I would like to compare the THD + N % of my self-built FirstWatt F6 with the factory measurements. I have connected this [the amplifier] but it is showing strange values. Do I have some settings wrong or is my amplifier simply poorly built 
I expected a higher RMS Power
Feeding the F6 with 1Vac should give a 5 Vac at the output, having the F6 a gain of 5. I also notice more third harmonic than second, unusual for Pass products.
You can lower the QA403 attenuation, and add RMS Volt in the measurements windows. You can double check the Vout at the load with a voltmeter, feeding the amplifier with a 400Hz sinewave from the QA403.
PS: did you set -12dB, as at page 19 of the QA451 manual?
Hi. There is one thing I don’t understand. The graph is of the right channel, while the measurement tiles refer to the left channel.
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The -12dB did the trick i think now i have the next measures. at 100hz a high peak, didnt expect that.
Very sharp! Sorry was clicking around and then take the screenshot.
Hi, if you can, try moving/rotating the transformer and power supply of the amplifier and see if anything changes. Then also try changing the connections of the qa403/451 and ampli and see if it changes. Otherwise it seems to me that the spectrum above 1kHz is definitely good.