For The Experts: What Kind of Weird Stuff Can Happen In This Scenario?

[Paul Marossy]

A guitar player with two Parker Flys wants me to build him a switchable box that will take the signal from the two different guitars with "stereo" outputs (piezo signal + mag pup signal) and split each signal to a mono jack to a pair of amps as in this line diagram:  http://www.diyguitarist.com/PDF_Files/DSSB-Schem.pdf

OK, not rocket science, but here's the part I'm wondering about:

A little more to get to my point. Only one guitar will be in use, and the one in "bypass" mode will have the hot signal switched off, but will still be connected to ground. That means that one guitar will be active and the other guitar will be sitting there, connected to a common ground, but no signal from that guitar can get thru the box because it's bypassed. BUT, can any weird stuff be happening in the guitar that's sitting on the stand that could cause problems thru the common ground - like an antenna effect of some sort? I don't see how anything could happen, but I thought I'd ask the question since I've never dealt with this before nor have I ever seen it discussed in any forums that I can remember. If there is a possibility of some weirdness, what could I do to prevent any problems?

Any help appreciated! 


EDIT: For those unfamiliar with the Parker Fly, it has magnetic pickups and active piezo pickups on the bridge, both of which can be routed to individual amps depending on what kind of cord you plug into the guitar (stereo or mono). The shared tone control is active and affects both types of pickups.

[Meanderthal]

 I'd be more worried about potential phase cancellation if you're mixing the piezo and magnetic signals... but you DID say stereo, didn't you. Should be ok... As far as the ground goes, I'm pretty sure as long as he's touching the strings all should be well, but I can imagine that when he lets go the hum might be twice as loud as normal.

2 reasons I think the Parker is super cool- the acoustic sounding piezo and that's my last name! Would be extra cool to have my name on a headstock...

Edit: oops, you wanted an expert... sorry...

[Sir H C]

Are you shorting the center conductors to ground?  Then you should be good, otherwise you might get some ground bounce that might not get quashed out elsewhere.

[Paul Marossy]

I'd be more worried about potential phase cancellation if you're mixing the piezo and magnetic signals...

Good point, but that's not a concern since the onboard Fishman electronics takes care of all those things.

As far as the ground goes, I'm pretty sure as long as he's touching the strings all should be well, but I can imagine that when he lets go the hum might be twice as loud as normal.

Hmm... that's a thought. But, I would think that you can't have more hum than from the currently active guitar since you aren't amplifying anything from the other guitar.

Edit: oops, you wanted an expert... sorry...

No worries.

Are you shorting the center conductors to ground?  Then you should be good, otherwise you might get some ground bounce that might not get quashed out elsewhere.

No, I'm not shorting them to ground. When you say "ground bounce", do you mean a ground loop? I've thought of ground loops in this scenario, but couldn't see how you could have one of any significance on the guitar side. More hum than usual is a possibility, I suppose.

[Sir H C]

What I am thinking is that there is signal on the other guitar, noise, what-have-you.  If the center is just floating, then often you will see that signal on the ground side of things, and it could possibly give some (albeit little) noise to the system.  So often shorting the center to ground in effect takes that whole cable out of your signal path.

[Paul Marossy]

I see your point Sir H C. I would like to short the hots to ground, but I've run out of poles on my 3PDT footswitch. 

[rockgardenlove]

I suppose you want it passive, eh?

[Paul Marossy]

I suppose you want it passive, eh?

You mean my box? Yep.

[Sir H C]

Would imagine it should not be much signal if it is noticable.  If the buzz is there, the other guitar will be screaming worse than the one that is off.

[MKB]

I don't think you'll have any grounding problems with the guitars.  When you think of all the grounded metal you have inside a regular guitar signal chain (stompboxes, cables, power supplies, the amp chassis), just a single guitar and cord hanging off the same ground shouldn't be a problem.  Noise shouldn't be as well since you have a relatively low gain need for the Fishman preamp, and since it is active it should be fairly low impedance.  I suppose it could act as an antenna if you have problems with an absolute ground at the amp though, but then all the other grounded stuff would act as an antenna as well.

Also, if you are using two amps on the same ground, you'll want to watch for ground interactions between the amps.  I'd personally be more concerned about that.

[Paul Marossy]

I don't think you'll have any grounding problems with the guitars.  When you think of all the grounded metal you have inside a regular guitar signal chain (stompboxes, cables, power supplies, the amp chassis), just a single guitar and cord hanging off the same ground shouldn't be a problem.  Noise shouldn't be as well since you have a relatively low gain need for the Fishman preamp, and since it is active it should be fairly low impedance.  I suppose it could act as an antenna if you have problems with an absolute ground at the amp though, but then all the other grounded stuff would act as an antenna as well.

Those were basically my initial thoughts, but OTOH, I want to make sure that there are no unexpected surprises!

Also, if you are using two amps on the same ground, you'll want to watch for ground interactions between the amps.  I'd personally be more concerned about that.

Already thought of that. I've got a ground lift on Output 1.

[Processaurus]

Also, if you are using two amps on the same ground, you'll want to watch for ground interactions between the amps.  I'd personally be more concerned about that.

Absolutely.  If you were to potentially have noise, that would be a better suspect.  I'd consider putting a toggle switch on your switcher box to be able to lift the ground on the signal going to one of the amps (using an insulated jack for that output).  The 2nd amp will get its signal ground through the power cords 3rd prong connection with the other amp.

As for grounding the unused guitar's signal, nah, it wouldn't make any difference, and you might not want to short it out because it sounds like the guitar has active electronics, and it would suck power to have it trying to drive the signal into the very low impedance of the ground path.

I don't think I've seen a performer with an amp for a piezo pickup and another for normal electric guitar, there could be a lot of advantages to that idea.

[Paul Marossy]

I'd consider putting a toggle switch on your switcher box to be able to lift the ground on the signal going to one of the amps (using an insulated jack for that output).  The 2nd amp will get its signal ground through the power cords 3rd prong connection with the other amp.

As I said, there's a ground lift on Output 1. 

As for grounding the unused guitar's signal, nah, it wouldn't make any difference, and you might not want to short it out because it sounds like the guitar has active electronics, and it would suck power to have it trying to drive the signal into the very low impedance of the ground path.

My intuition tells me grounding the second guitar's output is not necessary, and you also make a good point about the signal driving thing.

I don't think I've seen a performer with an amp for a piezo pickup and another for normal electric guitar, there could be a lot of advantages to that idea.

Welcome to the world of Parker Guitars! It opens up a lot of possibilities for those who play acoustic and electric a lot.

And now for some pics:

[Paul Marossy]

Well, I'll be sending this creation to its new owner in a few days for use out in the real world. Hopefully there won't be any problems with it...