Question about Adcom GFA-5802 2-Channel Amplifier

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Humming when no input in jack; works fine with rca inserted; worries me that somethings is not right even though works fine with RCA or XLR plugged in, right channel only

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Grubhead

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Sounds like it could be feedback hum. Is there anything near it that could cause a reaction when unpluged? Such as a turntable, cassette etc. If so move them and see if it goes away. Even then it won't hurt the amp. I think otherwise worry when there's a hum when they are plugged in. PS check any earth connections too!

Posted on Jun 23, 2010

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I'm thinking you have an open shield ground on the RCA input side. Connecting speaker inputs possibly restores the ground. Try connecting the high level inputs then disconnect the remote end of the cables (floating the grounds).

Then get out an ohmmeter and find that open circuitor or bad solder joint between RCA ground and real ground in the speaker's amplifier. Or.... if speaker ground kills the hum and you want to use RCA Line Level input to the sub, just connect one minus speaker output on your source amp to one minus on the sub's amp.

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You could make a rudimentary passive mixer by putting a series resistor say 1K ohms from each of the RCA's to a common UNBALANCED input of a subwoofer.

The XLR is balanced so that would not really work unless you also have an unbalanced input on the subwoofer.

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First, we need to get the terminology correct.

The DJM has "PHONO" (RCA) jacks, not "phone jacks" of which the current usage is normally the 1/4 inch diameter.

You would be going INTO the Berhinger 1/4 inch jacks on the back of the speakers since you are not using the three pin XLR connections that are used for balanced lines which the DJM does NOT have.

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