Pre-amp output grounding.

[marcoloco961]

Tried to grab a pre-amp output of off an amp directly after the tone stack out. It is a pretty decent signal strength for what I need but I have a huge grounding issue. It has nice tone but a nasty 60 or 120 Hz (guessing) hum coming from the amp I am using as the power amp. This amp has no issues and sounds great when not plugged into the pre-amp out of the other amp. The tube amp I am using for a pre is also quiet until I plug the two together.

Just for reference the amp I am using as a pre-amp is a 50 watt tube bass amp, and the amp I am using for the power section is a 350 watt Ampeg SS amp.

I am using shielded wire for the signal and took the ground from a spot closest to the jack. Should the ground be floating, or do I need to ground close to the OT outs on the other speaker jacks in the tube amp.

If I can whip this ground issue, I think I would be happy with the set-up.

TIA for any info or suggestions.

[ChrisM]

Sounds like a ground loop issue to me.

[marcoloco961]

Sounds like a ground loop issue to me.

That is what I had figured. Any way to avoid the loop between the two amps that you know of?

[Structo]

You can lift the ground of the power amplifier but it can be dangerous but if the amp you have your guitar plugged into is grounded then I think it would be OK.

[NickC]

You could try balanced interconnects between the pre and power amp.

http://www.dplay.com/dv/balance/balance.html#balance

[jjman]

Try lifting the ground (shield) on one end of the signal connector between the 2 amps.

[marcoloco961]

Thanks guys, I will try a few of these and see if it helps.

Will do them one at a time to see what works best, and then report back.

[rdjones]

First and foremost before lifting any grounds, make sure both amps are plugged into the same grounded outlet.

Is the tube amp's AC wired for modern 3 prong safety grounding ?

rd

[NickC]

Maybe not a permanent solution, but handy to have in the gig bag (and might help trouble-shoot the problem at hand):

http://www.ebtechaudio.com/llsdes.html

[Structo]

Or maybe try the Hum X.

[marcoloco961]

First and foremost before lifting any grounds, make sure both amps are plugged into the same grounded outlet.

Is the tube amp's AC wired for modern 3 prong safety grounding ?

rd

Yes, the tube amp has a 3 wire properly grounded chord. Neither amp has any issues with the noise until I hook the line between them then I get a loop.

At first I used a small chord that was a TRS chord converted to a TS chord. It has the two wires and the shield, kinda like the balanced idea. Noise was audible but not horrible until turned up over half. Then I tried a speaker cable, and it was much worse. Obviously the speaker cable was a totally improper choice for a signal.


Gonna try making a balanced chord like Nick suggested. If it still is there, I will lift on end of the shielding from the ground and see what happens. Thanks again for the great input everyone.

[marcoloco961]

Or maybe try the Hum X.

That was looking like the way to go until I saw the Price...............71 dollars????

Anyone got a schematic???

[marcoloco961]

Thanks all, I ended up putting a TRS jack on the pre out and sending the signal out the Tip and the ground through the ring. I left the shield unconnected on this end. On the power side I connected the signal to the tip and the ground from the pre amp and the shield to the sleeve. This didn't completely kill the hum, but cut it about 80% or better. It is now livable and can be used without an issue.

Once again, I sincerely appreciate the advice.