Grounding in unbalanced systems 2

[2sb18]

Hi,

I'm constructing an audio routing device that a musician on stage would use. They plug their guitar/keyboard/musical saw into one end of it, their amp into the other, and there are 8 send/returns that they plug their "effects" into. My question is in regard to grounding.

The system, because of the nature of music effects, has to be unbalanced. I've been lingering over the the following questions for a bit, and want some feedback from people who, unlike me, know what they're talking about. So thanks in advance.

Following questions:

1) Since both the input and output of a music effect has a signal ground, should i lift the ground on either the send or return jack?

2) Should the ground of the send or return jack go to the chassis or should it go to the signal ground of the circuit in my device?

3) Should the ground of the input and output jack go to the chassis...

4) Should the chassis connect to the signal ground or should I leave it floating? (I've heard with unbalanced gear this is an option)

My current half-baked answers that need refining:

1) I should life one of the grounds on either the send or return jack to avoid a ground loop.

2) Send the ground of the send or return jack to chassis

3) Send the ground of the input and output to chassis

4) Connect the chassis to the signal ground at one point on my pcb.

I'm pretty sure I'm right about 1) but the rest I'm sketchy.

Thanks again,
Steve.

 

[BrianG]

You are quite correct in that you need to avoid a ground loop. In my experience from "days of yore" with guitar amps and echo units, etc., there is no one right answer to this especially with an unknown mix of effects units. It can depend a lot on a) whether the various musical effects units are powered from the same mains supply (and phase) b) whether some or all of these units connect their chassis to mains ground or are "floating" (no ground connection), and c) whether any floating units have any appreciable leakage currents to ground.

I would say generally that the best answer is to maintain all screens since this is the signal return path; lifting the screen forces the signal return path around an unknown ground circuit. If you can connect all effects unit power supplies to a common power distribution box it should help as any units having a chassis connected to signal ground will share a close common ground point reference.

 

[VEBill]

If you end up with hum due to ground loops (very likely), then the solution might be to add audio transformers to convert between balanced and unbalanced. This would likely be a far better solution than leaving out various grounds at various points.

 

[itsmoked]

I wonder if the umpteen musicians who have been fried got that way because of grounding violations in search of no hum solutions. Some thought should go into this aspect.

Such as if the *effect* is line powered but not grounded, a fault in it does...what? Etc.

 

[2sb18]

Thanks for the help guys,

I think what I'll do is treat the signal grounds as another signal to be switched. I don't have the money for adding isolation tranformers to every send/receive (unless someone has a good source). I'll use isolated jacks and the signal grounds of the effects will never be connected to the ground of my device. Is there a reason why this would be a bad idea?

Thanks,
Steve.