Power amp humming - help please

could be a number of things (ground loop, bad caps or power supply, etc. etc.). however, i've often found the culprit to be a slightly loose transformer, which can be fixed simply by screwing it down tighter. make sure you disconnect the power from the amp for a couple of days before you screw around with it

Thanks a lot Johnson - I will have to try that - I will be careful as you warned me.

OK--typically it's a hexagonal nut in the middle of the top of the transformer,

Thanks again Johnson...

I have two of the Krell power amps (for home theatre) and they both make hums when I switch out and connect the same way, with the same cables.

Does that mean anything? Perhaps cables or something? (I am so ignorant when it comes to electronics)

If the hum is physically coming from the amp, specifically from the power supply transformer, that is DC offset hum. This is eliminated either with something like a Humbuster power strip, or by trial and error to find what is plugged in or wired that is causing the offset. 

If the hum is heard to be coming out of the speakers then this is ground loop hum. 

So as usual we start with precisely identifying exactly what it is we are talking about.

Gong, is the hum coming from the speakers or the amp directly? If it is coming from the amp directly then I agree, it is most likely the transformer. If it is coming from the speaker then you probably have a ground loop. Try putting a ground "cheater" on the amp's power cord and see what happens.

The volume doesn't change when I crank the volume up on pre, if that means anything...

mijostyn, excuse my extreme ignorance on Hifi, but how do I put a ground cheater on the amp's power cord?

The hum is coming from the speakers and not the amp, mijo...

Right!!!

Test the amp with a cheater plug, an adapter which removes the ground.
https://www.target.com/p/general-electric-2pk-grounding-adapter-polarized/-/A-14442316?ref=tgt_adv_X...

If it removes the hum then you have a ground fault somewhere.

Thanks so much lowrider!

I ordered it.

Later, please help me find the ground fault - I have no idea how to do that - excuse my ignorance on electronics...I just love music.

The cheater-plug will help troubleshoot the hum. If the hum stops when it’s plugged in then there’s a ground-loop.

To review, you disconnected all audio cables from the amp and you plugged the amp into different receptacles.
It’s possible they may be on the same circuit. Last thing to try, plug the amp into the refrigerator or air conditioner receptacle. They are possibly on dedicated lines, different circuits than your audio system.

Do you live in a house with your own service panel (circuit box)?