This This amplifier is an early MOSFET design with a built in pre-amplifier. Many of the later (these were made in 1978) Perreaux designs were separates, the power amplifiers were built to similar topology to the this power amplifier but were more powerful using more out put devices. This has the classic Hitachi 2sj50/2sk135 devices, a complimentary pair per channel, giving 80 watts into 8 ohms, per channel according to the manual published by Perreaux. The rest of the spec are also rather good. This example had a few issues, the electrolytic capacitors needed changing, the Operational amplifiers CA3100 all eight of them were noisy, and there was some hum that turned out to be an "earth loop" problem. I replaced all the electrolytic caps and removed the operational amplifiers, 8 pin DIL, and replaced them with IC sockets. I happened to have eight OP27,s handy to replace the CA3100's, these worked well and actually sound good; they do however have a very low slew rate and bandwidth compared to the CA3100, so its unlikely the amplifier will now meet the original specs, 10hz - 100k Hz...but now knowing all is well I can now buy better op amps to nearer match the originals. It seems the CA 3100 was almost unique at the time and has not got a simple drop in replacement, it also has external compensation not always found in newer op. amps, most have internal compensation. Anyway a near matching replacement appears to very expensive and may not "sound" any better. The operational amplifiers, CA3100's, in this amplifier run rather warm, the background noise was high, variable and erratic - cooling the op amps made them quiet and improved sound quality (temporarily, until they warmed up again) alerting me to the fact they were past their prime!
The hum (earth loop) problem. There was a persistent and annoying buzz coming from the speakers. The amplifier was completely untouched from new so far as I could see so no new earthing problem should have been created. I found the solution to the problem...when I removed the pre-amp "module" from the power amp "body" (the amplifier separated at the widening neck where the inputs and speaker terminals are, undo four nuts) the buzz became very loud, I measured a potential difference of five-ish volts between the body and the pre-amp module, the buzz completely disappeared when an earth wire joined the two, but that seemed odd as there is an earth connection between the two parts when bolted together ... well the bolts and join were not, electrically, made as well as they would appear, I cleaned the bolts and made a very good connection before reassembling the two parts and now the buzz has gone! I guess a small amount of corrosion creating a resistance had made the earth paths less simple; the original earthing strategy on this amplifier was obviously well thought out, earth wiring goes back to a single point and I expect this was significantly before the "star earthing" formula had been expressed, being a relatively modern term, but here it is, essentially.
The Sound Well no surprise really, it sounds extremely good, nice detail good bass, treble is bell like and not harsh...it's lovely to listen to, a little bit of warmth, typical of MOSFET amps... and all that with probably not ideal op amps in the pre-amplifier section and there is still a little more work to complete the restoration (I won't be "openly" modding it, only replacing "worn" parts with a best option as I can find)
Other stuff. The writing on the casework is worn off in several places. It is printed in a gold paint and seems to be a common problem from evidence I have seen in photographs of other examples. The amplifier is built like a tank, it has rack handles front and back and high quality fibreglass circuit boards as well as very high quality components. Another unusual feature the block diagram printed on the top panel of the amplifier. I did put they CA3100 op amps back in to listen again, they sounded very good with music so long as you ignored the noisy background or could not hear it due tot the music. I also forgot to take more photos when it was apart but will add more as I will be working on this amplifier a bit more.