Stereo Amplifier

Review: Pioneer A-400 Stereo Amplifier

All the small components are on a single printed circuit board occupying the right side of the enclosure; everything is on this one board so there is no additional internal wiring in the signal path. The two six-position switches are operated mechanically by flexible tapes from their control knobs and can therefore be placed in the most suitable position on the board from an electrical point of view. There are a considerable number of crossing links on the top of this board (a colleague calls them zero Ohm resistors), perhaps because the same board is obviously used for other products. The standard of components and construction is adequate rather than exceptional. Pioneer claim one or two unique features in the circuit, the major one being an arrangement of the output stages which works to cancel distortion by using a form of current mirror in their supply paths, thus permitting a reduction in the amount of negative feedback required to yield a particular distortion figure. (Once a universal panacea, excessive feedback has retreated under a cloud of doubt in recent years.) Typically this arrangement has to have initials, SLC, standing for super linear circuit. Then there is a "Clean Ground System" claimed to "prevent noise currents from contaminating the signal line"; this appears to be a regeneration of an old technique, popularly called star earthing, which has seen regular employment since the 1930s. Never mind what they call it - it all works well as we shall see.

How it performed

It so happened that I had mislaid the specification of the A-400 when I took it into my lab for measurements to be made. Comparing my tests with their's later (when the missing paperwork emerged from the pile I had searched through so diligently the day before) showed a most encouraging coincidence. The only real disparity was slightly higher distortion at very low frequencies, and past experience has shown that this would cure itself in time as the plus and minus power supply electrolytic capacitors (10,000 microfarads each) settled down to a working life. With the incoming mains sitting accurately at 240 Volts, clipping (both channels driven) into 8 Ohm loads occurred at 63 Watts and into 4 Ohms at 85 Watts. Distortion at this onset was about 1 per cent but easing back the power to the 50 and 70 Watts claimed reduced this to about one hundredth of that figure. The quoted input sensitivities were spot-on as was the input overload margin. Treating the dual volume control as one, aligned together at zero, showed maximum errors in balance of only 0.5dB, mainly in the area of 5.5 to 8.5 on the marked scale but with one sudden hiccup at 3.5. Altogether a most satisfactory set of results.

Equally satisfactory were the listening tests, conducted with three widely differing pairs of loudspeakers which were on the premises. Sources were Sony's CDP-X77ES CD player and for LPs Ortofon moving-coil and B&O magnetic cartridges, both in SME arms. I also spent some time listening to a NICAM stereo feed from my Bang & Olufsen TV. In all cases the sound was admirably refreshing and showed no real departures from my reference, except just possibly on the moving-coil input which registered as a shade held back. The sensitivity of this input was just about adequate for the lowest output moving-coils, but most would leave plenty in hand. The 150mV sensitivity of the line inputs means that the majority of sources require the volume control to be well down.

Pianoforte recordings, always a stumbling block at all stages from microphone to loudspeaker, were well handled by the A-400. The recent Schumann recital by Tzimon Barto on EMI CDCDC7 49970-2 was not much admired by Joan Chissell on all accounts in September: "Like the playing, the recording eschews the cold, clear light of day...". I remain doubtful that Abbey Road's Studio No. 1 is anything like an ideal venue for solo piano-there is always this compromise between over-closeness and excessive reverberation from what is a large empty hall, and this was clearly revealed by the Pioneer.

Another CD of inestimable value to us reviewers is the Tenth Anniversary Sampler from Hyperion (HYP 10). Here are no less than 27 excerpts from their current catalogue, amongst them a sequence of three piano pieces by different performers in different surroundings and with rival producer/engineer teams. Beguiled by the A-400, which has turned out to be an amplifier I could happily live with, I played right through the disc once again and then went back over several of the samples, deciding that I should have to acquire the complete works. Thus checking out equipment of this standard can prove to be rather expensive, but that is certainly not a word applicable to Pioneer's A-400.