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The Denon AVC-A1D has a host of operating features while still managing to look very clean and uncluttered. It provides all possible operating modes for home theatre systems. Denon's AVC-A1D surround sound amplifier When is an amplifier not simply an amplifier? Answer: when it is a surround sound amplifier made by Denon. This new model Denon is described as an A/V surround sound amplifier but that hardly begins to tell the story. Briefly, it has virtually every conceivable amplifier feature you could want in a Home Theatre surround sound system. By LEO SIMPSON These days one accepts that audio equipment for home theatre systems is going to be complex. After all, there are all the program sources which must be catered for, the minimum of five audio power amplifiers, the need to cope with Dolby Digital (AC-3), DTS (Digital Theatre System) and Dolby Pro-Logic sources plus DSP (Digital Signal Processing) to give surround 10  Silicon Chip sound effects like concert hall, rock stadium, jazz club and so on. This Denon product does all that and a whole lot more and it takes quite a lot of time to work out just how many functions and features it has. In fact, for a unit which has so many features, it is quite inscrutable at first sight. It looks more like a large conventional stereo FM/AM receiver than a surround sound amplifier, doesn't it? After all, it appears to have a long dial scale and a large knob on the right, just like a stereo receiver. Well, the knob is the master volume control but it is not a normal potentiometer and it is not motorised, as are many in systems with remote control. The other knob on the front panel is the input selector but it is not a normal switch and it can be rotated continuously back and forth, without stops. There is a large door in the control panel and it drops down smoothly to reveal 18 pushbuttons and two more knobs, for the bass and treble controls. And yes, the tone controls aren't normal pots either - they rotate continuously while the amount of boost or cut is displayed on the large vacuum fluorescent dot matrix display. So after tentatively trying out some of the controls or the multi-button remote you quickly realise that this is no ordinary surround amplifier. Then you take a look at the back panel and stagger back - just how many inputs and outputs can this unit handle? The number of inputs is increased because it handles video as well as audio - that's where the A/V designation comes from. But in addition it also handles optical digital signals from four sources such as DVD and CD players. As well, there are S-video sockets for six sources such as DVD and VCRs as well as outputs to two TV monitors and two VCRs. There is also an AC-3 Dolby Digital) input All told, there are no less than 77 RCA sockets, six optical fibres sockets and ten S-video sockets. And there are seven pairs of binding post terminals for the speakers: front, left, centre, and two sets of rear speakers. After doing this quick reconnaissance, the natural reaction is to retreat to the owner's manual and a cup of coffee for an hour or two's quiet reading. Well that's what you would normally do if you had the owner's manual; we didn't, as this was an early review sample. We did have some Denon publicity material and a brochure and fortunately, the brochure was pretty comprehensive. So what have Denon attempted to do in producing this very impressive looking piece of equipment? Not only does it incorporate very comprehensive input signal handling for analog sources but it also handles digital signals which can be connected to the RCA sockets via normal shielded cables or via optical fibre. And it is in the digital domain that Denon have really spent the money in developing 96kHz 24-bit digital-to-analog converters. In fact, there are no less than six of these D/A converters, one each for the five normal audio channels and one for the sub-woofer output. There is a major story just in the development of these chips but Denon have gone further and incorporated 32-bit DSP chips for the Dolby Digital and DTS decoders and for the THX and surround sound signal reproduction. Just so the system will not be outmoded in the future, the Denon AVCA1D also has provision for 6-channel and 8-channel audio sources. Did we forget video? We mentioned S-video sockets but not the RCA sockets for composite video or so-called "component" video (ie, Red, Green & Blue) for direct connection to a TV monitor or video projection system. AUDIO PRECISION FREQRESP AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 5.0000 04 NOV 99 09:13:14 5.000 4.0000 4.000 3.0000 3.000 2.0000 2.000 1.0000 1.000 0.0 0.0 -1.000 -1.00 -2.000 -2.00 -3.000 -3.00 -4.000 -4.00 -5.000 -5.00 10 100 1k 10k 100k 200k Fig.1: frequency response over the amplifier at a power level of 1 watt into an 8Ω load. This is taken in Direct mode which bypasses the tone controls. AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 05 NOV 99 08:47:48 1 0.1 0.010 0.001 .0005 0.1 1 10 100 300 Fig.2: total harmonic distortion versus power at a frequency of 1kHz with the two front channels driven. Maximum power is 180 watts at the onset of clipping (measured with a bandwidth of 20Hz to 30kHz). As you might expect, the remote control is pretty complex as well, with lots of buttons to control all the different sources. It is a learning remote so when you set up your home theatre system there will be no need to juggle four or five remotes. A feature we really liked is that the most often-used buttons, for things like volume up/down, play, stop and so on, glow in the dark. So you can see 'em before you press 'em and you don't have the concern about battery usage for back-lighting. But perhaps the best feature of the remote control, and the Denon amplifier itself for that matter, is the on-screen display. This is just like the DECEMBER 1999  11 AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 05 NOV 99 09:10:06 1 0.1 0.010 0.001 .0005 20 100 1k 10k 20k Fig.3: total harmonic distortion versus frequency at a power level of 100 watts into 8Ω (measured with a bandwidth of 20Hz to 80kHz). This remote control is a learning type so it will replace most remotes required for a home theatre system. on-screen display (on the TV monitor) which is a feature of today's VCRs. In fact, the on-screen display is a necessity, so that you don't have to remember all the settings you have made. After all, the front panel display only shows the control settings you just touched, not those which were previously set. For example, when you change the bass or treble control, the amount of boost or cut is shown on the front panel display, eg +7dB, but as soon as you select some other function, the boost setting disappears. Nor can you tell the tone control setting by looking at the knobs because they don't have any markings. With the on-screen display you can call up all these settings without having to touch them. 12  Silicon Chip By the way, you can bypass the tone controls by selecting the Direct mode although we could not measure any significant difference in performance when this was in use. Before we delve into the innards of the big Denon, I must say that I find the gold finish on the front panel a most refreshing change from the universal black or charcoal on most hifi equipment. It looks more at home in the living room to my mind although the unit is available in black for those who prefer the sombre look. Since the AVC-A1D is such a large and bulky unit I knew it would be crammed to the top with circuitry but I was not quite prepared for just how much there is. Removing the case shows it has lots of boards for signal processing on the righthand side (looking from the front) of the chassis and the big power supply on the left. Down the centre is the five-amplifier module, mounted on a heatsink tunnel and cooled with two fans. The power supply consists of the large toroidal power transformer and two large filter capacitors and these supply the five power amplifiers. Elsewhere in the chassis are the supplies for all the signal processing circuitry. Guess how all the analog signal switching is done? Not with CMOS gates but with relays, lots of them. So whenever you change a signal source or mode selection you can hear the relays clicking in and out. We don't how many PC boards there are in the unit because you would have to disassemble it to count them all but as you can see from the internal photograph, there are quite a few. All the signal processing boards are double-sided and are covered in surface-mount components on one side while the power supply bypassing and larger components are on the other side. Perhaps the most interesting (and understandable) module for me was that for the five power amplifiers. Each of these amplifier uses just two large power transistors in the output stages. These must be very rugged devices indeed because each power amplifier is rated to deliver up to 390W into 2Ω loads with normal program signals. They are plastic-encapsulated devices with the case measuring about 20 x 40mm, much bigger than the 200W-rated plastic devices we are used to seeing. Since we did not have any access to circuits, it is not possible for us comment in any detail but the power amplifiers appear to be conventional class-AB designs, albeit relying on those very large output devices. Each channel is rated at 140 watts RMS into 8Ω loads, with the power rising to 190 watts under dynamic conditions (ie, program rather than continuous sine wave signals). The rating rises again to 310 watts into 4Ω loads (dynamic) and 390 watts into 2Ω, as mentioned above. Rated harmonic distortion is .05%, for signals from 20Hz to 20kHz. Bench-testing the big Denon presented a real challenge. For a start we don't have a big load box which will cope with five power amplifiers simultaneously. This meant that we had to content ourselves with just testing the front left and right channels in stereo mode. Nor did we have time for a a full suite of audio tests and so we had to be selective. However, the tests that we did perform showed that the Denon amplifier comfortably met all its specifications, as you would expect. Fig.1 shows the frequency response of the analog circuitry in Direct mode, at a level of 1 watt into 8Ω loads. As you can see it is about 0.5dB down at 10Hz and just over 1dB down at 70kHz. At 20kHz it is about 0.35dB down - more than adequately wide for a hifi amplifier. Fig.2 & Fig.3 illustrate the performance of the power amplifiers and these were measured in Direct and non-Direct modes with negligible differences in performance, with both channels driven simultaneously. Under this stereo condition, the amplifiers comfortably exceed their power ratings, giving about 180 watts before the onset of clipping. By the way, the little zig-zags in the curves are an artefact of the attenuator switching The rear view is pretty cluttered though with no less than 77 RCA sockets (count them), plus S-video sockets and optical fibre sockets. in the Audio Precision test gear and do not have any significance in the amplifier's performance. One aspect we did have trouble with was the effect of residual noise at the lower power outputs. As you can see from Fig.2, the harmonic distortion is 0.03% for a power output of 100mW but that does not represent distortion; it is noise. This can be reduced by driving the unit with a bigger input signal and winding the volume control back and this is what we did, driving the unit with a 2V signal and measuring (for this test) with a bandwidth of 30kHz. Lest we give the wrong impression, the overall noise levels from the Denon are very low. Set for a sensitivity of 2V input and 140W output into 8Ω, the signal-to-noise ratio was -108dB This is the power amplifier module which has five power amplifiers mounted on a heatsink tunnel cooled by two fans. Notice the very large power transistors; only two of these are used in each power amplifier. unweighted. But they've done some clever things with their volume control attenuation because it drops to around -120dB at low volume settings and rises to around -92dB at maximum sensitivity, ie, with the volume setting flat out. This latter condition would never apply in practice so the overall noise levels are very low and even more creditable when you consider the amount of circuitry in the signal path. Fig.3 shows the distortion versus frequency at a power output of 100 watts into 8Ω loads, with both front channels driven. As you can see, the distortion is below .05% at all times and below .03% for frequencies below 10kHz which is a pretty credible result for power amplifiers with only two power transistors in the output stages. During the high power testing, the fans cut in regularly but they are relatively quiet and they cut out quite soon when the power level is reduced. They would probably not operate at all during normal listening. In any case, if you were listening at high levels you certainly would not hear the fans. We could go on with a lot more measurement results but you have the overall picture - this is a very impressive machine with a huge range of features. How does it sound? Very clean, very quiet and with heaps of power; what more can we say? The only drawback is that prospective buyers will need to be wellheeled; the recommended retail price is $5680 and it is covered by a 2-year parts and labour warranty. For further information, contact the Australian distributors for Denon equipment, AWA Audio Products by phoning 1 800 642 922 or by email info<at>audioproducts.com.au SC DECEMBER 1999  13