How to test amplifier

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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?
For what you're after, you don't need to get this fancy - here's what I would do:
Reverse left and right speaker cables at amp.
Reverse RCA input leads at amp in addition.

If the imbalance reverses, it's the amp (maybe protection relays). If nothing changes, it's something else (speakers, speaker placement, preamp, cables, ...).
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You can also play a 1kHz* test tone, set at some typical listening volume (say, 2 to 4 Vrms), attach a DVM set to AC volts to speaker terminals. (or a 10W 8ohm dummy resistor). Look at volts AC reading with the same test source connected to input of left or right channel. You now have quantitative evidence of the channel imbalance.

* try other frequencies like 100Hz, 500Hz, 2khz. Not all DVM's are capable of measuring 1kHz AC is they are set up for <200Hz line frequency.

There are online Youtube videos of test tones or use Audacity or REW to generate them.
Another easy method ...

Feed an identical tone into both channels from a mono source.
Connect your voltmeter on AC between the two live speaker terminals.

This will read the difference between the two channels.

If you get more than tiny readings you may have an issue.

Also... take note that on more than one occasion I've seen problems like this and it turns out the amp is fine, but the client's main system isn't. That is, he's gotten used to the imbalance and now everything else sounds wrong.
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