Designer Leo Spiegel and Jason Bloom of Apogee Acoustics, with their top model, the Apogee

Apogee Duetta

Full-range ribbon speakers are an Apogee exclusive. Now, with the arrival of the Duetta, their technology comes more cheaply - but remains very expensive. Alvin Gold interviews Apogee designer Leo Spiegel, then goes on to review this remarkable testament to US pioneering spirit

AG: What's so unusual about a full range ribbon loudspeaker, and how does it work?

LS: A ribbon is a thin, flat conductor in a magnetic field. We have the only woofer designed as a ribbon available today. In our case the low frequency ribbon is approximately 12 inches wide by 37 inches in length. Its trapezoidal shape is to get the Q of the resonance approximately right, so that when we tighten up the ribbon mechanically we are able to get the proper low frequency response shape.

How does a ribbon compare to other types Of flat diaphragm (eg electrostatic) physically?

It's heavier than an electrostatic diaphragm. The advantage however is that we have the structural properties of a metal. If you stretch a plastic film as used in other types of flat diaphragms it slowly returns towards its rest position, but never really makes it. If you stretch a metal, it obeys Hookes Law and does comeback. If you listen to a ribbon tweeter you'll notice that it's very very transparent. This transparency we achieve because we're dealing here with a mechanical spring.

Up to what frequency does the diaphragm stay rigid?

It will go to 2-3kHz before we get some HF oscillation. The crossover is at approximately 500Hz. The other interesting thing is that in the critical area of the crossover we have a nice, clean flat response. That means I can use a 6dB/octave (electrical and acoustic) first-order slope on the woofer.

Presumably another advantage of your ribbon is that you don't need a matching transformer.

Right, and that's the reason we've manufactured the Duetta ribbons to have approximately 5ohms impedance - we'd like to have it less so that we could pull more power from a normal solid state amplifier. Efficiency is around 80dB/watt at 1 meter, which is low, and we recommend a 100watt amplifier. But remember that a point source rolls off at l/distance squared whereas an ideal line source rolls off at 1/distance and sounds louder than you'd expect at normal distances.

I noticed that the midrange/tweeter ribbon flaps around like a sail in a breeze at a very low frequency. What effect does this have on, for example, Doppler distortion?

Because of the very complex construction of the midrange/tweeter it is at around 50Hz. That's the price for taking away blending problems between the two drive units.

Can you not damp down the resonance?

We've tried a multi-slope crossover, with 6dB near the crossover point and a higher rate elsewhere, and we found that this does a wonderful job of giving separation between the midrange and woofer. This may he something for the future. In my experience in the audio industry multi-slope crossovers are not done, but my background is in the control systems area, and there it's done all the time. When we tried it out in our laboratories we found we were getting an in-depth layered quality to the sound. It sounded absolutely stunning.

How is the Duetta constructed?

We have a wood frame which stays fairly stable, on which we screw down a perforated steel plate. Onto that we cement our ceramic magnets. On the top and bottom there's a steel angle iron which maintains alignment and dimensional stability. The side frame of the woofer into which the ribbon is placed is two squares of chipboard, then we have a spring loaded side piece which maintains tension. There's a very interesting point here in that the ribbon is always under tension. A plastic diaphragm is not always under tension because of the hysteresis of the plastic.

How uniform is the magnetic field under displacement given that the unit is not push-pull?

It's not perfect by any means. You get a change in gain as you move. What it leads to on a sine wave is greater displacement as the diaphragm approaches the magnets, and less in the other direction. At normal levels it's not an issue, but the magnitude increases at higher levels. If you go to a double sided magnetic structure acoustic impedance increases as you try to close the panel/magnet gap, and this acts as a partially compensating mechanism, though it's still not as good as a double sided design. A push pull woofer might be theoretically better, but it might sound worse qualitatively.

Review

It is often said that Britain can be proud of its record for making the finest loudspeakers in the world, but that we do rather less well with the esoteric technologies like Compact Disc, where Philips and Japan Inc share the honours (blame?). The facts, however, support neither proposition. We actually make both the finest and the second finest sounding CD players in the world (to the best of my knowledge; etc- the Cambridge Audio CD1 and the Meridian Pro-MCD, in that order) while our record on loudspeakers is pretty miserable when you compare ours to the undeniably exciting and adventurous 'shoot for the stars' attitude evident in the USA.

No, I haven't forgotten what Wharfedale and Celestion have done with metal domes, or what Mordaunt-Short achieve without crossovers. And I wouldn't dream of passing over the Quad ESL-63, which makes a tremendous contribution to the panel speaker art whose implications have still to be taken on board by rivals, though the design is substantially incapacitated by headroom and LF limitations.

The remarkable vigour and strength of US speaker design has been amply demonstrated in these columns before with the Snell Type A, the Magneplanar MG-I and MG-III, the Acoustic Research MGC-1 and others. The brand new Apogee Duetta (launched last June at the Chicago CES) is the perfect exemplar of the superiority of Uncle Sam. Apogee have taken what in this country remains a relatively obscure idea, getting rid of boxes in favour of flat baffle mounted drivers, and for the drivers they have used a principle that is quite unique - the ribbon, previously associated only with tweeters, microphones and, once upon a time, cartridges of the phono variety.

The Duettas are visually imposing. The framework is finished in a grey Nextel, and the gold finished ribbons glint through the panels in an unnerving fashion. At the end of my period with them I cannot make up my mind whether they've been beamed down from the 1990s or are a throwback to the '50s. Either way, the shape makes them look as though they're falling backwards, which is most disconcerting. They're heavy too at 115Ibs each.

There's a lot of ground to cover so I'll keep the description short. The interview has given much of the inside story anyway. In general form the bass ribbon, which operates up to 500Hz, is a large trapezoidal affair, and the midrange/tweeter unit is the long strip along the inside edge. The speakers sit on stands which have a tilt adjustment and are of reasonable structural integrity, though I have suggested spikes and been told this will be tried. The Duetta is equipped for bi-amping (or bi-wiring), but the crossover stays in circuit as usual with US speakers extensive rewiring would be needed for full active operation.

Let me make one thing clear. This is the finest loudspeaker I have ever heard, at any price or in any circumstance. Mind you, there are many loudspeakers I've yet to hear. However, not everything about the Duettas is sweet and lovely, and I'll be spending some of the remaining space on these pages explaining this. But above all is the (to me) astonishing revelation that here at last we have, in a pair of loudspeakers, a level of musical ability that when all the 't's are crossed and the 'i's dotted is good enough to make the flesh prickle. This happens regularly, reliably and often. The Duetta is a stunning achievement, far ahead of the Maggies I'm used to using at home, and you don't sacrifice maximum SPLs or bass extension in the process. The Duetta goes loud and deep.

Of course nothing on this scale is achieved without cost, and the price here is high. High financially, of course. In the States the Duetta (Apogee's budget model!) costs $2,300, but their extraordinary weight, bulk and fragility means that shipping costs are inordinately high, and other expenses are also incurred. But why should I make the excuses? Let Ricardo of Absolute Sounds (the importers) do so. He has assured me he is not profiteering, and he has quoted chapter and verse to support the proposition (like, f'instance, they cost within a few percentage points the same here as in other European markets) in the face of the £3350 UK price. But I can tell you one thing. If sound is the deciding criterion, they're worth every penny.

The bad news, however, is that the near three-and-a-half grand's worth of speakers isn't the only expense you're going to run into. There's the question of an amplifier to suit. Unlike the bigger of the two Apogee models sold in the UK, the £5,000 Scintilla, the Duetta has a nominal 3-and-a-bit ohm rating and is designed for use with 'relatively ordinary' amplifiers. But don't get too excited. My own 50watt Krell KSA-50, which by any normal standard is well able to cope with awkward loads, proved unequal to the Duetta when the chips were down, as was confirmed most dramatically when I borrowed one of the new generation Krell 100watt stereo power amps for a short spin. (Cost: If you, have to ask ... ) Make no mistake, the KSA-50 works and the speakers sound great, but the KSA-100 adds more than mere headroom, it also adds authority, consistency with level and a real feeling of grip. None of this is helped by the low efficiency of the speakers of course, but this proves to he less of a problem than you might expect for the reason given in the interview: a dipole produces an SPL proportional to one over the distance, where a point source obeys a one over distance squared law.

Apogee claim that the Duetta works well with some tube (valve) power amps, but I remain sceptical. It's something you'll have to try for yourself. The ground rules for choice, however, are clear enough. The amp must have high power and high current capabilities of a kind that no UK made solid state amplifier has under its belt at present. The manufacturer recommends 100watts a side minimum, and the required kind of watts don't come cheap.

The other point I'm sure you'll not he surprised to hear is that the Duettas act as a microscope to the sound of a system such as you have never experienced before. You need the very best amplifier, preamplifier and source money can buy. The deficiencies in my 50watt Krell have never been so glaring. I also note without further comment the observation that the qualitative gap that separates the best of analogue from the best of digital has never sounded so wide. I ended up using records almost exclusively.

Just for the record, equipment used during these tests included, among others, Linn and Roksan turntables (you'll be hearing more about the latter), Ittok arm, Koetsu Red Signature cartridge and Musical Fidelity MVT and DNM preamplifiers. The loudspeakers were bi-wired which brief comparative tests suggest is essential. I was unable to try twin solid-core cables in this case, but the reactive, current hungry nature of the Duettas means that ideally thin cables are not on, and the logic of thick wires for the bass and thinner ones for the midrange/tweeter panels is probably unassailable.

I have never in all my 21 years (honest) heard a loudspeaker like this one. I thought I had a pretty good idea what to expect from experience with Magneplanars, with which the Apogees share some general features though not, of course, the operating principle. I was half expecting that the treble would he a little sharper, and the midrange more rounded, but that the bass would be roughly comparable -after all, the major dimensions and geometry of the two models are about the same and these are two of the major determining factors. How wrong I was!

Split crossover allows bi-wiring or bi-amping

I poured myself a fat Havana cigar to get in the mood, lit a glass of Schlitz and settled back, a pile of vinyl at my knee for what turned out to be a fast and exciting (if bumpy) ride. Right from the first, the Apogees sounded magnificent. Had you been there at the time you would have been in no doubt about the transparency of the design, its consistency over the entire frequency band, and the extraordinary range and depth of tonal colour. The Duetta gives whole new meaning to the term 'separation', a quality it achieves in one hundred and one different ways.

But these things are mere generalities. The first real surprise was the sheer realism, power and, above all, quality of the bass. There is none of the dryness I expected, nor any of the overhang or one-note ringing qualities of so many box loudspeakers. The bass has a direct, open quality with a very clear sense of pitch and timing that makes of mockery of most other speakers. When the bass end drives like an express train you'd better get out of the way!

Among examples to illustrate the Duettas' superiority are some that are almost trite. I cite the track Smokin' (Empty, Try Another) from Joni Mitchell's Dog Eat Dog album which includes a recording of a cigarette machine repeated as the main feature of the track. It sounds pretty impressive on any good loudspeaker, but it was happening right there in the room with the Duetta. There was no mistaking the effect which sounded so real that even a non-audiophile would recognise the quality of the loudspeaker without hesitation. (I put it to the test more than once, so I'm not just puffing strawberries here.)

But this doesn't necessarily make the Apogees a very edifying prospect to sit down to listen to. Generally speaking you go to a concert because you're keen to hear the programme being played, but hi-fi is rather different. Often it will be used merely to play something quiet and soothing or while reading a book. A good Apogee-based system will not really do the job here. Even when played relatively quietly the speakers are just too distracting. In the past this has tended to indicate a limitation or problem of some kind, but here it was for the other reason. After all, if the musicians are actually playing in your living room you don't turn your head away, do you? This kind of realism, which springs directly from the extraordinarily explicit nature of the sound, can be a bit of an earful and occasionally hard to take. But listening to anything else afterwards has all the positive attributed of coitus interruptus.

I have to say that at the all-too-short end of the review period I did not feel that I had plumbed the full range and scope of the Duettas. I glimpsed nirvana through the use of different power amplifiers especially. I was not unaware of some limitations too, and they didn't meet with universal approval with visitors. The Man Who Does Not Like Loudspeakers (hello Ed!) was true to form. In truth the sound does tend to be thrown in your lap in an occasionally irritating way and, although its presence is almost palpable, depth images are curiously compressed. From the evidence I have gathered, a little extra space between speakers and listener would have helped, and equipment limitations elsewhere were certainly part of the problem. Additionally, although the Duettas were almost totally free of coloration in the accepted sense, the bass level (not quality) was often excessive, leading to a richness that was a little unreal. Finally there was a suggestion of unevenness in the midband which was associated with some losses of harmonic structure - and may also have been related to the flat imaging. On the other hand, the Apogees offered a usefully larger listening area than the Maggies which can be rather antisocial in this regard.

I have to call a halt here, or I'll overrun the page. I'll conclude simply by saying that whatever the specific pluses and minuses, if I had the necessary I'd buy a pair today. I believe the Apogee sets new and, in some respects, unprecedented standards in the areas of realism and musical truth, and is true state of the art. OK, I accept real people may not be able to afford them, but they may act as a spur to other designers to produce better and more affordable things in the future.

Alvin Gold

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