both.

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Positioning the Ariels

The best imaging and spatial qualities result with 50 to 55 degree spacing between the left and right speakers, with the tweeters placed towards the inside, and both speakers aimed at a point about 1 to 2 feet in front of the listener. In effect, this forms an equilateral triangle that ends about a foot short of your nose. When the speakers are correctly aimed, you will see the large-radius edge appear slightly closer to you than the small-radius edge, with an inch or two of large-radius sidewall showing.

Visualize a sphere with a 1 metre radius extending in all directions outward from the tweeter; keep this imaginary sphere free of obstructions of any kind. If your room is big and uncluttered enough to provide a free radius of 1.5 metres, so much the better. The less junk there is in the immediate vicinity of the speaker, the better it will sound. (No, various room-damping tricks won't help if the free-space criteria can't be met. You can't fool Mother Nature!)

If you can set them up like this, and have a pretty good system, you'll hear a smooth and even halo of sound extending about 2-3 feet outside the speaker pair and about 5-10 feet in height. If you're lucky enough to have top-quality directly-heated triode amps, these figures double, providing spatial qualities approaching the best and most natural multichannel systems.

By contrast, if the speakers are too far apart, or not toed-in enough, the center-fill will be weak and "phasey," and the intermediate center-left and center-right localizations will tend to jump towards the speaker. If the speakers are too close together, or toed-in excessively, the stereo image will compress, lose its natural quality of airiness, and off-axis images will disappear.

When the toe-in and spacing are correct, you will hear a perfectly even distribution of sound and a well-proportioned impression of space on nearly all stereo recordings. You should be able to move left-to-right over several feet and not have the image shimmer, wander, or collapse back into the speakers. If it does, you'll experience listening fatigue just from muscular tension trying to keep your head in the "sweet spot."

Unfortunately, most systems I see in homes, dealerships, and the CES have the speakers aimed with zero toe-in and spaced too close together. This gives the worst of both worlds: narrow, unstable images that collapse off-axis, intermittent depth perception, and a very limited impression of the spatial texture of the original recording.

Since many people, including audiophiles and reviewers, have never heard stereo as it's intended to be heard, try a simple experiment with a boom-box that has detachable speakers. Go ahead, put the thing on a kitchen table, space the speakers 60 degrees apart with yourself at the tip of an equilateral triangle, and aim the speakers at a point about a foot in front of you. For once, don't listen to the awful sound quality; just pay attention to the stereo impression.

The point of this simple experiment is to demonstrate that good, in fact, excellent stereo can be delivered by the crudest and most basic systems. There's no reason you can't have this same experience with much larger systems in your living room.

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A Brief Note about Efficiency

There seems to be a misunderstanding out there about transmission lines and efficiency. Keep in mind that driver efficiency is almost purely a function of cone mass, radiating area, and magnetic strength in the voice coil gap, and very little else. Believe it or not, for a given driver, the box design has no effect on the driver efficiency. (Read the Neville Theile and Richard Small articles in the 1973/74 AES Journal if you don't believe me.)

If the crossover is really greedy, it can attenuate a lot of upper-band energy, but very few designers try to remove energy in the 100-500 Hz region, where any correctly operating driver is naturally flat.

So how did TL's get a reputation for being inefficient? Well, if you mass-load the driver by putting damping material too close to the cone (3 inches or less), that will indeed remove a couple of dB ... as well as altering the frequency response. Also, the heyday of TL's was in the mid-Seventies, when driver efficiencies were at an all-time historic low of 81 to 84 dB/meter.

Actually, TL's are well-suited for very efficient drivers that would otherwise have high cutoff frequencies if you used them in a conventional vented or closed-box Thiel-Small alignment. For example, the Vifa P13WH-00 drivers used in this system typically provide an F3 of 83 Hz in a conventional vented QB3 alignment, while the Ariel transmission line system is still strong at 55 Hz. As you might imagine, there's a pretty big difference between 55Hz and 83Hz of bass response!

In addition, the transmission line rolls off more gently than a 4th-order vented system ... indeed, if the TL has the appropriate rolloff frequency, it can actually match the low frequency room lift and not seem to roll off at all. (To do this, though, the Ariels would have to roll off at 25 to 30 Hz. That's another project altogether.)

The Vifa midbass drivers in the Ariel are operated in parallel and produce an efficiency around 92 dB/meter (calculating directly from the Theile/Small parameters). The Scan-Speak D2905 tweeter produces a measured efficiency around 90 dB/meter. These numbers are consistent with the target slope of an overall 2 dB tilt from 100 Hz to 10 kHz.

By measuring efficiency in the power center of the musical spectrum, which is a band between 200-800 Hz, the Ariels provide an efficiency of 92 dB/meter ... more efficient than most audiophile-class loudspeakers, and much more efficient than mini-monitors. My joke on the Internet is the Ariels occupy the "golden mean" between a Klipsch LaScala and a BBC monitor!

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The ME2

Over time, several OTS members asked me about a simpler and more compact version of the Ariels. I though about this for a while, and turned on my 486/33 PC and designed a compact near-Bessel vented enclosure for the Ariel system.

(I selected the near-Bessel alignment because it has the least overshoot and time-ripple, and is less sensitive to dynamic variations in driver Qt than the typical commercial QB3 alignments. The Bessel alignment is optimized for a Damping Factor of 8 when they are connected to the 4 ohm tap of the output transformer. They are usable with Damping Factors from 4 on up.)

Front View Since the new versions are minimonitors, efficient, and 2-way, and are basically a smaller twin of the Ariels, Karna suggested christening them the ME2 (pronounced me-too). Why not? So ME2 it is.

The ME2 is 18" high, 8" wide, and 8.5" deep, with an internal volume of 521 cubic inches, and a pair of 1" diameter vents that are 4.125" long. It retains the large diameter radius (1.25" on top, bottom, and one side) of the Ariels. Everything else is the same as the transmission-line Ariels ... same drivers, crossover, front panel layout, composite MDF/plywood construction, etc.

The LEAP modeling system shows a -3 dB point of 70 Hz, a -6 dB point of 53 Hz, and a box tuning frequency of 48 Hz. Your initial reaction might be: 70Hz! That's not a lot of bass! Well, actually, it's not too different than lots of other minimonitors ... many of them are actually spec'ed at the -6dB point, which gives a better match to the perceived low-frequency limit.

Still, I agree, you're not going to make the room shake with these speakers. The ME2 is aimed at three groups: folks who like the unadorned minimonitor sound, those with multichannel systems, and/or systems that use one or two biamplified subwoofers.

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A Multichannel Digression

Part of the reason I designed the ME2 was listening to the Cogent Research SPI, which is a modern sum-and-difference 4-channel decoder using psychoacoustically optimum decoding coefficients (whew!) This unit offers high-resolution "frontal quad" without the listening fatigue that most people associate home theatre systems.

My preliminary listen to the Cogent showed a lot of promise ... very wide and spacious soundstage, stable imaging, with reasonable but not great sound. But I listened to the Cogent unit with a quad mid-fi Rotel amp and four mid-fi speakers at $300 each. Comparing this setup to the two Ariels and the resident triode amp was ludicrously unfair ... there was a ten-to-one difference in price against the Cogent setup!

Which brings us right to the ME2 speaker. It's more compact, less expensive to build, and offers the same sound quality as the Ariels, with 20 Hz less bass. It's a prime candidate for an audiophile-quality multichannel system using the Cogent decoder, particularly with a high-quality pair of stereo subwoofers, which leads us right to ...

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The ME2 and Powered Subwoofers

I recommend trying a 80 or 100Hz crossover as a starting point, and using no more than a simple high-pass capacitor for the high-frequency amplifier. I also recommend a simple, high-quality tube, or better, triode amp for the high frequencies, and a good transistor or MOSFET amp for the lows. This lets each amp function in its best frequency range ... in particular, the triode amp will sound far better if it doesn't need to amplify low frequencies, which is what the input capacitor accomplishes.

Stereo subwoofers are a good idea, but not for the reasons you might think. It is indeed true that localization is very difficult to perceive when the wavelengths are longer than 10 feet. However, there is an effect known as "Spatial Impression," which conveys the impression of size, or space, and it is quite important at very low frequencies. A mono subwoofer, in other words, won't distort the image, but it won't sound as spacious as stereo subwoofers! In addition, it has been demonstrated in psychoacoustic research that a 2-speaker stereo image grows narrower as the frequency is lowered ... which means that it is desirable to place the stereo subwoofers much further apart than the normal 50 to 60 degree stereo-pair. Good results have been reported with 120 or even 180 degree spacing of stereo subwoofers.

Level-setting for the subwoofer shouldn't be a big chore ... select piano music that uses the bass register (I use the full-keyboard scale on the ProSonus test CD) and adjust the subwoofer level (and phasing!) for the most realistic and "quick" sound. Using piano music as a reference should curb the natural tendency to let the subwoofers creep up in level, which leads to the dull and soggy bass that is so common in home-theatre systems.

The biggest challenge with any subwoofer system is getting the bass to sound quick, natural, and well-integrated with the rest of the system. That's why I'd avoid big (10" or larger) polypropylene woofers and select a really good paper or Kevlar 8" driver instead: for example, my first choice would be a Scan-Speak 8" driver in the factory-recommended vented box. (Madisound or Just Speakers can help you with the directions on building the subwoofer cabinet.)

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ME2 Cabinet Construction

The drawings pretty much tell the tale. Use Baltic Birch or Apple-Ply plywood for the internal members, and premium-grade MDF for the outer shell. The 1.5" thick front panel plays a significant role in quieting down the cabinet and is worth the hassle of bonding two 0.75" panels together. Be sure to make the speaker in mirror-imaged pairs and try to achieve the 1.25" radius shown on the drawings ... I found out that there is a pretty big sonic difference between 0.75" and 1.25" radius when I made the Version 1 and Version 2 Ariels.

Lightly fill the V-shaped rear half of the cabinet with long-fiber wool (best for mids) or crimped Fortrel (like Acousta-Stuf), but don't cover the inlets of the vent tubes.

Note: If you're using the ME2 with subwoofers, fill the vents with tightly-packed wool or Fortrel. This improves the clarity of midrange (by stopping the mids from coming out of the vents) and damps the low-frequency cone motion of the ME2 drivers, which further improves the midrange clarity by keeping the voice-coil in its most linear region.

You'll notice that there isn't any room inside the cabinet for a crossover ... that's intentional, since vibration of crossover components can degrade the clarity of sound. A small external box for the crossover is the best solution ... but keep it at least 12" away from steel stands and transformers. When you lay out the crossover, remember to keep the inductors at least 8" away from each other, and to place them at right angles to each other as well. Crosstalk between the inductors, or any adjacent ferrous objects, will seriously degrade the sound and possibly even affect the reliability of the drivers.

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Positioning the ME2's

I recommend placing the ME2's at the tips of an equilateral triangle with your listening position at the bottom of the triangle. Aim them at a point about 1 foot in front of you, with the tweeters on the inside of the stereo pair. You should be able to see just a little of the side of the cabinet that has the large radius.

Next, adjust the height of the stand so the centerline of the tweeter is between 38" and 40" high. If the tweeter is below ear level, the stability and overall size of the stereo image will be degraded, so adjust the height so the tweeter is at or a few inches above ear level. (Many compact speakers that "sound bad" can be cured simply by elevating the tweeter to ear level! This is a psychoacoustic effect that doesn't appear in any measurement.)

When the ME2's are correctly set up and used with the right electronics, they will have a very big, spacious sound, a stable stereo panorama well off-axis, and will sound quite natural and lifelike even from another room. They are "tuned" for the best possible voice reproduction, so use recordings of male and female singers to do the final in-room tuning with crossover levels, cables, damping material, location from the back wall, etc.

When you are all through with the tune-up process, the ME2's will have all of the sonic quality of the Ariels ... they're just as solid, use the same drivers and crossover, and are just as efficient.

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Amplifiers and Sound Quality

As mentioned above, the Ariels and ME2's have an efficiency of 92 dB for 1 watt at one meter, which is 2 to 5 dB higher than most other audiophile speakers. This means that the amplifiers you use with it will not be operating in the usual power range, so some amps will sound better, and some worse.

With typical music having a 20 dB peak-to-average ratio, and the peaks set to 105 dB, the average power level delivered to the speaker will only be 200 milliwatts. That's not a whole lot of power. This means the amplifier will be operating at quite low levels for most of the time, in a region where many high-end amplifiers are not at their best. As you might imagine, the Ariels really shine with modestly-powered triode amplifiers, but other amps can also sound quite good as well. Here's how the Ariel sounded on two amplifiers I had in my home; with a few exceptions, the sound ranged from quite good to extraordinary, depending on the amplifier.

At the time of writing, I've heard about twenty different power amps with the Ariels. Some were conventional Class AB transistor amps, some were push-pull pentode, and some were Class A direct-heated triode amps. Surprise ... they all sounded quite different, and some very expensive amps were duds, and some modest ones sounded really good. The three standouts were the Audio Note Ongaku, the Reichert Silver 300B, and the RE Labs LNPA-1. The RE Labs is a fully-regulated complementary Class AB transistor amp, so don't get fooled by the fads that sweep through the high-end.

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A Second Opinion on the Ariels

Karna and I both enjoy listening to the Ariels, and so do our guests from the wild. Squirrels and birds sometimes drop by when I have the porch door open and listen to the music, and fly off when the piece is over. One time the resident squirrel joined in with a ten-minute duet, something I wouldn't have believed possible. Aside from these notes from nature, though, I haven't had a clear idea how other human beings (particularly those of the audiophile persuasion) reacted to the Ariels.

So when Phillip Griffin, General Manager of The Parts Connection in Canada, called me out of the blue and said he had gone ahead and built the Ariels, I was naturally curious about the outcome. When you write an article in a magazine, it's like throwing a bottle in the ocean - there's no way of telling where it will go, or who will be the first to pick it up. It was a pleasant surprise to meet Phil on the phone and hear that the Ariels had gone well for him. That was a year ago; every now and then, I get a phone call from another PF reader who tells me about their experiences building the Ariels or the ME2.

A few weeks ago, some questions popped up on the Joelist (an Internet mailing-list for ultra-fi audio), and Phil jumped in and said good things about the Ariels. I'm not shy about asking for small favors, so I asked Phil if he'd mind writing a few short paragraphs for the book describing his experiences. A week later the following e-mail letter appeared in my computer:

To: lynno@teleport.com (Lynn Olson)

From: pgriffin@sonicfrontiers.com (Phil Griffin)

Subject: Ariels

A couple of years ago, I was in a bit of a quandary as to what to do with my system. I had recently moved to a small loft apartment, with a small living/listening room of a mere 11' x 14'. I was also getting more interested in lower powered, (<20W) simple circuit topology tube amps, and while I was enjoying what I was hearing from these designs, it just wasn't enough power for the Spica Angelus that I was using at the time. So, I needed a more efficient speaker, but didn't have the room for horns or Lowthers or anything else that had the requisite sensitivity. (Besides, I was getting enough grief over the presence the Spicas had in my diminutive room.)

But, what's this I see in Positive Feedback Vol. 4 No. 4-5? A 92dB sensitivity speaker that only takes up 8" x 11.5" of floor space? Has a flat impedance curve? Won't cost a fortune to build? Has reasonably extended bass response? A design that actually seemed to make sense? Well, it seemed too good to be true, but I thought I'd build it anyway. Luckily for my natural procrastination in projects like this, I hadn't started the cabinets before Lynn's next design article in PF with the labyrinth transmission line. (Lesson: Never build anything from a magazine article until the next issue is out with the revisions!)

With Lynn's article in hand, along with some AutoCAD drawings for the panels that I did, off I went to my cabinet maker friend's place to try to browbeat him into building a pair of cabinets for me. After a look at the plans, lots of cussing, and the suitable bribes (Canadians will agree to almost anything after a few beers!) he agreed to build a pair for me. The cabinets went together with surprisingly little fuss, even though this is a MUCH more complicated woodworking project than your average reflex box with a brace or two. I hereby reinforce Lynn's assertion that accurate cuts are essential.

My prototype crossovers were professionally assembled on a slab of MDF, drivers were installed, and the whole works connected up with alligator clips and jumper wires! Time to listen! WOW! I was very impressed right from the start, the Ariels had more extended, controlled, articulate bass than the Spica Angelus, imaging at least on par, a much more extended, airy, detailed treble, and a sense of dynamics and leading edge attack that I had only previously heard from horns! We were on to something here! (The Spicas were only heard once after that, when their new owner came to audition them!) After a couple of months of break-in and tweaking, the Ariels proved to be all that they seemed, with solid bass down to about 45Hz in room, an exceptionally dynamic and tuneful presentation (that rhythm and pace thing if you prefer) excellent imaging capabilities, and a slightly warm tonal balance that makes for fatigue-free long listening sessions.

The Ariels' bass performance is quite sensitive to the stuffing in the line and labyrinth, and some experimentation may be necessary to get the best bass in your room. The adjustable height line exit is also of great help in tuning the speaker and gives more flexibility in room placement. Small changes can be quite noticeable, so proceed slowly.

Crossover component changes are also quite noticeable, rest assured that premium quality parts are not lost on this speaker. After a few months of experimentation, I ended up with something very close to Lynn's Version 2.2 crossover. (Much to my surprise when he mailed me the schematic!)

I ended up using the Hovland MusiCaps in all critical areas, with Solen's in the impedance compensation network on the woofers, SOLO Copper Foil Air Core inductors, and Caddock MP series metal foil resistors. The premium quality parts simply added to the Ariels' natural sense of ease, dynamics, and improved resolution of low-level detail without ever sounding etched or analytical. I also used 0.1uF Teflon by-pass caps on the series tweeter caps that gave a slight increase in "air" and apparent upper frequency extension.

While I didn't experiment extensively with speaker cables, I did get excellent results using Goertz MI2 bi-wired from amp to crossovers, MI2 as continuous pieces from internal crossover wiring to the speaker inputs, and MI1 as internal speaker wire. The Goertz seemed to work so much better than anything else that I tried, I just didn't pursue it any further.

The Ariels also seem to love low-powered tube amplifiers. With their very flat impedance curve, high output Z amps are simply a non-issue with the Ariels. Most of my listening and "voicing" of the speaker was performed with a triode wired push-pull EL-34 amp of about 25 watts. (Modified Sonic Frontiers SFS-40) This provided plenty of power in my smaller room, even full-range. A triode wired, modded Dyna ST-70 also provided very pleasing results, proving you don't have to spend megabucks on amplification to get very enjoyable music out of the Ariels. I even tried a stock Dyna SCA-35 integrated just for kicks, and the Ariels still never failed to please, just not quite as much so. Excellent results were also obtained with a pair of vintage rebuilt Quad II amps.

A few listening sessions with a DIY SE 300B amp provided a gloriously large soundstage, with images extending well beyond the speakers, but not enough punch and control in the bass. I bi-amped this setup using a cap at the 300B amp input to roll off at 80 Hz (6dB/octave), a third order (18dB/octave) active low pass crossover at 75 Hz, a pair of Audio Concepts DIY 10" downfiring subwoofers, and a Hafler DH-200 amp (or whatever else I could borrow). This was a system that was very easy to live with, with open, liquid, and detailed mids and highs and a powerful bass flat to almost 20 Hz.

This setup also played VERY loud without any sense of strain. I measured approx. 110dB peaks cleanly reproduced in the listening room. This is WAY louder than I ever listen for any period of time, and unless you are a true volume freak, should be plenty for all but the largest listening rooms. It's surprising how far 8-9 watts per side can go when you use a powered sub of some kind. I continued to play with the subs after going back to my push-pull amps, but at least with the Audio Concepts subs, I had to keep the crossover point below 100Hz as a maximum, otherwise there was an audible discontinuity between the Ariels and the subs, no matter what their position.

A pair of forward firing, "fast" sounding subs, possibly sealed or transmission-line loaded, placed close to the Ariels may allow a higher crossover point and greater volume potential. For me, I'm more than satisfied with them, even full range, in my smallish room. I've had the Ariels now for over a year and a half, and am not even beginning to tire of them. I even broke down and built ANOTHER pair of cabinets, but these are actually finished instead of plain MDF! (Thanks to my partner Madelaine, who actually finished them for me in a beautiful "Marbleized" blue glaze finish!) A friend is now finishing my first pair for himself, and my cabinet making friend has built a pair for himself as well.

My audiophile friends have been very impressed with the Ariels, but that's not the real test. When our normal, um, I mean, non-audiophile, friends drop over for the first time, they almost never fail to comment on how good the music sounds even before they make it into the living room. To me, that's passing the ultimate blind listening test with flying colours!

Best regards,
Phil

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Resource Guide

Positive Feedback Magazine
A Journal of High-End Audio
2939 NE 155th Ave.
Portland, OR, 97230
(503) 256-1300 Business Office
(503) 254-3866 Business Fax
(503) 257-2002 Editorial Office

E-Mail: davidr@ix.netcom.com

Website: http://www.audioweb.com/newsstand

As an Associate Editor of Positive Feedback magazine, this is the place where much of this book first saw the light of day. PF has a broader reach than the three mainstream audio magazines, covering design, philosophy, reviews, recordings, and just about anything interesting and/or funny. Each issue has many articles from industry insiders, enthusiasts, and people who just enjoy writing about audio.

Sound Practices
A Journal of Audio Technology
Box 180562
Austin, TX, 78718
(512) 339-6229

E-Mail: sp@tpoint.com

Website:http://www.soundpractices.com

The groundbreaking magazine that just about single-handedly created the triode revolution in the USA, as well as reviving the DIY spirit in audio from decades past. If you subscribe, be sure to get the first four issues, which are required reading if you're serious about building your own triode amplifier. Issue 8 describes the construction of the Reichert 300B amplifier reviewed above.

Speaker Builder and Glass Audio
Dell Publishing
PO Box 576
Peterborough, NH, 03458-0576
(603) 924-9464

E-Mail: audiotech@top.monad.net

Website:http://www.audioweb.com/newsstand

Two unique and outstanding resources for information. I've subscribed to both for many years now, and the quality has steadily improved the whole time. For direct-heated triode fans, check out the Nobu Shiseido article in Vol. 6-1 of Glass Audio; his comments on using the voice as a reference standard is worthy of close examination, and one that I agree with completely.

North Creek Music Systems
Main Street, PO Box 1120
Old Forge, NY, 13420
(315) 369-2500 Fax/Voice

North Creek is now offering the Ariel and ME2 in kit form (so I'm told by George Short, the principal of North Creek). They also offer top-flight drivers, Sprague 730P and Hovland capacitors, tapped air-core inductors, Ohmite 1% resistors, binding posts, glue, etc. as well as excellent information on subtle points of cabinet construction. In addition, they have several interesting designs of their own, which appear to be serious high-end speakers, and far better than the typical DIY project seen in the pages of Speaker Builder magazine.

Madisound Speaker Components
8608 University Green
Box 44283
Madison, WI, 53744-4283
(608) 831-3433 Voice
(608) 831-3771 Fax

E-Mail: madisound@itis.com

Website: http://www.itis.com/madisound

The folks in Wisconsin are friendly, have detailed spec sheets for the wide range of drivers they carry, and offer complete ready-to-build kits. Madisound also has the other bits-n-pieces needed for a complete loudspeaker, including the essential Deflex damping pads for the interior of the loudspeaker cabinet. The FTP-able Madisound BBS has many public-domain and shareware programs for loudspeaker design.

Just Speakers
4075 Sprig Dr.
Concord, CA.
(510) 685-5252 Voice
(510) 603-2724 Fax

Website: http://www.justspeakers.com/

Friendly, oodles of interesting drivers, but no spec sheets unless you ask. They carry Audax Norsorex gaskets, Solen Perfect Lay inductors, Hovland MusiCaps, and many other goodies. Up-to-date info (or lucky guesses) on which vendor is using what drivers in their commercial speakers.

ORCA Design & Manufacturing Corp.
1531 Lookout Drive
Agoura, CA, 91301
(818) 707-1629 Voice
(818) 991-3072 Fax

E-Mail: orcades@aol.com

They sell a broad range of products, including the French Raven R1 and R2 true-ribbon tweeter, the Italian CLIO speaker measurement system, and the Sofia computerized curve tracer and distortion analyzer for vacuum tubes. Lots of European high-end drivers, crossover components, etc. etc.

Michael Percy Audio Products
P.O. Box 526
170 Highland
Inverness, CA, 94937
(415) 669-7181 Voice
(415) 669-7558 Fax

Michael specializes in the hard-to-get parts that appear in the most exotic audiophile and ultra-fi equipment in the world. Want to get Yamamura wire, Rel-Cap polystyrenes, Black Gate carbon-fiber electrolytics, Caddock and Mills resistors, and Deflex damping pads? It's all here.

Michael does not accept VISA/MC, so a prepaid check or money order is the only way to order. Use your fax machine to avoid the very busy telephone.

The Parts Connection
2790 Brighton Road
Oakville, Ontario, Canada L6H 5T4
(800) 769-0747 Order Line
(905) 829-5858 Voice
(905) 829-5388 Fax

E-Mail: TPC@sonicfrontiers.com

Website:http://www.sonicfrontiers.com/TPC

Really big catalog here, with even more high-end and ultra-fi goodies. There's only a minor overlap with Michael's product line, so you'll want to get both catalogues. TPC has more of a build-from-scratch orientation; you can build up a complete tube amplifier with what's in here, including output and power transformers from MagneQuest and Hammond, as well as chassis and other major parts. Since TPC is affiliated with Sonic Frontiers, a long-time vacuum-tube manufacturer, they have considerable buying power. That's probably the reason they carry Rel-Cap Teflon caps ... these are really hard to get! This is the only place outside of Tektronix where I've ever seen them.

MagneQuest, Peerless & Acrosound
1404 East Bristol St.
Philadelphia, PA, 19124
(215) 288-4816 Voice/Fax

E-Mail: MQ@magnequest.com

Website:http://www.magnequest.com

Michael LeFevre was a disciple of the top engineers at Peerless transformers, and eventually took the plunge of buying the archives of the Peerless Transformer Corporation and shipping them 3000 miles to Philadelphia. Since Peerless was part of Altec for many decades, he has design information and construction techniques going right back to the Western Electric archives, as well as selected designs from Acrosound, Chicago, UTC, and others.

The MagneQuest series are original designs of his own, and are used by Gordon Rankin, Noriyasu Komura, and other SE triode amplifier builders that appear in the pages of Sound Practices magazine.

Supermods
Stan Warren
Eugene, Oregon, USA
(541) 344-3696 Voice/Fax
(no answering machine, so be patient)

Stan does modifications for Audio Alchemy, Monarchy, and other well-known brands. Stan, like Michael LeFevre, is very modest, and "mods" very much understates what Stan really does. He redesigns large portions of the original circuit, removing bugs and design oversights, and upgrades the parts quality of the most important components. When Stan gets through with a unit, it doesn't sound anything like the original ... it sounds a lot better.

As the owner of a Stan-modified Monarchy M-33, I can attest that Stan can take a unit that is already quite good value-for-money and elevate it to the highest levels of the art. The only drawback to Stan's work is that he charges about one-quarter of what audiophiles expect to pay, so they don't always take him seriously, simply because he's not charging thousands of dollars. If you can live without an endorsement from Stereophile and a fancy faceplate, give Stan a call.

Stressless Chairs
Ekornes of Norway

At better furniture stores from $900 to $1200

Hey, stop slouching in that couch! You've devoted all that time, money, and energy getting your hi-fi to sound good, so why not relax and enjoy it? Besides, if you're physically tense, your hearing won't be as attentive and pleasurable (really true, folks).

The Stressless chairs are to a sofa or ordinary chair what a ultra-fi system is to a car stereo. All I need to do is get a review sample for a (very extended) in-home review, and I've got it made.

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Links to the WWW

AudioWeb Home Page
http://www.audioweb.com
The CES of the Internet; this is where a wide range of vendors, magazines, audio clubs, classified ads, and links to hobbyist pages can be found. It also has a very unusual feature: a chat area, something I've never seen before in a audio-related Web page.

Hi-Fi on the WWW
http://hydra.unik.no/~robert/hifi/hifi.html
Fascinating Hi-Fi Show Reports with photos, product reviews, articles, and many audio-related links.

Tom Dunker's Home Page.
http://invalid.ed.unit.no/~dunker/index.html
Lot of interesting horn & triode-related links. The photos of Tom's speakers are amazing!

Craig Stark's DIY Page
http://www.psy.cmu.edu/~stark/audio/DIYaudio.html
Lots of interesting projects here. The links have informative descriptions and do a good job of covering vacuum-tube amplifiers, high-quality DIY speakers, and high-end audio.

Roy Viggo Pedersen's Speaker Building Page
http://bundy.hibo.no/%7Erpd/Speaker/
Kits, complete plans, and detailed specifications and curves for advanced drivers such as the new magnesium-cone Seas Excel series.

The Poly Natalia
http://www.blackdahlia.com/do_poly.htm
The sound of Dick Olsher's speaker is closer to the American high-end audio mainstream than the Ariels. To put it another way, if you like Wilson Audio, Avalons, and Theils, you'll like the Poly Natalia; if you like the original Quad ELS, BBC monitors, and ProAcs, you'll like the Ariels.

Dan Hildebrand's Do-It-Yourself Speaker Project
http://www.qnx.com:80/~danh/speakers.html
Interesting design published in Speaker Builder magazine, and a mind-boggling number of links when you click on "The Audio Info Site List."

The BASS and DIY Loudspeaker List
http://reality.sgi.com/employees/pdc/bass/index.html
Archives of the list going back several years.

Kevin's Vacuum Tube Audio Page
http://www.ultranet.com/~kennedyk/
Fellow PF editor Kevin Kennedy has "written the book" on modifying the Citation II amplifier. Current projects include a 6CK4 triode amplifier, hard-to-find info on the 6C33, and a review of the new Svetlana 6550C.

Dave Cigna's Audio Tubes.
http://www.phy.ohiou.edu/~cigna/tubes/tubes.html
Plate curves for lot's o' interesting and useful tubes. Good links.

The Farnsworth Chronicles.
http://songs.com/noma/philo/intro.html
The REAL Inventor of Television.

View of Portland
http://www.tek.com/PDX_Pictures/picture.jpg
Speaking of television cameras, this is what the hometown of Positive Feedback magazine, Nutshell High Fidelity, and yours truly looks like right now. Thank you, Mr. Farnsworth!

Monk's Guide to Portland
http://www.neo.com/Monk/Portland/guide.html
And for a completely different view of the City of Roses ...

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The article above is a chapter from the forthcoming book, "The Soul of Sound," by Lynn Olson, and is based on material previously published in Positive Feedback magazine. All material herein is Copyright (C) May 1996, all rights reserved, by Nutshell High Fidelity. Permission is granted to Webmasters, readers, and any others to link and mirror this HTML file, along with the associated image files.

lynno@teleport.com