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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
9 responses Add your response
williewonka
3,075 posts
 
What amp/pre are you using and do they both use grounded leads

Did this just start happening? - you could be experiencing a real problem in the amp or pre

Try grounding the chassis/case of the preamp

In my case I have a Naim amp, because of its design all sources must be grounded. When I connected a Pioneer DVD player it hummed - grounding the case of the player fixed that.

Regards
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.
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.
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?
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
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
 
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.
stereo5's avatar
stereo5
6,388 posts
 
The turntable ground wire can also be connected to the pre-amp chassis. That is how I set up my turntable.
brf
1,715 posts
 
Try moving your TT away from your pre-amp.
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