I would like to know how to measure that. 30% is hard to imagine. I test
amplifiers all the time with ordinary power cords and have never seen
this. This reminds me too much of Bruce Brission's white paper which was
a total embarassment to his distributor who promptly dropped his line.
Set up the amp on a variac with an appropriate speaker load and sine wave input. Measure the voltage drop across the power cord with the amp at full power. A 3 1/2 digit DVM is sufficient for this. Then boost the variac voltage to compensate for the voltage loss. You may have to do this several times because as you increase the output power of the amp, the load on the variac increases too.
This came to our attention with our MA-1 amplifier back about 1998 or so. Bascomb King had measured the amp as only 100 watts where we get 140. I was wondering where the measurement error was and found it easily enough. We spec the amp at 120 volts (which is what Bascomb claimed he had) since these days that's more common than 117V. We normally set the incoming voltage by measuring it at the IEC connection, since there is typically a voltage sag in our variac. We have IEC connections on all of our products since we know that audiophiles will want to use or audition different power cords from what we supply; the idea that power cords make an audible difference is nothing new- its been around for decades (this is also why our MA-2 employs *two* power cords per chassis, to reduce the effect of the power cord). So the power cord isn't regarded as part of the amplifier in our test. Bascomb King didn't do it that way- thus the difference. By our measurements he was seeing about 117 volts at the AC input of the amp.
So it follows that the heavier the load on the power cord, the more audible the differences between power cords will become. This ignores the higher frequency aspects of any power cord's current delivery of course, but cynics of the fact that power cords can make a difference are usually silenced by the simple revelation that they never bothered to measure the power cord's voltage drop! It is easier to measure the effects the cord has on the product with which it is being used.
For example on our MP-1 preamp, which has heavily regulated power supplies, even though we can measure differences between power cords, we can't measure any differences in the performance of the preamp, nor can we hear any differences. So we can also conclude that regulation in the circuit plays a role here as well.
