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Old 19th February 2013, 01:46 PM   #1
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Default 1st post, issue with a Adcom GFA-555, 60 cycle hum

I picked up an Adcom GFA 555. I have placed the amp in two different systems, at two different residences. The amp hums.
It doesnt make any noise just sitting idle with the speakers connected and no inputs connected....but as soon as RCA cables are plugged in (connected to either of two pres or just the cable to nothing) the amp exhibits a 60 cycle hum. This has been taken out of my house and retested to confirm at a friends. Same thing.
Already tried lifting the ground to the whole system. Already verified no dimmers. Unhooked cable entering house. Used a speaker wire to ground the amp chassis to the pre chassis. Nothing fixes it.
I need some more suggestions. I am handy with a multimeter and soldering iron if there is something inside to test and replace.
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Old 19th February 2013, 05:28 PM   #2
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I have the same amp and the exact same issue - I have gone through a host of tests and configurations however the humming is getting worse. It increases as I increase the volume (with nothing playing) of my Adcom GFP-710 preamp.
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Old 19th February 2013, 09:41 PM   #3
cihtog is offline cihtog  United States
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Sounds more like the cables and/or pre-amp are a possible source of the hum. Have you tried swapping out the preamp/cables/amp etc. to isolate the problem?
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Old 19th February 2013, 10:42 PM   #4
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cihtog - Thanks for the helpful advice - it is the preamp after all - the amp has a basic tolerable buzz but it is normal and it does not increase as the volume increases. I really appreciate you taking the time to respond!
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Old 22nd February 2013, 02:21 PM   #5
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Not a preamp issue at all with mine. We have tried 2 different preamps, a new peachtree nova pre and manley shrimp, hum is there with either preamp.
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Old 22nd February 2013, 02:52 PM   #6
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Quote:
Originally Posted by Vedder323 View Post
Not a preamp issue at all with mine. We have tried 2 different preamps, a new peachtree nova pre and manley shrimp, hum is there with either preamp.
ok, let's do this test: connect speakers to the amp only, turn on amp without preamp!
Do you still hear the hum? if you do not, it can be cables, preamp, ground loops etc.
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Old 22nd February 2013, 03:15 PM   #7
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Originally Posted by lanchile View Post
ok, let's do this test: connect speakers to the amp only, turn on amp without preamp!
Do you still hear the hum? if you do not, it can be cables, preamp, ground loops etc.
Hi Lanchille, thanks for the response and the help, its appreciated.

From my memory, there is little to no hum with no preamp plugged into the amp. HOWEVER, we tried 2 preamps and even a dvd player and then for grins and giggles, just the cables with nothing attached to the other end and the hum persisted.

Not sure if this helps but with the preamp connected but not turned on, the amp does hum, when the preamp is turned on, it becomes quite a bit louder.

Last edited by Vedder323; 22nd February 2013 at 03:37 PM.
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Old 22nd February 2013, 03:50 PM   #8
gootee is offline gootee  United States
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Are you sure it's 60 Hz hum, and not 120 Hz?

Have you tried:

1. different RCA cables?

2. shorting the inputs (signal shorted to ground) at the jacks? (Will probably still hum.)

3. looking inside the amp?

The amp might have been modified. It could be the RCA plugs' grounds' insulation, or re-routed signal/ground or transformer wiring, or a bad reservoir cap, among other things.

Last edited by gootee; 22nd February 2013 at 04:02 PM.
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Old 22nd February 2013, 04:00 PM   #9
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Quote:
Originally Posted by Vedder323 View Post
Hi Lanchille, thanks for the response and the help, its appreciated.

From my memory, there is little to no hum with no preamp plugged into the amp. HOWEVER, we tried 2 preamps and even a dvd player and then for grins and giggles, just the cables with nothing attached to the other end and the hum persisted.

Not sure if this helps but with the preamp connected but not turned on, the amp does hum, when the preamp is turned on, it becomes quite a bit louder.
in that case...IT IS NOT THE AMP!!! use different cables,different outlets or different preamp.
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Old 22nd February 2013, 04:07 PM   #10
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Originally Posted by lanchile View Post
in that case...IT IS NOT THE AMP!!! use different cables,different outlets or different preamp.
It has been in two completely different systems, in two completely different homes. The amp has to be the problem.
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