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Old 13th July 2003, 07:07 PM   #1
JoeBob is offline JoeBob  Canada
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Default Buzzing, could it be a ground loop?

I have a tube preamp, all worked fine until I switched the output capacitor (same value, just a higher voltage part) then when I pluged it back in I get a very loud buzzing sound. When not connected to an amp it measured 0.5V 60Hz coming out of the preamp output with volume turned to the min.

Could this be a ground loop? I ask because the voltage seems high and the buzzing gets amplified in any later stages, it seems to me ground loops would only occur when the preamp was connected to something else and it wouldn't get amplified... right? Well, just looking for advice I can't figure out what happened here.

BTW the preamp is wired up with a star ground and hum wasn't a problem before...
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Old 13th July 2003, 08:32 PM   #2
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Could this be a ground loop?
Not likely.

What may have happened is the you somehow lost a ground connection when you replaced that output cap.

Have you checked for cold soldering joints?

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Old 13th July 2003, 09:05 PM   #3
JoeBob is offline JoeBob  Canada
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Well, thanks for heading me in a direction, let's hope it's the right direction. I haven't checked everywhere for cold joints, because of the star ground that's going to be alot of checking.

But if that's a possible reason it would make sense, maybe I bumped something when changing the cap, for I am pretty darn certain that cap didn't change anything.

I'll hunt for a ground conection that got loose.
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Old 14th July 2003, 02:24 AM   #4
JoeBob is offline JoeBob  Canada
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Hmmm... all of the ground points in the circuit have good contacts and all measure a resistance of 0 to ground, so I guess no ground points got loose.

After looking further I notice that there is also a 1kHz signal present as well, could this be oscilation? And if so, what are common reasons for this (that could explain it spontaneously happening)?

BTW I'm using a 6922 as a common cathode stage followed by a 6AS7G as a cathode follower.
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