Hum Super Sleuth Needed


OK guys, not my first rodeo but this one has me scratching my head.  I have a pair of Coincident Frankenstein-MkII monoblocks.  One is dead quiet, one has a psudo 60Hz hum with the amp hooked to the speaker and no input hooked up.  Here is what I have tried thus far:

- Cheater plug on power cord to lift ground (no change)  If this had worked I would just get a Hum-X and be done.

- Swapped all three tubes between amps (no change)

So if it's not a ground loop on incoming power and not a ground loop on the RCA input (unhooked) what can it be?  Possibly a bad filter inside the amp?  Anyone have ideas?



tinear123
It should not be a standard ground loop problem if you just have one component plugged in with no other connections. Are your speakers powered? That doesn’t mean you don’t have some kind of grounding problem internally. I would look inside and retighten / reseat all of the power connections. Pay particular attention to where the ground is connected to the chassis. And a shot of Deoxit on the connections before you retighten them wouldn’t hurt.

There are lots of other things that might cause a hum it but this is easy enough to try first.




delkal...  Thanks, the speakers are not powered though they are horns thus any hum is very noticeable.  Note that the other monoblock is dead quiet.
A couple of questions, first, did this just start or have you had the problem as long as you have had the amps? If it just started, did any system parameters change?

When you say that the hum is present with the amp not connected to the preamp, are you shorting the input to the amp or just leaving it with nothing plugged into it?

What is the difference between a 60HZ hum and a “pseudo 60HZ” hum?

My sense is that it is one, or more, of the power supply caps in the offending amp. Often there is no visual indication that the caps have gone wrong, but sometimes they bulge, or leak. Have you inspected them for irregularities? If you can measure the leakage in the caps that would be useful, otherwise my sense is that a trip to the stereo doctor is in order. I would look at the cathode bypass caps while I was at it.