How to test balance between right and left channel

Well then, let me enlighten you. It's not the room. It can exacerbate the problem, but it is never the source. The OP is on the right track. It absolutely has to do with signal level as it relates to the output devices within your amp. If both channels aren't set 'exactly' the same, this problem is the result. I'm referring to both bias and dc offset. You've probably never heard it functioning properly. And if you've taken your amp in to have it biased, you probably didn't notice any difference since sonic issues are never attributed to these functions by even the tech that spent no more than a half hour on the bench with it. In reality, it takes days or even weeks to properly set these adjustments because the circuit has to settle down and numerous adjustments are necessary until it reaches spec, finally. First bias, then offset, every time. Until you actually do this procedure, you will never realize the potential of your gear, any of it. That's when you will recognize just how little an influence all the other things you've been doing actually has on your system.

I've spent years trying to discover the source of this problem. Since everything but the room has been eliminated, that must be the problem. Although with left and right volume controls, the problem is easily corrected; like a foolish "audiophile", I still wonder what the source of the problem is.

SPL meter is probably different from your ears. Since human hearing is almost never balanced perfectly between left and right ear, the surest test is by ear IMHO.

A Stereophile Test CD has L & R mono tracks if I'm remembering correctly. You will need a sound pressure level (SPL) meter and you should be set. There are also many other tests that are helpful.

You can also play some white noise and use an spl meter. Sometimes if your room is an odd shape or doesn't have consistent lighting, it can be hard to judge. Kind of like an optical illusion.

If your system has the ability to adjust the balance level, I would just do that and enjoy the music, after all, that is what the balance control is there for.

And if no balance control, I have had success in different rooms by just moving one speaker in or out, in both cases by only an inch or two.

But if you want to make sure your equipment is functioning properly than I can understand.
If testing with equipment isn't practical, I would play a mono recording and listen to where the image is centered. Then I would swap cables down the chain to see if the image stayed in the same spot or moved left or right.

If your system has the ability to adjust the balance level, I would just do that and enjoy the music, after all, that is what the balance control is there for.

Thanks for your replies. Bfr/Rrog, you may be absolutely correct. My dedicated audio room has a sloping roof - 8ft high on one side and up to 20 ft on the other. But before I tackle this problem, I need to be sure that the slight imbalance I am getting at time does not come from my equipment. I know my cd is ok as i can read my the balance levels on it. But i need some sort of equipment to individually test each component of the chain to identify the source of my problem.

Brf is correct. In my experience even though the equipment is balanced equally from left to right the room is where the problem lies. This can also be frequency dependant.

If you have a mono source you can measure the output of each channel, BUT measuring the output will tell you nothing about the system’s music balance as the measured signal cannot take into account the room’s acoustic properties.

A SPL meter plus a source that provides separate left and right channel signals. I believe there are cell phone apps that provide common SPL meter functions.

if it has vu meters, than you can play mono record on stereo pick-up.