Amplifier harmonics and load stability
There are repeated claims made about the sonic abilities of certain amplifiers, around which folklore develops and legends are born. Scientifically structured blind listening tests conducted over the years suggest that, within their specified operating power ability (which must be somewhat speaker load dependent according to circuit design) when amplifiers are compared like-for-like these sonic differences vanish.
A couple of years ago I measured one channel of three power amplifiers driving a certain speaker load. The amps didn't have any volume controls, so their gain or loudness was fixed by the internal circuitry alone. I can't recall what the speaker was, but if it was one of ours it was certainly a benign, easy load. The graph attached shows variations in voltage at the output terminals of three power amplifiers (blue, black and red) across the 20Hz to 20kHz audio band (horizontal scale). The vertical scale shows the voltage in dB. A perfect amplifier would be a perfectly flat line from left to right raised to some vertical scale marker. For example, the black amp has a fixed gain of about 39dB and it is as flat as you could wish for (or need) except the slightest and utterly inaudible droop at the very highest frequencies.
The black and red traces tell a different story. Both black and red are one channel of two QUAD 405 (or 405-2? can't recall) power amps. The black amp shows a very slight (perhaps +/- 0.5dB) variation from a perfectly speaker-independent flat line and the gain is slightly less than the blue amp at about 37dB. So, if the blue amp was exchanged for the black amp it would sound about 2dB louder and this alone would be more than enough for the listener to attribute some marvellous extra 'punch', 'dynamics' or 'rhythm and pace' to the blue one. All of that would be expected for an amp that is somewhat louder. And if we started off with the blue amp and switched to the quieter black one, we'd probably note that the black amp was 'thinner', 'softer', 'has less guts' ... even 'more detailed, refined'.
The red trace is a warning to us all. First the overall sensitivity is lower than the black sister 405. This one is down at about an average of 35dB. That means it is about 2dB quieter than the other 405 and a whopping 4dB quieter than the blue amp for the same music input signal. For no more than the reasons of the difference in loudness between these three specimens they will definitely sound different. Of that I am certain. But what of the red trace, it's far from flat. Given that it's average loudness is about 35dB (a visual guess) across the frequency band for the very same speaker load it is about +/- 2dB either side of that 35dB line. That's a very big variation indeed. Since the black and blue curves are the very same brand, model and age of amplifier how is this possible? It's possible because the red amp is faulty - it still works and may continue to do so for some years with ever more significant change in performance until it fails. And then it may destroy the speakers going 'DC'.
As the black and red amps are about the same age, how would you know that one was faulty from the serial numbers? You wouldn't. You could cheerfully buy on on Ebay and it would be pot luck. Fortunately QUAD can still service these and presumably could pick up this fault ... but there again, maybe not as it is rather subtle and in the lab, they may well test the amp with a fixed resistor, not a real speaker load.
Conclusion: amplifiers can sound different solely for reasons of ageing. And as I've said before, get your amp checked regularly by a qualified engineer, best of all the makers themselves. Capacitors age horribly and as they are the heart of the amp, when they fail mayhem results. And remember, when comparing power amps (or indeed any audio equipment) unless you equalise the loudness (the gain) any conclusion you draw just won't stand up to proper scrutiny when the gains are equalised.
Alan A. Shaw
Harbeth Audio UK