Film-Tech Forum ARCHIVE: Need help tracking ground hum

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Posted by Martin McCaffery (Member # 37) on 11-20-2013, 09:53 AM:

I've got a light hum in my Left Surround Speakers. I think the monitor is causing this. If Louis is checking in, he may know just what to check. We will be getting a new monitor in a few weeks, so I can live without the surrounds for what we are showing, but want to eliminate other sources.

Here's what I've checked.

The hum is audible in the monitor channels LeftBackSurround (we have two of those on the monitor, maybe one should be labeled RtBackSurround). Not audible in the monitor channels left or right surrounds.

Hum vol will increase in monitor when monitor gain is increased. Will not increase in house speakers.

Hum will not increase in either monitor or speakers when Dolby 650 gain is increased.

buzz is consistent across all formats.

Disconnect RS Output results is loud ground hum in RS monitor channel. No change in LBS channel.

Disconnect LS Output, same result except LS monitor

Disconnect inputs, no change.

Hum does not follow output cables (ie: plug outputs into right channel amp the Right speaker does not hum. However the same disconnect symptons occur in the monitor in the LS & RS channels.

Suggestions?