Richard Clark $10,000 Amplifier Challenge - Why Couldn't Anyone Pass this Test??

Everyone knows that well built solid state amplifiers designed to stringent specifications and tolerances are going to sound very much alike at low volumes.

It would be a huge surprise if that weren’t true - it would mean that all those measurements and engineering principles and the consistent improvement in amplifier performance are completely wacky and wrong.

It is when you stress a SS amplifier that differences may be audible - at higher SPL and with challenging loads or with dirty power or when driven close to clipping and where distortion rises rapidly.

That said, there will be very small differences even at low volumes but these will extremely difficult to discern reliably with most music sources unless using very specific test signals.

So no surprise to me.

@bdp24   

Not surprised by your finding as Quad ESL can be an extremely tough load. Differences in amplifier damping (output impedance) would be quite audible. This only proves one of the major design issues with Quad.

@almarg    

+1 that is what I stated I even clarified that damping equates to  "(output impedance)" 

@erik_squires   

+1 the link you gave states this about Quad:

"The second downside is that they are a difficult load for the amplifier to drive. They are highly reactive loads (primarily capacitive). Their impedance drops as the frequency increases. This makes the Quad ESL a bad choice for single ended tube (SET) amplifiers."

+1 Everyone 

Good thread with good points by all. There is much knowledge among you Audiogoners!

What Ralph said

https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt

This explains why my amps are designed as Class A to 2/3 power. The switching distortion at high output is insignificant compared to everything else. However at very low levels switching distortion is audible (at least on test signals) - so it is preferable to be in Class A at low levels (of less than 1/10 of a watt)

The article confirms to me that only carefully designed listening tests at very low levels will lead to audible differences between well designed high quality power amplifiers. Alternatively, stressful loads or extreme power requirements would audibly differentiate the amplifier with better performance.