What could be wrong?

Reverse the cables from the turntable to the preamp. If the volume reduction moves to the right channel, it's not the preamp.

Dick

And.....if nothing happens above (which tests turntable), then reverse from phono preamp to audio preamp to see if it is the phono preamp.

If not, then plug CD player into audio preamp where the phono preamp is currently, or plug phono preamp in where CD is to see if it is the inputs on preamp..

In all likelyhood it's not the line preamp at all because that would mean one input on it went bad and there's no electronics associated with a single input (just wires). I'd suspect the cartridge/stylus. Good luck

I've seen selector switches go bad in spots, so while there are just wires, things can go wrong with individual inputs on preamps. Take the extra minute and check everything.

You know the how it goes. The one thing you don't check is what ends up being wrong..

But yeah, I suspect the cartridge as well...

Update...
I changed the turntable connection to a different input on my linestage preamp and there was no change; the right channel was still much louder than the left. I then swapped the left/right channels from the tonearm cable to my phono preamp, and the problem switched sides. At this point, I figured the problem was either the cartridge, tonearm cable (which is relatively new), or the phono preamp. I then swapped back the left/right channels on the phono preamp, and the problem disappeared. Everything is back to normal now.

I suppose I'll just wait til the problem occurs again (if it happens again) to see if I can determine what's wrong. If it was either a cartridge, tonearm, or phono preamp issue, isn't it odd that the problem just fixed itself? Strange.

calgarian