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This isn’t a DIY amp, but I want to test it/measure it myself so it’s kind of a DIY question.

I had a Mark Levinson amp (model 335) sitting unused in a closet for about 15 years.

I’ve started using it again, and it sounds great. But there seems to be a slight channel imbalance. I want to test the amp to make sure I’m not imagining the channel imbalance, and to make sure its the amp with the problem and not something else like the pre-amp or the cables.

How do I do this? Do I just play pink noise, or a single-frequency sine wave through it, and just use a volt meter to measure the output of each channel?