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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
8 responses Add your response
lak's avatar
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lak
4,628 posts
 
Glad it was an easy fix!
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tinear123 OP
241 posts
 
Found it!  Freaking cable box sitting one shelf under the noisy amp.  Must be broadcasting RF.  Moved and solved.  I was just before taking the back off the amp.
atmasphere's avatar
atmasphere
11,804 posts
 
If nothing is hooked up to the amp this issue points to the amp itself.

What happens when you short the input of the amp? If it goes quiet, then the real issue is whether it has a hum when you connect the preamp to it.

If it does at that point, then it needs to be serviced. If there is no hum or so slight that you have to be within 6 inches of the speaker to hear it, I wouldn't worry about it.


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tinear123 OP
241 posts
 
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.
delkal
110 posts
 
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.




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