Reprinted from Hi-Fi News & Record Review, December, 1988
APOGEE DUETTA SIGNATURE
Apogee
enters a new era with the Duetta Signature: Martin Colloms enthuses
Based on earlier Duetta models and related in technology to the Caliper and Diva, the Signature's design principles involve the use of a freely suspended open ribbon element, of medium resistance and made from thin-film aluminum conductors bonded to a Kapton substrate. No step up transformer is required and a reasonable sensitivity can be obtained. In the Duetta the ribbon is a substantial 1180mm high (46½ in) by 25mm (1in) wide, and is complemented by a large, flat diaphragm woofer which is also a kind of ribbon - but is suspended on a tensioning system and driven by an array of magnets over its surface. This design resembles the Magneplanar form of bass unit.
The sheer size of the LF panel is impressive - a trapezoidal structure 35cm wide at the bottom tapering to 27cm wide at the top, and again some 118cm high. This is roughly equivalent to the effective radiating area of ten 12in bass drivers, though the allowable excursion of the Apogee diaphragm is probably only one third that of a 12in piston, reducing the equivalence to three such drivers - but still capable of delivering a sizeable wallop.
The Signature's outer frame is also trapezoidal and stands an imposing 1.5 meters high (almost 5ft) on its spiked feet. With a 72cm (28½in) wide base, it tapers to 62cm (24½ in) at the top, but is only 7cm (3in) deep excluding the projecting feet. Electrical connection is via two pairs of three-way binding-posts, one for bass and one for mid-treble, which may be used directly for bi-amping or strapped together for single wiring. It will be possible to connect Apogee's new electronic crossover, which, if of genuinely high quality, could improve the performance of the system beyond the passive case. On unpacking, the two rigid aluminum feet must be fitted using four socket-head bolts for each foot. Once the best position has been found, spikes can be fitted to lock the system to the floor.
The speaker is available in light or dark grey, as well as Taupe, a yellow beige, which I find, shows finger marks too easily. Weighing 95kg, these models generally need two people to shift them and require dedicated positioning for best performance. Purchase should not be lightly embarked upon, as compatibility with one's house and spouse is essential!
Technical details
Rated at some 200W (8ohm rating) or 400W at 4ohms, these speakers are of moderate sensitivity and require and deserve healthy output powers. However, the sensitivity is actually rather higher than with previous Apogees, and a solid 100W per channel will deliver satisfactory sound levels.
With a free-air resonance of only a few Hertz, the mid/treble ribbon possesses a very extended bandwidth making for easy 6dB/octave crossover design, comprising simply a high quality polypropylene series capacitor. Wire wound resistors permit setting of the tweeter sensitivity, and these and the associated switch are probably the weakest link in the design. In an ideal world the tweeter balance would be predetermined and the switch attenuator omitted.
The Kapton ribbon has three vertical conductors of aluminum film, arranged with the same direction of current flow and thus requiring looped interconnections, achieved with thicker aluminium straps. The ribbon is gently corrugated in the horizontal plane to prevent it bending about the vertical median axis, while the 1½ in wide magnet gap has a linear field capable of driving the ribbon as a single plane surface, thus representing an ideal transducer free from breakup over the whole audible range. A low-mass foam element bends the ribbon through a small angle over one third of its height, so improving directivity in the vertical plane. At normal listening distances the radiation from such a driver approximates to that of a bi-polar vertical cylinder, with very little rolloff in the lateral plane.
The crossover element for the woofer is a single low loss air-cored 2mH inductor feeding the 3.8ohm diaphragm, and the latter is horizontally corrugated like the treble unit to help keep it operating as a single plane. It effectively works only in the fore and aft mode. The current path is a zigzag engineered by hand cut slots in the foil conductor, while the current lines are asymmetrically energized by loops of magnetic field generated by rows of alternate ferrite bar-magnets located behind the diaphragm. These are founded on a heavy, well-perforated steel plate. Once in place, the main resonance is hand tuned by controlled tensioning of the suspension membrane at the diaphragm edges. The combination of the trapezoidal shape and differential tuning provides a relatively smooth and extended LF response, with the lowest resonance set at 30Hz and providing extension to 25Hz in larger rooms. The short, simple conductor paths, open construction, and lack of matching transformer make this kind of panel speaker potentially very transparent.
Testing
Given the time allocated for the review, it was not possible to take these speakers to an anechoic chamber. In any case, with the exception of the point source Quad 63, open panel speakers generally require measurement at 3m or more, a distance out of the question even in the test chamber. Accordingly, the speaker was analysed in the listening room, and the measurements should be seen more as offering an exploration of its performance than a definitive view of its frequency response.
It became clear that as more components were tried the more one learnt about the Duetta Signature's potential. The starting point for the power amplifiers was my own Goldmund Mimesis Three, which gave fine results to a respectable maximum listening level; but the Signatures were clearly not being stretched despite the amplifier's quite decent peak current and 180W peak programme capability. In fact, though the Duetta can be regarded as a fairly kind 4ohm load of quite moderate reactive component, the standard Mimesis could be driven to cut-out long before the speakers. Used with discretion by an experienced owner, the Duetta Signature can take up to 350W peak programme without distortion or limiting (8ohm speaker rating), and such powers will be required to exploit the speaker fully.
The Duetta can certainly take advantage of both power and quality, and while the ARC D125 certainly demonstrated the latter (see separate review), one wished for the impossible: D125 sound in an A470 power package. The D125 would play to modest levels from full orchestra, and provided one didn't become greedy for more volume it delivered the highest quality sound, further extended in the split 4ohm secondary bi-wired connection. We tried bi-wiring the Duetta with considerable success and moved on to bi-amping in conjunction with my second Mimesis Three. Significant advances in sound quality were noted - tighter, firmer, more open bass, greater apparent LF extension, more stereo depth and focus, plus a more delicately transparent treble with greater air and sparkle. It sounded almost too good to be true.
Yet another power amplifier was then tried - the new Krell KSA-200 - a further real power-house with bass to match. It worked very well with the Duetta, and one was tempted to consider that the optimum solution for the Signature would be a starter system based on a bi-wired KSA-200 (£6000), to which a second KSA-200 could be added for bi-amping as (or if!) subsequent finances permit. Rated at 200W per channel, this amplifier delivers around 320W unclipped programme into 8ohms (over 60OW into 4ohms) and will provide ample power.
Cables were borrowed from a number of sources, including van den Hul and Siltech, and it became increasingly clear that the finest sound resulted from the use of monoblock or stereo bi-amped power amplifier units located close to the speakers, using the minimum of cable. Top-class interconnect is relatively more economical than the equivalent quality of speaker cable.
My final location was with each speaker normal to the side wall and facing almost perfectly straight-ahead, spaced l8in from the side wall at the base and set about 4ft from the back wall, with the latter undamped and undraped. Curtaining tended to reduce air and ambience. The speakers were set 7ft apart, tweeter ribbons inwards, with the listener sitting at the apex of an equilateral triangle based at the centres of the midbass panels. A listener position 1-3ft from the rear wall proved fine. The main listening room was 19 x 13½ x 9ft lwh, but previous experience with other Apogees and Magneplanars has shown that such large panel speakers can sound better in still bigger rooms. Mine would be classed as fairly small for this model.
These speakers are able to throw a soundfield a considerable distance, since they do not obey the inverse-square law which controls the output of smaller box systems in larger rooms. Big panels sound unexpectedly loud at larger distances in bigger rooms, and continue to improve in matters of bass extension and overall coherence as well as stereo focus, due to their bi-polar directivity.
Sound quality
Once all the preliminaries had been dealt with, we could get down to serious auditioning. At last, that marvellous 'Scintilla effect' was re-created in my listening room by another Apogee model, a living testament to the innate rightness of this loudspeaker's performance. This amounts to a strong appreciation of the Signature's midrange, while a still greater level of achievement was apparent in the bass. The Duetta is no longer bass-heavy and under damped. Quite the contrary: Signatures bass was crisp, tight, and highly tuneful and conveyed hitherto unexpected levels of detail. In my room the bass was well maintained to 30Hz, even at high power levels, and I can willingly trade the superficially more extended but less well controlled output of the earlier Duetta's for the performance of the Signature.
In general the bass was reminiscent of a well set up Tympani IV - high praise indeed - while the Signature goes further and appears to define the word 'slam' in an absolute manner. Free from audible compression at any subjective sound level, this Duetta puts together correctly the weight, bang and edge of a transient bass note. Once you have heard it, you know it is right. All manner of drum sounds, large and small, were played with conviction, and so obviously free from the boom, overhang, ringing, boxiness and blurring which are the norm with lesser speakers. Moreover, when wafting large quantities of open, free bass into the room, this speaker showed no loss of detail in the mid and treble.
As is the wont with good speakers, this design did not impose itself on first hearing; rather it sounded smooth, unexaggerated and apparently unassuming. If you are looking for superficial excitement, head-banging dynamics, or an exaggerated clarity and attack, the Duetta may well disappoint. It has inner qualities whose strengths can only be realized with the help of the finest sources. It would be easy to misjudge the Signatures, ascribing fault where the blame really lies with the cable, power amplifier, or earlier source components. Despite some minor flaws, this speaker's behaviour shows most clearly that the major aspects of its performance are relatively unbounded, given the present state of the art. By this I mean that the music-making and the information-retrieving aspects of the Duetta Signature are superior to the best of present-day sources and electronics, and thus barely impose any limit upon them. There is clearly scope to accommodate future improvements, while an existing system will perform to its practical limit with the Signature.
So far I have covered the bass, which was truly excellent, and in practice may be surpassed in quality only by the bigger amplified sub-woofers such as those supplied with the larger Infinity designs. With an extended bandwidth design such as the Signature, the music is reproduced with a natural degree of foundation and authority - the performers are properly located on the floor of the auditorium, and do not appear to lift off the deck as can occur with smaller speaker designs. This natural LF weight and extension carries with it a sense of scale and power a step closer to a real performance. Very occasionally it did sound a mite heavy in the low bass, but see the comments on 30Hz behaviour in the Lab report.
As regards the mid-range, this was undoubtedly very good, almost but not quite beyond reproach. Remembering the liquid, open transparency of the Scintilla, the early Duetta was slightly 'closed-in', a little less pure and transparent. Conversely, the Duetta Signature holds to a very high standard of detail throughout the mid-range, and while slightly dry-sounding, gave an excellent transient response. A shade less perfect than a large pure ribbon or the finest open-construction electrostatics, the Signature mid-range remained satisfying for its consistent quality, which was held over many octaves. Its reproduction of piano was more convincing than from most of the true high-end models I have heard.
As listening proceeded one became aware of an absence of those odd background noises, distortions, what-have-you, which are a fact of life with most other speakers and which ultimately confuse and fatigue the ear and mind. In this respect the Duetta Signature can be considered a low-noise system, or put another way, it has very good dynamic range. When the main 'tune' is playing, spurious sounds are held to a very low level. The worth of this aspect is not immediately appreciated, but it grows with extended listening, and is more than obvious when one returns to ordinary systems.
The treble was extremely detailed; it was clearly in the top class for information retrieval, though I have a trace of doubt concerning its absolute sweetness. Major aspects were handled very well, such as mid-range transient edges, the rasp of brass instruments, sibilants on vocals, percussion, etc. But two aspects caused mild concern: (i) orchestral strings occasionally showed a slight nasal coloration, a little too much rosin on the bow, with a whiff of grain and zing heard in the high treble; (ii) a slight exaggeration of breathiness on massed close-miked choral passages, and a little too much 'edge' on a hi-hat cymbal with rivets. Since such effects have often been identified as the unwanted artefacts of imperfect sources, it is hard to come to a certain conclusion. Is the Signature revealing such distortions more clearly, are they a feature of the speaker, or a combination of the two? At this stage I would vote for the latter, with 67% blame placed on the sources and 33% on the speakers. Duetta Signature treble is alive, transparent, and very low in non-linear distortions of the harmonic variety. It sounded unstressed and dynamically unbounded, impossible to drive to overload either thermally or mechanically, even at high domestic listening levels. No trace of buzz was detected on 200W peak of pure Steinway.
The Signature can certainly play loudly. It can be driven to a stage where the floor shakes with the bass and you have to shout at your listening partner, yet it does not sound loud in the usual hi-fi sense of the word because it is so smooth, aided by the audible absence of unwanted stored energy. Energy for music dynamics must come from the programme itself, and with the Signature orchestral climaxes build effortlessly. The final statement is arrived at without causing one to wince. Many large speakers need to be driven at high levels to make them come to life; used softly, they appear thin and anemic, lacking in clarity, not so these Apogees. Over a wide range of sound levels a consistent clarity and accuracy of balance was maintained.
Finally we come to the stereo image, and here the Signature was state-of-the-art. Yet I find it hard to envisage quite how it achieved this, since the specific focus of simple sounds was not that wonderful, at least not in the context of a near-perfect point-source miniature such as the Wilson Watt. I think it has something to do with the complete believability of the Duetta's performance. So many natural cues were present concerning place, level, acoustic and tonal colour, all uncomplicated by coloration, phase problems, time-delay and the like, and as a result the Duetta projects a most convincing sound-stage. For example, one can choose a track containing a heavily reverberant massed choir, imaged over the whole width of the room, and find this perfectly contrasted with a life-sized solo voice at centrestage. No matter how loud or complex the chorus, the soloist appeared with near-perfect presence and clarity as well as excellent perceived focus.
Stereo stages have considerable scale, while the image has very good width, appreciable apparent height, and ultimately very good depth. I use the word 'ultimately' as this speaker sometimes gives an impression of less than perfect transparency and then surprises one by reproducing a particular track with as much depth as one could wish for. As with the treble, I wonder whether these minor inconsistencies are in fact the responsibility of programme sources rather than the speaker.
Homogeneity was the watchword for the Signatures. They sounded skilfully blended and provided a harmonious texture over a wide frequency range. Their sound was laid-back in a classically correct manner, and on naturally recorded orchestral material from both LP and CD their ability was certain. Aggressively forward, grainy rock material was not spared, and sounded as rough as it really was. On the other hand, high quality rock was reproduced with all the dynamics, power and pace you could wish for, though not with disco-like sound levels. Transients sounded fast, and start-stop quality was exceptional, defining much of the pace and rhythm shown by good programme. This speaker also sounded consistently tuneful, enabling one to grasp the pitch of complex sounds with ease, while normally hidden counterpoint was clearly revealed.
Set against normal expectations, I must note some especially memorable aspects of the Signature's performance. Church organ was excellent (Mendelssohn Sonatas, Argo 414 420-2), with the pedal bass accorded full measure and extension and the upper pipework reproduced with considerable authenticity.
Complex registrations did not collapse into a congested reediness, as is so often the case. This speaker could be bought for its organ quality alone! Choral music (Handel Messiah, Decca 400 096-2) was handled with complete confidence, all vocal ranges sounding natural and clearly defined, well separated and with sufficient body. Speakers submitted for review so often appear to lighten and emasculate bass voices. It must be stressed that this performance was achieved without a hint of chestiness or boom, while BBC studio voices were also handled very well another good sign.
This speaker sounded genuinely aperiodic at low and mid frequencies, free from significant bumps or group-delay effects. Acoustic guitar and harp were strikingly natural, yet wood-block had the correct clack and associated short ting. Belts sounded almost too real to be true, while orchestral brass had a most authentic character. Small bells, triangles and tambourines all sounded lively, 'airy', and tonally correct. Piano was exceptional, cellos and bowed double-bass had excellent texture and sang without apparent restraint, free from the usual boxy emphases. One danger with the Duetta Signatures is that the more you listen, the more you like them. They were certainly seductive and habit-forming!
Lab report
Equated to 3m, the Duetta operates at an approximate sensitivity of 86dBW - a usefully average level. This explains why it is able to deliver a decent 105dBA peak programme for a stereo pair at the listening position - assuming that sufficient amplifier power is available. For comparison, an ARC D125 would provide 100dBA and a Mimesis Three 103dBA.
Response testing a speaker of this size in a listening room is rather a headache, and the curves here should be regarded as rough guides only. Graph 1 shows a 'snapshot' of the reference response at the recommended lateral and vertical listening angles. Given the circumstances, the result is quite remarkable, extending easily from 30Hz to 20kHz, and meeting approximate ±3dB limits above 200Hz. Some lumpiness is present, due mainly to proximity effects in the mid-range, and this did improve at more realistic listening distances. The treble is exceptionally even, especially from 2kHz to 16kHz (measured in 1/3 octave bandwidth resolution).
The driver outputs were measured separately and showed a smooth integration at the 1kHz acoustic crossover point, with well-controlled out-of-band responses contoured essentially to 6dB/octave slopes. The whole system is thus very close to linear-phase, while auditioning the drivers individually showed that they were singularly well balanced. The positioning recommendations were then explored via a higher resolution analysis above 1kHz (Graph 2). This plot shows good extension to 20kHz (in contrast with the Duetta II), while a minor glitch of ±2.5dB is seen at 10kHz, a likely cause being the deep ribbon slot. Moving round to an incorrect position (i.e. axial to the HF ribbon or 20' off the preferred angle), the response was disturbed by a serious irregularity at 4-5kHz, while the smoothness of the tonal balance and extension was impaired. Apogee evidently understands the workings of this ribbon tweeter very well.
Graph 1. Duetta Signature: In-room 1/3 - octave response at 1m
Graph 2. Duetta Signature: High-resolution in-room response above 1kHz measured on recommended axis
If the speaker's vertical angle is out of line - i.e. it is not properly tilted back - the response suffers, and comparing the curve taken at + 15' (Graph 3) with Graph I shows that the upper-bass loses power, while both the lower‑mid and treble are more peaky and the frequency balance is impaired. Subjectively it sounds thinner and nasal.
Save in the 30Hz 1/3-octave, the Duetta performed quite remarkably well in the listening room using the standard integrated RAR measurement (Graph 4). In fact over most of the range it is virtually flat, rolling off above 16kHz and is correctly 'brighter' than the measurement for a system with a point-source dome-tweeter, since the Duetta approximates to a cylindrical radiator. Confirming the listening results, the output is exceptionally homogeneous, meeting tight ±2dB limits from 80Hz to 12kHz. Down to 40Hz the bass is unusually even too, within ±3dB of the 1kHz reference level; but at 30Hz the Signature moves into higher gear, with an undamped output rising 7dB or so above the adjacent bands. It didn't sound as bad as it looks, probably due to the low incidence of genuine 30Hz in programme; such effects placed at 40 or 50Hz are distinctly audible. The effect of the room must also be taken into consideration, and a larger space may well moderate this 30Hz effect.
The spectrum analyser was pressed into service for some distortion measurements. Given that 30Hz is the main resonance, when driven at this frequency to the usual higher 96dB sound level (8.9V RMS) the distortion was likely to be at its worst. Here 3rd-harmonic predominates with a satisfactory reading of 10% (-20dB). The 4th-harmonic is down at 1%, with no significant higher modes. Given that full acoustic output was available to 30Hz, this is a good result - below the threshold for audibility on programme and comparing well with the industry average at this frequency. Only very large woofer systems can surpass it. By 55Hz the Duetta shows improving linearity, with an inconsequential level of -32dB for the 2nd harmonic, and higher overtones at -60dB (0.1%) or better. Remaining at 96dB and moving up the range to 500Hz, 2nd was moderate at I% and 3rd very low at 0. 15%, while at 2kHz the tweeter ribbon results were excellent at 0.1% of 2nd and little else, with 2nd harmonic held to 0.12% at 8kHz These frequencies were not specifically chosen and illustrate the exceptional performance of this large panel design, while when the sound output was reduced to a cruising level of 86dB the speaker showed even lower levels of distortion, down into the regions associated with purely electronic devices.
Graph 3. Duetta Signature: In-room 1/3 - octave 1m response at 15' above horizontal
Graph 4. Duetta Signature: Room-averaged response (RA R)
Set to 'normal', the impedance plot (Graph 5) gives a most uniform 4 ohms, with a minimum of 3.6 ohms around 2kHz and a maximum of some 5 ohms at 450Hz. No rise of any kind was visible at the bass resonance, and the reactive content is very low.
Graph 5. Duetta Signature: Impedance plot
Accepting that it is four ohms, not eight, the impedance is close to a pure resistor, and given sufficient amplifier current it represents a very straightforward load. With the tweeter set to 'high' the load drops to 3 ohms above 3kHz and when set to 'low' the upper range rises to a moderate 5 ohms. Incidentally, these HF switch positions were a bit fierce at +2.5dB and -2dB respectively. Steps of 1dB or less would have been nearer the useful mark. The individual impedance curve trends are also shown, these possessing 6dB/octave first-order slopes with an electrical crossover indicated at around 400Hz Interestingly, the acoustic crossover - taking into account the driver responses lies at I kHz.
With a speaker impedance minimum of 3.6 ohms and a 200W (8ohm based) power rating, an amplifier with a peak current capacity of ± 16A will be satisfactory. If 100W (8 ohms) maximum is to be used, then ± 11A would be the required level just within the compass of a 4ohm tapped ARC D125.
The Duetta Signature is Apogee's finest effort to date. Other models may surpass it in specific areas, but in my opinion none provides as complete a performance. It excelled in terms of tonal balance, coloration and uniformity, and sounded both consistently and convincingly truthful. The promise first shown by the original Scintilla has now been realized.
Summarizing its performance, this is a system of wide frequency response, average sensitivity, and good dynamic range. It offered excellent dynamics with high power-handling. In general the distortion was very low, and it offered an excellent transient response with linear-phase operation. While the loading is 4 ohms, this is almost purely resistive and will not be too critical of cable impedance or too stressful on either solid-state or valve amplifiers if they are suited to 4ohm loads. The speakers' directivity worked well in my standard room, delivering a most uniform output at the listening position. The 30Hz excess was easily controlled if desired by a baffle of rigid card, 14 x 7in, horizontally disposed and pinned on the lowest section of the rear panel just above the connector plate. The card's smaller dimension may be altered in the range 4-10in to fine-tune the 30Hz output.
I regard this speaker as a major design success, establishing a new reference standard for its price category. At dollar prices, US customers will have something to crow about! Even at the inevitably higher UK imported pricing, this speaker must be said to have done very well on test, and much greater sums would be required to establish a better performance. The Duetta Signature easily qualifies for full recommendation as a true audiophile component.