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Very Specific Ground Loop Question.

I have my turntable set up as follows:

Technics SL1600>Little Bear tube preamp>USB external soundcard device>PC

I am getting a ground loop buzz. The turntable ground wire is on the screw on the outlet that the turntable is plugged to. I have disconnected absolutely everything else and isolated just these connections above.

The interesting thing is that if I take the tube preamp out of the sequence, and substitute a little solid state preamp from radio shack, the buzz continues UNTIL I disconnect the ground wire from the outlet and don't connect it to anything else. The the hum goes away. The little solid state preamp uses 2 AA batteries.

However, when I have the tube preamp inline, disconnecting the ground wire from the outlet does not stop the buzz. I have also tried touching the ground wire to places all over the tube preamp to ground it there, and nothing stops the buzz.

So to summarize:

TT>tube preamp>computer (ground wire on outlet)=buzz
TT>tube preamp>computer (ground wire not connected)=buzz
TT>battery preamp>computer (ground wire on outlet)=buzz
TT>battery preamp>computer (ground wire not connected)=NO BUZZ

Can anyone help? I do not want to use the radio shack battery preamp.
xtom68
xtom68 OP
3 posts
 
Here is even more information. In an effort to see if this had to do with the computer, I reconfigured to go through one of my receivers. Same effect:

TT>tube preamp>receiver (ground wire on outlet)=buzz
TT>tube preamp>receiver (ground wire not connected)=buzz
TT>battery preamp>receiver (ground wire on outlet)=buzz
TT>battery preamp>receiver (ground wire not connected)=NO BUZZ
williewonka
3,075 posts
 
Have you tried diconnecting the computer ?

Best thing to do is start with the amp/pre only - then add each component
to see what causes the hum

Regards
tls49's avatar
tls49
1,351 posts
 

When using the receiver, is the preamp and receiver plugged into the same outlet? Also, I'm assuming the computer is not connected in any way?
xtom68 OP
3 posts
 
Yes. I have taken the computer completely out of the loop, and same effect.

When I start with the amp/pre, there is hum. When I have just preamp>amp there is hum. Maybe I just have two bad preamps.
xti16
1,525 posts
 
I had a similar problem with a Meridian G08 cdp and Bryston B100sst. If I isolated the ground from the G08 (2 to 3 prong adapter) on the power cord the hum went away. Same if I isolated the ground on the Bryston. So I ran the G08 ungrounded (preferred to keep the component with the highest current draw grounded). Same problem with a Sim Audio Super Nova cdp. When I replaced the Bryston (with an Octave V70se) I never had the problem again with everything grounded. My initial thoughts the fault was with the G08 but in the end it was the Bryston. So my guess is the receiver is most likely the culprit. Again it's only a guess. Have you tried a 2 to 3 prong power cord adapter for the tube pre? Also from your description the TT should not be grounded with the ground wire.
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