Kudos for the QA402

I thought I would start a thread where we talk about how we used the QA402 to solve a problem “or something”. There was some discussion in one of the threads a few weeks ago that I commented on which led me to do what I suggested, which I will describe here…I have an integrated amp from the mid 80’s that I have been testing in order to learn about how to use the QA402 as well as the amp’s performance. One channel had about 6dB of noise that came and went when looking at the THD/SNR, pretty much regardless of the level of the signal going into the dummy load- this was with the QA402 feeding the main in to the power amp. You can see the noise on the FR plot below (reference was to 24dbV) for the channel in red:

It does not look like a lot of noise, but is much more noticeable when looking at the SNR, plus it is not there in the other channel, and I am retired. I set the QA402 for a -10dBV 1Khz signal into the main inputs and connected one of my oscilloscope probes, set to x1, to the Right + input of the QA402 and just started stepping through the signal path with the probe. If there was any noise moving up and down I could see it. I did not need to hook a scope probe up to the other channel for what I was doing. I was able to track the problem down using this method. I should mention that I was careful to measure the DC voltage at each point prior to hooking up the probe so as not to damage the QA402. I do not think I would have been able to solve this problem without the QA402, hence my kudos.