My Bass Guitar Preamp and Balanced Output Ground Loop ?

[frankenp]

Hello,

I've recently built my own rack mounted bass guitar preamp. Connecting to a power amplifier and bass cabinet my preamp works great.

Schematic:
**broken link removed**

However, when I connect my preamp to my pc to record, there is an annoying ground hum. It occurs on both the balanced and unbalanced outputs. I've tired connecting and disconnecting chassis ground to signal ground and disconnecting the ground pin on the balanced TRS output. No luck. I've gone through everything connected to my computer trying to find a ground loop.

The best success I've had so far is chaining my preamp into another preamp that has an isolation transformer on the output. No Hum !

I'm hoping that someone can verify my circuit and perhaps recommend a high quality transformer and how I might connected it to my balanced output (U10).

More information on my preamp can be found here:
**broken link removed**

 

[Nigel Goodwin]

Disconnecting the screen on the lead connecting preamp to PC should cure it, this breaks any ground loop. Ensure though that there is a coupling capacitor in the signal output of the preamp, to break any DC connection through that.

 

[frankenp]

I've tried disconnecting the ground lead on both the balanced and unbalanced outputs. I go from an annoying buzz in the background to an overwhelming buzz. C29 should take care of any DC (on the unbalanced output anyway). I'm at a loss. Maybe the noise isn't 60Hz but it sure sounds like it. The noise doesn't occur when I'm connected to my power amp. My poweramp ties signal ground to chassis/earth ground so in my preamp I keep the two isolated. One path to ground = super clean signal.

 

[frankenp]

Finally figured this out!! My design and cabling are all ok. The extra ground path causing the hum was from the television cable plugged into my PC's tuner card. Crap, it took me way to long to figure this out.