How to fix "hum" in Carver Preamp & Hafler Amp?

Given the age of those components(Over 20 yrs), the power supply caps have no doubt seen better days. I start there, and replace them all.

Rodman may be right, but first I would try reversing plugs and/or check for a ground loop. I have a Carver power amp that had a hum until I reversed the plug.

Ground loop is indeed the first thing to check. Always run power to your entire system from only one single circuit. if you are plugged across 2 or more different outlets then you are likely going to have problems.

All the older Carver amps I have owned hum a bit. I asked Carver tech about it long ago, they replied "they all do that". They did update some caps to help it a little. My hums were only audible with my ear up close to the speakers while idling. Did not increase with volume change, so I never worried about it too much. What helped the most for those switching designs was to just leave them powered up 24/7, would usually settle down. They don't get hot, I had mine up for 12 years one time. Nice sound too.

If you are only hearing the buzz in the room and not through speakers it can be caused by transformer buzz. I have had this with Hafler and Ashly amps and lots of other equipment. If you are on an online battery conditioner (true sinewave dual inverter, not a switching unit), that can make it worse in some cases.

Steve