Amp Hum

You could try attaching a bit of speaker wire as an earth strap between the chassis of the amp and pre-amp and see if that helps.

*(1) Disconnect the input ics from the amp.
Turn on the amp and check for hum through the speakers.

I did this as well and still got hum, so I was pretty sure it was the amp itself. The caps were replace shortly after I bought it (used) back in the mid 80's.

Yes there could be a bad one. Need to find someone in Orange County that can check and replace.

Thanks for the test procedure. I followed the first item and came to the same conclusion but wanted a more educated opinion.

I think Jim has given you some very good advice. Your amp is older and there is a good chance the electrolytic capacitors in the power need replacing. Follow his advice first.

Recently, I've noticed a significant hum coming through the mid's on my speakers.

Key word recently..... "On my speakers" indicates both right and left channels. Could be a bad electrolytic capacitor/s in the power supply of the amp or preamp.

If your hum problem is caused from a ground loop you need to find the source of the problem through the process of elimination.

*(1) Disconnect the input ics from the amp.
Turn on the amp and check for hum through the speakers.

Hum? Problem is with the amp. Could be a bad cap/s in the power supply.

No hum? Go to the next step.
>>>>>>>>>>>>>>>>>>>

*(2) Connect the preamp to the amp via the ics.
Make sure no ics from any source are connected to the inputs of the preamp. And no open ended ics connected to any inputs of the preamp... (only preamp > amp > Speakers)

Turn on preamp then amp and check for hum.
No hum? That is good go to step #(3)

Hum? For a test install a ground cheater on the power cord plug of the amp. Turn on the amp and check for hum.
Still have hum? Could be a problem with the preamp. Maybe a bad cap/s in the power supply.
>>>>>>>>>>>>>>>>>

*(3) No hum with only the preamp and amp connected together by ics.

Next step add one front end source to the mix. Connect the source to R&L line inputs of the preamp. Turn on the amp check for hum.
If no hum, add the next front end source to the mix and so on.

Post back your results.
Jim