Where to place isolation transformers in circuit?

[jet_atx]

I have a conceptual question about ground loop prevention that's doing my head in...I want to make an all-in-one pedal that takes a "stereo" (i.e., two signals via TRS) input from a bass, etc., splits it and runs each of the resultant signals separately through its own compressor and outputs them as separate TS outs.

To prevent ground loops when going from the box into amps, I planned on using 1:1 isolation transformers. That's easy enough if we're talking about three pedals: an isolation box and two separate compressors. The part that's doing my head in is trying to figure out where I should put them in relation to the two compression channels and how to handle grounding when they all live inside the same enclosure.

Specifically, if I were to put the isolation circuit in the very beginning of the chain (i.e., before the comps), wouldn't I effectively just reintroduce the ground loop by virtue of the common grounds shared between the separate channels' compressors? Is it just as simple as putting the iso transformers last in the circuit and then making sure that one of the TS output's 'S' connection is isolated from the chassis?

[kraal]

Hi,

The part that's doing my head in is trying to figure out where I should put them in relation to the two compression channels and how to handle grounding when they all live inside the same enclosure.

in -> buffer -(split everything from here 2x)-> compression -> transformer -(no more connection to ground from here) -> out (insulated jacks)

Specifically, if I were to put the isolation circuit in the very beginning of the chain (i.e., before the comps), wouldn't I effectively just reintroduce the ground loop by virtue of the common grounds shared between the separate channels' compressors? Is it just as simple as putting the iso transformers last in the circuit and then making sure that one of the TS output's 'S' connection is isolated from the chassis?

Yes and yes

[antonis]

And, of course, Welcome.. !! 

[kraal]

And, of course, Welcome.. !! 

Damn... yes welcome 

[jet_atx]

Awesome - thanks for the replies and the welcomes!

in -> buffer -(split everything from here 2x)-> compression -> transformer -(no more connection to ground from here) -> out (insulated jacks)

I know you mentioned insulating on the input jacks, kraal, but I just wanted to confirm: should I float the grounds on both of the TS output jacks, or just one of them?

[PRR]

> "stereo"...input from a bass, ... ... ... by virtue of the common grounds shared between the separate channels' compressors?

So you have one(?) bass. And "one" box of compression (a dual compressor, or two compressors in one rack).

I don't see a ground loop.

Tack it up and prove me wrong.

[jet_atx]

> "stereo"...input from a bass, ... ... ... by virtue of the common grounds shared between the separate channels' compressors?

So you have one(?) bass. And "one" box of compression (a dual compressor, or two compressors in one rack).

I don't see a ground loop.

Tack it up and prove me wrong.

First off, thanks for the help!

I'm specifically talking about Rickenbacker's Rick-O-Sound TRS output, and I want the ability to apply per-channel compression (or really whatever effects chain you can imagine) and then output via two TS jacks into two separate amps. I would assume – but not steadfastly claim – that if I merely split the hot leads of the respective channels coming from the TRS and don't manage the grounding appropriately, that when I connect the "dual mono" TS outputs to two separate amps, that I run the risk of a ground loop.

My question was specifically about where/how to isolate the ground, but it sounds like you're skeptical of there being one to begin with. Stripping back to basics, if we dispense with effects chain and simply consider the use case of routing from 2-channel TRS guitar output into two separate amps via 2 x TS jacks, would I not have a ground loop issue? I know it's not exactly the same situation as when you're running an A/B/Y box, but I still thought I'd need something similar to this:



...where one of the TS jacks is isolated. (http://www.ultramagnetics.org/splitter.html)

If it helps, here's the schematic for the Ric: http://www.rickenbacker.com/pdfs/19507.pdf

[stallik]

If you were to rig up a simply Y lead and connect your guitar to both amps, you'd soon hear if you had grounding issues when connecting both amps together. If you do, you need to isolate one from the other, usually with a transformer.

So far, using bi-amp setups with many different amps, I've never experienced a ground loop. Maybe I've been lucky. What I very often get is phase issues between the amps. For this reason alone, I use an isolated splitter but with the critical addition of a phase reversal switch on the isolated output. The model you illustrate does not appear to have this feature.

I'll put in a recommendation for RG's hum free ABY if you are going to build your own and add your own phase reversal switch

Oh, and welcome to the forum

[merlinb]

I know you mentioned insulating on the input jacks, kraal, but I just wanted to confirm: should I float the grounds on both of the TS output jacks, or just one of them?

Ideally you would isolate both outputs, each with its own transformer. But as other note, you could just build it without, and use a separate ground-loop-isolator-in-a-box if you discover a problem.

[R.G.]

Ground loops happen when the "ground" wire carries current and the wire resistance makes the "ground" wire have a voltage across it that inevitably gets mixed with the signal carried on the signal wire. As I posted in this forum a number of times, grounding is all about where the (ground) current goes.

In your case, the problem comes down to the two amp problem, I think. Amps are plugged into the AC line and have their chassis/signal ground connected to the AC mains, either at the safety ground wire (highly preferred) or to the neutral wire through a "death cap". More importantly, the two amps simply cannot have the same voltage on their signal/chassis ground because of the voltage losses in the AC wiring. So connecting the same signal ground from guitars or pedal chains to two amps establishes a wire/connection/resistance path for that voltage offset to travel through, and causes the two amps to get slightly different signals, each of which is offset by the voltage differences between the two amps' "ground".

Bottom line is: isolate the signal into one (or both if you're a belt-and-suspenders kind of guy) amp. This breaks the ground wire resistive path, makes current not flow through it to cause a signal offset between the amps, and stop ground offsets by floating the transformer isolated secondary signal. The problems with this happen because there are no perfect transformers, so you have to take a lot of care with the transformer itself to get good frequency response, no hum pickup in the transformer's coils (a coil acting as a "pickup" for radiated AC wiring and RF signals) and a reasonable price. This is because the amps typically have a high input impedance and guitars/pedal chains are not designed as transformer primary drivers. Design of a good transformer isolator gets complicated, and then it gets expensive.

Try one isolator in front of one amp. Don't sweat the isolator between compressors >unless< you are using two different AC powered compressors. In that case, the compressors are in the position of acting like an amp in the discussion above. Better yet, post a diagram of the various boxes you're trying to connect.