ground loop hum elimination circuit

[tempus]

Hey all;

I'm in the process of redesigning my current pedalboard with some improvements. As it is, I have an electric guitar with a piezo bridge (for acoustic sound). This stereo plug plugs into my pedalboard which consists of 5 pedals mounted on a box which contains the power supply and switching matrix. The pedalboard also has a preamp for the piezo. This output feeds the PA, while another output feeds my guitar amp. As you can see, there's a lot of potential for ground loops in my system, so I was wondering if it was feasible to build a circuit to eliminate the possibility of ground loop hum. I'm thinking something along the lines of a DI box on both outputs (would I need one on the inputs?) but mounted inside my pedalboard. Any suggestions? I've thought of using transformers, but I'm not sure how that would affect the sound quality. Is there another way to implement this?

Thanks

[Radamus]

I'm pretty new to all of this, so you may know more than me, but from what I understand, there are two ways of getting rid of ground loops if they're bugging you. The first is to break the loops, which I think means disconnecting ground from your output jacks so the loop doesn't carry over into the next effect. The second is using a different transformer for each effect, providing each with an independent ground.

You have all of the effects in one box? Just make sure the ground doesn't get looped anywhere. Don't connect any point to ground by more than one other point and that should work. I think you only have to worry if you make a full circle in ground wires.

You might want to do a search on power supply DIY on the board. You'll find a couple good links to more information.

[R.G.]

I'm in the process of redesigning my current pedalboard with some improvements. As it is, I have an electric guitar with a piezo bridge (for acoustic sound). This stereo plug plugs into my pedalboard which consists of 5 pedals mounted on a box which contains the power supply and switching matrix. The pedalboard also has a preamp for the piezo. This output feeds the PA, while another output feeds my guitar amp. As you can see, there's a lot of potential for ground loops in my system, so I was wondering if it was feasible to build a circuit to eliminate the possibility of ground loop hum. I'm thinking something along the lines of a DI box on both outputs (would I need one on the inputs?) but mounted inside my pedalboard. Any suggestions? I've thought of using transformers, but I'm not sure how that would affect the sound quality. Is there another way to implement this?

Read http://www.geofex.com

[Paul Marossy]

I build little boxes for Parker Fly players that accomplish more or less the same thing as what you want to do.

As long the guitar itself doesn't have any grounding problems, I would route the piezo pickups thru a standard passive DI box circuit, with the XLR output going to the PA system. Since a standard passive DI box circuit has a ground lift switch, in theory that should be all that you need to avoid a ground loop between the two pickup systems thru the amplification systems (in a perfect world). You can treat the magnetic pickups as you normally would. This works most of the time if the place you are in has an electrical system that is wired correctly, but sometimes you get weird stuff happening with how a building is wired (or how their sound system is wired), so it's not 100% foolproof.

And, as R.G. suggested, do some reading at www.geofex.com - I learned a great deal from his website.

[tempus]

Thanks for the replies guys.

R.G. - Can you point me to a specific article on your (most awesome) site? I checked there before my initial post on this topic and could only find the Hum free a/b/y circuit. What did you want me to read from your site? Also, in the a/b/y design, do those trannies from Mouser do a good job as far as uncolouring the tone (I'm assuming yes, since I know that you are also concerned about maintaining tone, but they're only like $2.45!! I thought you needed some expensive Jensens or Sowters or the sound was crap).

Paul - so you're suggesting just a DI on the piezo. There is also a ground loop hum possibility from my amp being plugged into 1 wall socket and my pedalboard being plugged into another. Should I also put a DI circuit on the output to my guitar amp?

Thanks

[Paul Marossy]

Paul - so you're suggesting just a DI on the piezo. There is also a ground loop hum possibility from my amp being plugged into 1 wall socket and my pedalboard being plugged into another. Should I also put a DI circuit on the output to my guitar amp?

You could... but I assume that the guitar amp does not have an XLR input, right? Most don't anyway.

I would try to avoid a ground loop on the power side of things by using a grounded extension cord with the all of the equipment that you have being served by a single grounded power strip.

There's still no guarantees that this will work in all situations, however. If you will be playing several different venues, there's too many variables to say that this is all bulletproof. 

[Andi]

R.G. - Can you point me to a specific article on your (most awesome) site? I checked there before my initial post on this topic and could only find the Hum free a/b/y circuit

That includes ground loop elimination.

[tempus]

Why does it have to have an XLR input? I thought the ground loop hum problem was fixed by the transformer breaking the physical connection between the 2 ground points, not by a balanced line.

[Paul Marossy]

Why does it have to have an XLR input? I thought the ground loop hum problem was fixed by the transformer breaking the physical connection between the 2 ground points, not by a balanced line.

True, you could just use an audio isolation transformer. I was thinking in terms of a DI box that has XLR jacks on it...