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Author Topic: Possible earth problem, but only in one side of an amp  (Read 774 times)

Jonathan Goodall

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Possible earth problem, but only in one side of an amp
« on: June 21, 2009, 06:02:24 pm »

Hi all, just wondering if anyone can offer some suggestions of where to go next with this problem (I think I have narrowed it down to the amp itself but I might have missed something).

I have a new (6 months old) Australian Monitor AM2600NG amp and all has been great with it until yesterdays setup for a theatre show.
Set up the system, powered up the amp, turned up ch A (on amp) and got nice clear silence.  Turned up ch B and got a nasty buzzy hum, not good.
I checked all the connections in B side of the signal chain (all ballanced), everything was fine.  I then swapped ch A's input to ch B's input and B's to A's.  The output from ch B is still buzzy, ch A is fine (so it doesn't appear to be a problem in the signal chain from the desk through to the amp as it didn't switch sides).
Next step was to swap the A and B outputs and the noise followed the swap (noise now coming from left hand speaker instead of right).
Tried the ground lift switch on the amp and the noise got worse (got a kind of pulsing noise which reminds me of a backing truck, but at a lower pitch.  Have been told that this has something to do with the ripple control for hot water cylinders but i'm not sure about that one).
By this time I was getting a bit peeved off so as an experiment I switched the amp to linked mono.  Noise still there on B output.
Last step was to try bridging the amp and chaining the speakers, what do you know, silence !!!!!!!!
I am running the show like this but it’s not really the way I would like to do it.

Note: amp only makes the noise when something is connected to the B input.  The system is all being run from the same power source.

Ok, so is there anything I have missed before I go back to the supplier and suggest to them that I have another dud?  (this is the second amp of this model that I have seen issues with.  The first one did this right out of the box, and again only from the B channel and was replaced by the supplier)

Cheers
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Matthew Knischewsky

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Re: Possible earth problem, but only in one side of an amp
« Reply #1 on: June 21, 2009, 06:13:21 pm »

you followed all the right troubleshooting steps. The problem is in the amp.
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