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SWR SM500 Ground loop

Hey all.

New member here. Hoping someone might have an answer to this question.

I have an SWR SM500 amp, about 10 years old. Anyways, at some point in the last 2 years, the amp has developed a ground loop problem whenever anything is connected to the Direct out. Ground lift doesn't work, good power conditioning doesn't stop it, doesn't matter where or what is connected to it, it will get an unstoppable, and unusable 60hz buzz. However, the buzz usually takes between 10-15 minutes to set in, but once it starts, it won't stop.

Have started using a DI, but I really enjoyed the preamp output, so I am missing this. Everything else works fine. Hoping someone out there might have a suggestion on how to stop, or fix the issue.