Our Issue 5 evaluation (Sept/Oct) of four recent and highly significant Class D amplifiers has raised something of a controversy and we are now subject to heated discussion on the audio forums including Pinkfish.

pinkfishmedia.net forum link

We began this evaluation with an open mind and took scrupulous care in trying to answer the many questions raised about sound quality and to subsequently seek useful information which might help to explain the results.

All models were well run in. They were auditioned prior to the lab testing, so that the listening findings would not be prejudiced by prior knowledge of the test results.

None of the amplifiers did well in the auditioning. All showed numerous artefacts in lab testing which past experience has shown may have been responsible, at least in part, for some of the observed variations in sound quality.

While we hoped that these recent examples would begin to justify the voluminous hype seen in the media over their claimed value and performance, particularly sound quality, we could not verify the reports and claims which been made.

Two kinds of audio listener?

However the project did raise an important issue concerning how audiophiles and audio designers perceive sound, how they interpret music, and what qualities they value in both live musical performance and in sound reproduction.

Without wishing to appear excessively judgmental, the sound quality results from our team, when compared with the much more positive results reported by some other listeners seem to suggest fundamental differences in perception.

According to the first kind of listener: "I can hear clear sounds, the bass is powerful, solo instrument focus is highly defined. It drives my difficult speakers well, and, this amplifier is cool, clever, compact, convenient, ecological and good value."

Conversely, the second kind says: "I want to tap my feet, I want to hear the life, expression and natural dynamics which help make it sound more like real music. I want the image to be stable, with deep three-dimensional perspectives; I want sparkling high resolution treble sounds which image in depth. I want a natural timbre and tonal balance."

From personal experience I know that to be a member of the second group you need to have had experience, ideally on good quality familiar recordings, of how well these qualities can in fact be expressed in a well tuned and matched audio replay system.

Those who have not experienced these music performance qualities from reproduced audio may never know or understand what is possible. They probably run a system which has not been selected to reveal these qualities, and are therefore unlikely to be able to detect whether they are present or absent in any other component introduced into the system. The system becomes a self sustaining closed loop encompassing the listener.

It's important that the audio system in which a product is evaluated has the potential to reveal most of the important parameters, both those helpful to listener involvement and those responsible for detail, bandwidth, power and spatial effects. Only then will the musical performance differences which we consider so important be sufficiently revealed.

It seems that the first listener group are more accepting of the balance of performance provided by Class D amplifiers, whereas those in the second group may appreciate certain aspects of Class D performance, but viewed as a whole find them dissatisfying at present.

We hope to explore this listener divide in future issues of HIFICRITIC.

Technical comment:

Some of the technical reasons why this might be so are explained in that fifteen page Class D feature, issue 5, covering ProJect, Bel Canto, NuForce and Channel Islands Class D amplifiers, and including comment on earlier Yamaha, Sharp, Sony and Flying Mole designs.

Briefly summarising the test findings, 'switching amplifier', Class D designers seem to be perfectly happy to discard several decades of audio engineering wisdom which have helped shape the creation of many great sounding pure analogue products.

From a technical viewpoint it now seems permissible to:

1. Feed broad-band radio frequency noise into the power supply outlets



2. Feed broad-band radio frequency noise to the line and ground connections



3. Drive broad-band radio frequency noise into the speaker cables and loudspeaker. (up to 500MHz with up to 2V generally measured at around 50kHz)



4. Define the output impedance using a significant passive filter, with a result which varies with frequency and is dependant on speaker loading



5. Allow the amplifier to be marginally or completely unstable with either high or open circuit output loading



6. Employ soft compressor clipping circuits prior to full power clipping to prevent feedback saturation.



7. Employ high order negative feedback to improve in-band distortion figures and low frequency output impedance.



8. Specify numerically high damping factor at low frequencies and claim that this guarantees fine bass.(regardless of the interface to the loudspeaker or any other property of the circuit)



9. Use steep low pass filters to limit the upper high frequency range, partially negating the purpose of wider bandwidth, source material e.g. SACD, while the resulting filter phase shifts may be audible in the working band.



10. Have low bandwidth input circuits which are highly susceptible to stray high frequency input signals, including upper band noise shaper signals and DAC artefacts. The result is poorer treble sound quality and measurable distortion.



11. Have power output circuits with poor high frequency resolution resulting in high levels of intermodulation products at the high frequency end of the spectrum



12. Have 'sampler' noise-shaped noise floors. The latter vary dynamically with the level, frequency and complexity of the input signals.



13. Have comparatively small power supply reservoirs, in the light of their low frequency output current potential and available power.



14. Have thermal dissipation limitations due to the small power module size which means that dynamic variations are present in the performance with time and temperature.



15. To protect the fragile output stages all kinds of pre-clip and aggressive fold back protection regimes are included which are frequency dependant and are also programmed for duty cycle. Unexpected sound quality variations may result when operated at higher powers and with more difficult loads.



16. Operate at an equivalent sample rate which is insufficient for good resolution above 7kHz. DSD, 1 bit pulse-width modulation operates at 2.4MHz, nearly ten times the usual rate presently used in Class D amplifiers.



17. Deliver high, constant DC voltages (up to 70V) relative to local ground at the output terminals and hence also the loudspeaker connections and cable. (not of course between the +,- terminals as both of these are at the same dc potential)



18. Use a high feedback switch mode power supply which has to react dynamically to the power draw variations of the power amplifier with the music programme. Generally these are designed for supplying dc and are demonstrably imperfect faced with near audio bandwidth loading at a wide dynamic range. Essentially the supply constitutes a form of audio amplifier yet it was never intended to be optimised for such duty.

Comment:

For a linear amplifier designer the presence of even one or two of the above eighteen listed integers would generally be regarded as potentially prejudicial to sound quality of the design.

From this viewpoint the odds seem to be stacked against the switching amplifier as applicable to high quality audio applications.

Perhaps instead of criticising, we should instead be applauding the Class D designers for achieving what they have in fact achieved against all the odds!

HIFICRITIC 2008

The Bose SoundDock

Martin Colloms

After reviewing the Audica mps-1 desktop stereo system for Issue 7 HIFICRITIC, and finding it in context surprisingly plausible for casual, desk top music listening, I obtained a Bose SoundDock, for which there is some equivalence, not least in price and size.

While equally compact, by contrast the latter is a one piece device which has proved extraordinarily popular, riding the storm of iPOD sales and dedicated to these devices. This Bose is designed with what appears to be a textbook implementation of the perfect user interface.

Clearly a high quality consumer product, one is left in no doubt concerning the Bose approach to industrial design, professionalism at every level, including the finish, the fit of parts, and its likely considerable durability. Intended for 'normal' people the technical side is completely concealed, there is no significant specification or technical story, and like a good transistor radio this product is intended to be easy to understand and use, right out of the box. It is implicitly understood that if you pay this sort of price for a famous name item it will deliver as expected and require no further thought. With other brands this is not always to be found.

In fact packing the subjective scale and weight and power of sound which this improbably small piece manages requires extensive use of technology, several components of which are protected by patents. It is the depth of design in which Bose is prepared to invest which results in something of universal appeal worldwide, providing a superior acoustic punch and a perceived quality which has endured far longer than the competition. This general approach has enjoyed huge world success with the extant, pioneer product in this arena namely the Bose Wave Radio.

The advertised 'wave' component is not just marketing hype, it concerns a clever system of long, resonant wave acoustic ducts which are coupled to the back directed, low frequency energy from both speaker channels. Stagger tuned, these ducts are resonate in harmony down to the lowest frequency required and greatly increase the very modest power handling and bass response of the necessarily tiny, full-range cone speakers built inside this product. However some time delay and stored energy effects are to be expected with such high Q, resonant duct loading. Sophisticated equalisation is applied to the speaker drive to attain a nominally uniform subjective frequency response from the system. However at higher power the speaker and duct system will overload and this must be avoided at all costs. The resulting distortion would be considered unacceptable to the user. As a result, the heavy 'loudness' equalisation which is employed to good a firm bass effect at low volumes is also constantly adjusted according to signal dynamics and volume level. The electronics track a calibrated, calculated 'envelope' which maps out the distortion free headroom of the system for loudness versus bass response. This is why the bass extension is progressively cut back as the volume level increases.

It is also usual to electronically apply soft compression to the audio signal to avoid overload. Some competing products employ an alternative, a psychoacoustics based processor which synthesises a range of bass harmonics from simple tones. These are intended to convince the ear that a more extended bass response is actually present and these use the theory of the missing fundamental.

It is likely that the SoundDock does employ some form of compressor circuit to avoid unpleasant 'clipping type' overload, given the limited driver and amplifier headroom present in this minuscule physical space. With its external switch-mode power supply and compact dimensions I expect that its cool running internal amplification is switch mode i.e. Class D. (As for the Audica mps-1)

SoundDock measures a diminutive 12 inches wide, 6.5 inches high by 6.5 inches deep, though this figure includes the rear 'pedestal' and visually it is quite shallow. I quote here the marketing material: '-a host of proprietary technologies add vitality and realism to your tracks'.

Unpack the dock and plug it in, and nothing happens, not even an 'on' light illumination. For any action you need to drop an iPOD into the matching receptacle, a snug fit as matching adaptors are provided for all compatible types, (and 'pods come with spare dock adaptors too) and then it springs into life with an electrical click. The 'pod also powers up automatically, even beginning its battery charge refresh via the 'dock power. The neat little Bose remote control works just fine, even at a good range, and you get track change and pause in addition to a really finely graded control of volume. By the way decent lengths of cable are provided to the sizeable switch-mode power supply box, and then on to the 'dock.

I tested it with my iPOD Classic 80GB on both 320k AAC and a selection of lossless tracks and was truly mystified by the resulting sound. The device was certainly capable of almost filling a room (though the much larger 'Wave Radio' manages this trick with considerable ease). For the record, in my opinion neither of these is a match for in-room drive and dynamic performance for the exceptional and undervalued, but large and costly stereo box from Apple, which really did try to be a portable Hi Fi. (The Apple M9867LL/A iPod Hi-Fi Home Stereo)

My scientific training informed me that the SoundDock was an impressive performer, and there was surprising levels of mid bass present, belying its small dimensions. In theory, with no significant bass driver or enclosure volume to support it, this device then ought to have overloaded rapidly at higher volume levels, but it did not. In fact it played louder and louder without harshness or clipping sounds until you felt that it was finally beginning to compress and become progressively mushy.

It was really trying to sound like a compact shelf top 'Hi Fi', the kind with separate speaker enclosures. With a short frontal dimension, the stereo image was understandably rather small. However it showed class in both available bass and volume. Further, there was quite good clarity, significant detail, the tracks sounded busy and well filled; so far so good.

Maybe I am just too fussy, but with continued listening I experienced some misgivings. It was hard to put a finger on it at first. However I found my attention wandering from the music as I began to wrestle with perception issues in order to focus my mind on what was going on. Firstly there was that initially impressive bass. At low levels there was worthwhile extension to perhaps 60Hz, but it was a rather insistent almost one-note bass, a bit synthetic, and moreover the bass extension varied a lot with loudness, losing perhaps an octave when moving up to at higher if still fairly moderate volume levels, the 'dock now relying on harmonics, though clearly remaining free of 'electronic' distortion. Thus I came to recognise that the bass was being modulated by level, or programme volume.

The treble was fairly average, not really clear or sparkling but good enough, but it was the mid range which proved confusing. Vocals sounded quite strong and firm, quite well weighted and not thin and tinny as is often found with competing docks. However the singers were somehow not very familiar. If this were a Hi Fi speaker I would have said that there was a significant coloration, but for this Bose was more subtle than this. I can only describe it as an unfamiliar shift in 'tone' which made a performer less recognisable, some of their essential character, which for example easily makes it through a simple FM transistor radio, was diluted. The faintly 'pseudo' singers still sang clearly enough, were well articulated, but were a cause for concern. The other aspects were rhythm, timing and dynamics. While you should ask too much in these areas from low powered portable equipment I was fresh from my reasonably rewarding experience with the equally diminutive Audica design (which incidentally does not attempt the deeper bass available at low and moderate volume levels from the Bose product). By comparison the SoundDock appears to compress musical dynamics, and somewhat dilutes the contrast between soft and loud sounds, an aspect which makes music replay interesting. The SoundDock bass lines seem to be subtly separated from the rhythm section, and the result does not sound like good timing. Rhythm subtleties seemed to be smoothed over, this Bose design favouring a rather gentle, more constant flow, even with proverbial 'Musak' tendencies. (Musak programme is generally intended not to draw attention, rather, to gently set a positive mood) Listener involvement is significantly reduced.

I concluded that the SoundDock is very sophisticated in creating the 'sound' of music, a high proportion of all the noises and frequencies that go to make up a track, but to some degree they seem to arrive in an unfamiliar order, with unfamiliar dynamics, and a sense of altered pitch and timbre. Singers and performances which you know really well, and which your iPOD, if set to high resolution, has little difficulty in reproducing via earbud cans or when jump started into the aux sockets of a Hi Fi system, come out of the SoundDock with a certain strangeness about them, with a vague aural suggestion that they might somehow be cover versions and not the originals.

Instinctively, I suspect that it is the particular, complex and clever signal processing which makes this much larger than life sound and response range jump right out of this little box, but which is also its musical undoing. The avowed action of the processors and of the system as a whole is to fool us into thinking that this product is much larger than it really is. I feel that it may be the processing which also alters the way the music is reproduced, its subjective flow, its timing and dynamics, and which gets in the way of communicating that vital feeling of involvement and deeper enjoyment.

To conclude, in its market led way, the SoundDock is likely worth its premium price. Technically it delivers an almost magically large 'sound'. Its interface with the user and iPOD is very well thought out and beautifully executed. The build quality is exemplary. Huge numbers have been sold and will no doubt continue to be sold, but even for casual use with music which I treasure; this is one product which I just have to pass over.

Martin Colloms
HIFICRITIC

Note: While the standard SoundDock remains in production and may now sometimes be obtained at a discount (e.g. £200 UK) it has recently been joined by a generally similar Mk II version which now includes a 4 hour lithium battery providing portability, also an auxiliary input socket, a useful addition, and some further internal improvements said to raise sound quality. The price of the new SoundDock II Portable is about 25% more than the original.

Sumiko Cartridge setup instructions

In writing up the cartridge reviews for Issue 7 (Vol2 No 1)  model were so helpful that we have obtained permission to reproduce the general text on our site, and  many thanks to Symmetry and Sumiko for helping make this happen. Download Sumiko Cartridge setup instructions here.