Amplifier design matured during the 1970’s and the better ones reached sonic perfection at the time. Of course, such technical instruments age, and the most problematic components are the power supply capacitors. Quality of those at the times was not as good as it now, and after 10-15 year many had already aged dangerously. So if you have an amplifier from those days, it pays to get it recapped. If you don’t, one day you may destroy your speakers.
The vast majority of the amplifiers mentioned here were never on the European market, so my experience has been with Quad: a 303, a 405-2 and a 606-2. I still have all three of them, and all three completely refurbished. The 303 is a bit old fashioned and is less usable for the lower impedance speakers that only came on the market after its introduction. But it was the ideal amplifier for my Quad esl 57 speakers. The 606-2 and 405-2 were bought second hand quite recently, and are now used to drive my Quad 2805 main speakers and Harbeth P3ESR desk top speakers respectively. Sonically, there are no differences at lower levels, but the bigger ones do sound better at higher levels.
The design criterion for ampifiers was defined long ago by Peter Walker, to be ’a straight wire with gain’ and that was indeed achieved by the better amplifiers of the period. After all, there are only three ways in which a well designed amplifier can sound different:
1 input sensitivity that is not matched to the output sensitivity of the source. This was a problem in the early days with no effective fixed standards for home audio. Hence Quad pre amplifers like my Q33 had variable inputs to precisely match the output level of the pickup cartridge, the tape recorder etc. If this is not done, there can be clipping of the input stage. This is quite common when you use these older amplifiers with modern cd red book standard sources with their 2.0V output (or more if the designer wanted to cheat). 2.0 V is way more than their rca inputs can handle. The solution is simple: inline attenuators.
2 Insufficient output power to avoid clipping of the output during dynamic peaks. Real dynamic music like a symphony orchestra may occasionally need hundreds of watts. Here, big power output rules, so beefy power supplies and big output capacitors. You can upgrade the latter, which is well worth it on the older Quad amps if you have them recapped anyway. Modern capacitors can be much smaller, so you can now fit bigger values in the same case.
3 load dependent frequency response. Real speakers are not just e.g. 8 Ohms, but their impedance varies quite a bit. The more the impedance varies, the harde the speaker is to drive for the amplifier. What this means is that the frequency response is not flat, but varies with the impedance at that particular frequency. To avoid this, amplifiers need a low output impedance, for a high damping factor (i.e. speaker impedance divided by output impedance). If this is not the case with real speakers of varying impedance over the frequency spectrum, the frequency response of the amplifier can vary quite a bit, giving that amplifier a signature sound like extra bass or a high frequency roll off for more ’warmth’. Many audiophile amplifiers are like this, thus deviating from the ’straight wire with gain’ criterion. Here, solid state amplifiers have a great advantage over most tubes.
So, to come back to the original discussion about older amplifiers, yes there are real beauties. Get one with sufficient power (there were some really big ones at the time), have it at least recapped, and probably get some inline attenuators to connect modern 2.0 V sources.
