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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

Showing 3 responses by xtom68

xtom68 OP
3 posts
 
I tried moving the TT away from the preamp, and no change. I also tried putting the ground wire everyplace on the pre-amp I could see, including chassis, power supply, etc. No change.

Interestingly, I discovered something else. I still get ground-loop hum when the TT is disconnected completely, and this is the configuration:

Power outlet>Preamp>external Soundcard>computer

I don't know what to make of that. The computer is in a separate room from the preamp and plugged into an outlet that is on a separate circuit from the preamp, as far as I know. This is frustrating.
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
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.
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