Double neck wiring question

[Rock64]

Hello, First post

I have a double neck with a piezo in the top side, and magnetic pickups on the other. I'd experimented for a long time with two sets of wires hanging off the guitar into two separate destinations. Once I was happy with my arrangement I decided to wire it up to a stereo jack and use a TRS cable divided at the end.

Problem I'm having now is that despite absolutely zero contact with each other, (the exception being the ground), I'm getting a 90/10 bleed both ways from the guitars. This is happening in the guitar as I have tested the continuity of every piece of the chain many times over and the crossover bleeding is inside the guitar. I test the stereo jack, cable, and the splitter, and all of them combined before soldering and it's 100% perfect. As soon as I solder the leads I have bleeding. I'm getting bleed across the A and B. Just to be clear, I have been testing this arrangement for a year and the signal path works as intended so my gear settings and routing are not the issue.

I have two totally discreet signal paths (The exception being the ground). No matter how I wire it it bleeds in some way back and forth. I thought a piezo was to blame but I tried a single coil instead and still it bleeds. In addition, I have tried soldering both sides directly to the jack with no other connections and it still bleeds.

Is it possible that I'm mistaken in assuming you can run two separate signals through one cable (stereo) and with a common ground have no bleed? If that were the case then my headphones would forever be mono.

Imagine for a moment you had an SG with it's two volumes and two pickups. Imagine switching to the bridge. The neck pickup has no chance of coming through. Now imagine you took that guitar and instead of a 3 way switch, you wired both pickups directly to a stereo jack and all switching control was outside the guitar, but when you cut out the signal from the bridge pickup you still hear it coming through the neck side. This is what is happening to me. What on earth am I doing wrong?

[Hugh Robjohns]

I suspect the problem you're struggling with is crosstalk within the cable due to the very high impedances of the pickup sources and of the destination amplifier inputs.

This crosstalk isn't such a problem with headphones because the impedances involved are hundreds of Ohms at most, and often only tens of Ohms. In the guitar you're looking at hundreds of kilo-Ohms or even Meg-Ohms. These very high impedances means that the inherent small capacitances between signal wires within the cabling act to couple the sound across quite efficiently.

Using a cable with two individually screened cores would undoubtedly help, but I expect you'll either need physically separate outputs and two separate cables (as per your trial rig), or active buffering circuitry inside the guitar to convert from the high-impedance internal wiring to provide low impedance outputs.

[Wonks]

Hi and welcome.

That sounds very strange. Of course, there is a possibility of the signal transmission from one 'hot' cable to another 'hot' cable within the TRS cable itself. The high impedances of guitar pickups and piezo circuitry make this far more likely than with the much lower impedances involved in headphones, though there still is some measurable bleed in headphones between channels - which is why some headphones use a 4-wire system.

The longer your TRS to TRS lead, the bigger the cross-talk signal is going to be. Have you got a short TRS to TRS patch lead that you can use between the guitar and the splitter box to see if the cross-talk reduces significantly?

[Rock64]

I suspect the problem you're struggling with is crosstalk within the cable due to the very high impedances of the pickup sources and of the destination amplifier inputs.

This crosstalk isn't such a problem with headphones because the impedance's involved are hundreds of Ohms at most, and often only tens of Ohms. In the guitar you're looking at hundreds of kilo-Ohms or even Meg-Ohms. These very high impedance's means that the inherent small capacitance's between signal wires within the cabling act to couple the sound across quite efficiently.

Using a cable with two individually screened cores would undoubtedly help, but I expect you'll either need physically separate outputs and two separate cables (as per your trial rig), or active buffering circuitry inside the guitar to convert from the high-impedance internal wiring to provide low impedance outputs.

Thanks for the quick reply. I think this cable is about 10 ft. I had a 3 footer that did exactly the same thing. I did try placing a homemade 9v eq circuit I had here on the electric side and that seemed to stop the piezo side from making its way into the electric side. I think I tried it on the piezo side and it still bled. I might have to try that again to be sure.

Fishman powerchip claims to be able to bring a magnetic and piezo pickup together or separate them to a stereo trs cable but that lil thing is 130 bucks.

So I'm correct in assuming that this passive arrangement is not going to work as is? There need to be some type of buffering involved if I don't wan the two signals to push their way into each other? And this all happens through the common ground. Why then didn't they do this when the two sets of wires I used to have, hit the common ground of my gear? I place a meter across those contacts and they're a cleaner continuity than anything I've tested.

[Hugh Robjohns]

I did try placing a homemade 9v eq circuit I had here on the electric side and that seemed to stop the piezo side from making its way into the electric side.

Yes, I'd expect that to work because it provides a low impedance output to drive the cable.

I think I tried it on the piezo side and it still bled. I might have to try that again to be sure.

Try it again, as it should work!

So I'm correct in assuming that this passive arrangement is not going to work as is?

Only if you use separate cables.

There need to be some type of buffering involved if I don't wan the two signals to push their way into each other?

That would certainly be the best way.

And this all happens through the common ground.

No, it happens because of capacitively-coupled crosstalk between the two output wires inside the cable due to the very high-impedance environment.

Why then didn't they do this when the two sets of wires I used to have, hit the common ground of my gear?

Because they were physically separate wires, so the capacitive coupling was negligible. The common ground is completely irrelevant.

[Rock64]

Thank you for the crazy fast replies, but most of all for the truly helpful details and concise pinpoint answers. I'm making people mad elsewhere as they are trying to help but aren't getting it.

[Sam Spoons]

I'm guessing this is a problem because you have two amps with the mag pickups bleeding into the acoustic amp and the piezo bleeding into the electric amp? Can you not prevent the signal going to the amp you are not using with an A/B/Y box?

[Rock64]

I'm guessing this is a problem because you have two amps with the mag pickups bleeding into the acoustic amp and the piezo bleeding into the electric amp? Can you not prevent the signal going to the amp you are not using with an A/B/Y box?

Once the two signals leave the guitar thru the trs, they're split and sent into two separate inputs on a pedal board and the expression pedal is programmed to pan between two inputs. This worked beautifully and still does but the bleed ruins the 100% end product I used to get with two separate cables. If that makes any sense.

[Sam Spoons]

Yes it does. Why do you need to pan between the two signals, do you ever blend them together (not sure how you could given that that would have to play both necks at once?). Can you gate the two inputs so that when you reach say 15% the gate activates to silence the bleed?

[Wonks]

Once the two signals leave the guitar thru the trs, they're split and sent into two separate inputs on a pedal board and the expression pedal is programmed to pan between two inputs. This worked beautifully and still does but the bleed ruins the 100% end product I used to get with two separate cables. If that makes any sense.

If you aren't playing the piezo neck, how does it produce any sound if you are just playing the magnetic neck (apart from sympathetic vibrations being picked up)? Or do you alternate between the two necks quickly? Or are you doing something like tapping on both the necks together at times?

I could understand more easily it if it was a single neck with piezo and magnetic pickups.

And presumably within the pedalboard/multi FX you've got one set of FX treating the acoustic sound and another the magnetic sound, and the bleed is affecting the tone of the signals.

[Rock64]

Yes it does. Why do you need to pan between the two signals, do you ever blend them together (not sure how you could given that that would have to play both necks at once?). Can you gate the two inputs so that when you reach say 15% the gate activates to silence the bleed?

Last question first. The bleed is in the guitar signal so I cannot affect by that point. Its literally coming through the wrong side all the time.

I used to play just one guitar onstage and if ever needed an acoustic sound I simply used a sim. That was fine for what I did, but I always wanted to do more. I just didn't believe the sims. I once played two guitars strapped on my carcass, one electric and my acoustic. It was a simple tune and it worked. When the other guitarist quit I had to condense so the simulator became king.

I've been toying with this idea for a very long time and I convinced the wife to buy me this doubleneck for Christmas. I actually paid for it but she's run out of gift ideas for me so it was win win. I've become totally dependent on expression pedals for almost anything onstage. I make them do 10 things at once so I don't have to look down for buttons and I can't get forward/backwards wrong. I can feel on and off or anything in between.

I don't always want to pan but sometimes I find a little cross-fade is nice with this arrangement as I'm not two guys with 4 hands. I never play both at once and never foresee wanting that but you never know. I could do that too if need be with this arrangement.

[Rock64]

If you aren't playing the piezo neck, how does it produce any sound if you are just playing the magnetic neck (apart from sympathetic vibrations being picked up)?

You got it. The biggest problem is the electric contaminating the acoustic. I use a string damper to quell that. I had the acoustic vibration problem under control but now it's sneaking in through the electronics and that is why you find me here

There's a ghost in the machine