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Question about hum from speakers

I know there's at least one amp engineer here... I'd like some help.  I bought a used solid-state amp from ebay; the ad read " McCORMACK POWER DRIVE DNA-.5 Deluxe REV B POWER AMPLIFIER - EXCELLENT CONDITION!"
Well, I swapped it in, replacing a Dynaco Stereo 120, and there's a hum from both speakers.  I have to believe that the seller was well aware of the hum.  What I'm wondering is, is the hum a result of some electronic component(s) in the amp failing (or having failed), such that it's salvageable by replacing the bad component(s)?  Or would that be hoping too much?
bhakti-rider
bhakti-rider OP
44 posts
 
Yogiboy:  I've requested a return for full refund; I'm thinking in terms of what if I can't get that refund.
Thanks for the responses so far.  So it could be a ground loop even though there's no hum from the Dynaco, all else being equal?
yogiboy's avatar
yogiboy
5,642 posts
 
You should try a cheater plug and see if that works!
bhakti-rider OP
44 posts
 
Erik & Yogiboy:  I did disconnect the inputs, and there was no hum.  So I found a spare cheater plug and inserted that, connected the inputs again, and the hum was gone.  Thanks!
erik_squires
16,016 posts
 
Do not use the cheater plug as a long term solution. It does indicate you have a ground loop however.


It also absolves the previous owner. :)
bhakti-rider OP
44 posts
 
Erik, what is the long-term solution?  I'm using one for my PrimaLuna preamp too, and if there's a correct approach, then I need to employ that for both of these.
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