So once again, while characterizing my vintage NAD 3130 Integ amp, I have run into a strange problem with the QA402, that only happens when the QA402 is connected to the Phono input (MM selected, though the problem is the same if MC is used) and the Pre Out is connected to the Main In via the jumpers. An 8ohm dummy load is connected to each speaker output, where it can be monitored by the QA402. If I test the THD/SNR with a 1Hhz signal level set to -46dBV, and monitor directly at the Pre Out (no load connected, obviously), this is what I get:
The SNR was spec’d years ago to be >75dB, but at least the THD is good. The preamp volume is set such that 420mv gives 1w to the 8ohm load if the load is connected. The problem happens when I connect the Pre Out to Main In and start turning up the volume- at about a 2/10 the 120Hz spur starts growing and the whole trace goes to pot due to a very large 60Hz signal now at the output. I did not pay attention the first time this happened and my load heatsinks got very warm. The amp works fine with any other input not named PHONO. The amp works fine when powered by the QA402 directly in to Main Ins. Here is a link to a video I made of what happens:
As a side note, I have replaced every Electrolytic cap in the unit.
For reference, here is the THD/SNR using the CD input at the dummy loads:
So, after scoping and measuring voltages of different parts of the phono ckt, I am not finding anything. On a whim, I decided to hook up an audio generator I have into both of the Phono inputs (set for ~5mv), instead of the QA402’s. I connected my scope to the right channel load instead of the QA402’s R+ input, and monitored the scope as well as the QA402’s Left Channel (which showed my 1Khz generator as 2Khz…?), and as I cranked up the volume, I was able to get my 30w/ch without any problem. Next, I set the volume to 0, removed the Right Phono input and connected the QA402’s Right input and got that same ugly result shown it the video. Am thinking some kind of a strange ground loop issue…Thoughts?