Ground loop with outboard gear

[crholguin]

I recently finished a Dumbleator to go with a non-HRM ODS clone. A power conditioner feeds AC to both. When I plug the D'Lator in, however, I get a 60Hz hum into the amp. The hum is there even with the D'Lator off. I can cure the problem by lifting the ground lug off the male end of the D'Lator's AC cord. Not ideal insofar as safety is concerned, but it works.

Everything I've read indicates that the proper solution is to use balanced interconnects everywhere, which strikes me as a rather impractical option with guitar gear. Are there any other possible fixes I'm missing?

TIA,

Carlos.

[briane]

This wont happen unless there is a ground loop in the amp or dumbleator.

I've built many, never had that issue. Suspect a problem in the build.

[crholguin]

On their own, both amp and D'lator are very quiet and sound great. They even sound right together so long as the AC ground is lifted on the D'lator. I do have two PTs in the D'Lator, one at 1:1 feeding the voltage doubler, and another for the 6.3v filament supply. Would you suspect some screw up in polarity in wiring the two up?

TIA,

Carlos.

[briane]

You do have the 2 100 ohm heater ground reference resistors in, right?

I've never had polarity issues like that, though does not mean there's something going on there. I always use 1 pt for the dumbleator, its such a small build.

Lifting the ground and having the humm go away is a guaranteed result of a ground loop.

[iknowjohnny]

well, i don't know if this is something you've done or WOULD do, but if you lift the ground shield from one end of any cables connecting the 2, that should do the trick. Same as lifting the plug, but assuming you don't want to do that for safety reasons, this will accomplish the same thing, and the plug's ground will act as the single common ground between the 2.

[crholguin]

Yes, I have the 6.3v secondaries lifted 100 ohms off ground.

I tried an interconnect grounded at one end only into the power amp, and it was noisier than the one grounded at both ends.

I starting to think I'm just being anal about the additional 60 Hz hum. There's definitely more there than with the AC ground to the D'Lator lifted, but I need to put my head next to the speaker to hear it. Maybe what I'm hearing is just some minor eddy currents or kindred despicable gremlins running around the chassis.

Anyway, I much appreciate the kind advice.

Best,

Carlos.

[aclempoppi]

I don't know what a Dumbleator is, but guess it's a preamp. If it's only audible when you try to hear it, I'd say " live with it." If on the other hand, it increases with volume applied to the units, I'd try a Hum-X by Ebtech. The other alternative would be going the isolated ground and filtered ground setup in the Dumbleator's chassis. Either method gives you a safe ground. Art

[drz400]

I recently finished a Dumbleator to go with a non-HRM ODS clone. A power conditioner feeds AC to both. When I plug the D'Lator in, however, I get a 60Hz hum into the amp. The hum is there even with the D'Lator off. I can cure the problem by lifting the ground lug off the male end of the D'Lator's AC cord. Not ideal insofar as safety is concerned, but it works.

Everything I've read indicates that the proper solution is to use balanced interconnects everywhere, which strikes me as a rather impractical option with guitar gear. Are there any other possible fixes I'm missing?

TIA,

Carlos.

If you keep your power cords short and plugged into the same plug strip you can make it minimal sometimes. If both are third pin ground to chassis as well as the audio cables you have multiple grounds. You can let the Dumbleator chassis stay grounded to third pin and put a switch in to lift the audio from the chassis (if you built the dumbleator with a separate ground path allowing you to do this). You can also use isolation transformers but I would prefer not if I could avoid it. You also may need to lift the effect from 3rd pin ground. You should have 1 path to ground.

The problem you describe is VERY common in the rack world. Ask any rack builder. There is nothing wrong with lifting the third pin as long as you can assure the connecting cables are not unplugged. You hear the hum when you have two grounds. Also dont use those silly polarity caps on the AC, they cause way more trouble than they fix.
If you want to get further into it
http://www.rane.com/note110.html

[Bruciep07]

The easiest option to eliminate the ground loop is to lift the ground on the Dumbleator. With the dumbleator grounded and the amp grounded, once you connect the 2 you now have 2 paths to ground creating a huge ground loop. With the dumbleator's ground lifted, it's path to ground is now through the inter connect to the amp, and shares the amps ground. I read an interview with Mike Soldano where he talked about the GTO pedal, and about how people were complaining about the hum created when it was connected to the amp. His advise was to use a 3 prong to 2 prong adapter on the GTO pedal to lift the ground...
Hope this helps...
Bruce

[jjman]

I've read that one should lift the ground on the signal wire rather than the power wire. Safer.

[Bruciep07]

Where in Central NJ are you JJman? I grew up in Hightstown but live in Hamilton Twp now...