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.
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.
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:
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.
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:
in that case...IT IS NOT THE AMP!!!
use different cables,different outlets or different preamp.in that case...IT IS NOT THE AMP!!!
It has been in two completely different systems, in two completely different homes. The amp has to be the problem.
adcom gfa humm solved
We had one come in recently and the engineer here solved the problem. These amps are quiet when new but tolerances drift. It is not the caps.
We improved the design slightly and the result is a noise floor nearly -80 dBV.
that is a lot better than new and now as good as the Levinson No436 monoblock.
I know this because we are also one of 4 Levinson regional service centers. We have also been an Adcom warranty station for many years. and Hegel, Lexicon ( love the LX-7), Yamaha ( own M-2 and M-4 amps) Onkyo (awesome M-508 supported) Denon, Marantz (I work on the 240/250 and owned a 500 that I miss).
We had one come in recently and the engineer here solved the problem. These amps are quiet when new but tolerances drift. It is not the caps.
We improved the design slightly and the result is a noise floor nearly -80 dBV.
that is a lot better than new and now as good as the Levinson No436 monoblock.
I know this because we are also one of 4 Levinson regional service centers. We have also been an Adcom warranty station for many years. and Hegel, Lexicon ( love the LX-7), Yamaha ( own M-2 and M-4 amps) Onkyo (awesome M-508 supported) Denon, Marantz (I work on the 240/250 and owned a 500 that I miss).
Last edited:
What changes did you make ?
I had problems with an old Maplin design with hum.
I added RC decoupling to the front end on both rails and this killed the hum dead.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.