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Analog help: Splitting guitar signal and eliminating cross-talk

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Scott D:
Hey everyone,
First ever post, so be kind. I am hoping somebody with analog experience would be able to help. I have taken some advice from an EE I know, but have exhausted his analog knowledge.

I am not an EE, but know enough to be dangerous.
In simple terms, I have an integration problem. I have designed and built an analog circuit that will be housed inside a guitar and requires the AC source input from the guitar, whilst maintaining its standard output. So, I want to split a guitar signal from inside the cavity in more or less a "Y".

In essence, the circuit is for all intents and purposes, an amplifier in its own right, which does frequency filtration. As expected, the output power drops when split. I also understand there are inherent issues with respect to both amps may attempt to drive each other etc. (although the theory/math hurts my brain).
The issue I have seems to be one of cross-talk between the circuit board and the guitar output to the amp. There is a noticeable fuzz/distortion which is often talked about in this scenario of splitting signal/driving two amps at once.
Relevant portion of circuit:

I ran a "Y" split through 10k resistors on the active/hot line to circuit/guitar output. When scoping the circuit board vs the guitar output, I can clearly see one signal seems to be out of phase with the other. Which does make sense, as I am hearing a classic "fuzz" distortion sound through to the guitar amp.


The leading theory so far, had been that the passive pickups (and its field) are picking up the circuit board and introducing noise this way. I have tested the circuit boards at least ~1m away, with the signal being introduced by alligator clips directly to the POT, needless to say, it doesn't seem to make any difference.

To anticipate some questions:

* In terms of my circuit, it works as expected independently, 9v battery power source
* 9v power source and circuit input source etc, ground's are all common to guitars ground
* Circuit does have DC noise filtering and is not in and of itself 'noisy'
* Guitar uses passive pickups, not active pickups - 500k pots.
* guitar is fully shielded, noiseless in its own right, HSH pickup configuration (with awesome custom switching  ;D)
* Audible distortion/humming etc is removed if grounding the circuit's signal input, when switched on
* No distortion or interference when circuit is turned off
* no distortion, or loss of signal etc if circuit input source is substituted, eg from piezoelectric or signal generator
What I would like help with, if possible - is splitting and isolating the guitar signal to prevent cross talk.
Any solution needs to be able to fit in an already over-crowded guitar cavity, So a big chunky transformer wont work.
It would also need to ensure the same frequency response, ideally from 50h to ~1.6khz
it would also, preferably involve passive components and not "another preamp" or filter, or otherwise turn the pickups to active. - That being said, I would still like to know!

In other terms, how can one SPLIT and ISOLATE an AC signal?
is there a level of impedance matching or system that must be in place for normal operation of the amp etc, and if so how do you do it? - I don't have an LCR or way to measure impedance.

I have done some reading on opto-couplers / transformers etc but not sure how best to proceed. My alternative would be to use an isolated and separate input. that is a Piezoelectric sensor/element but having just tested that today, it does not have the frequency response/power required to drive my circuit - although I only tested one.

I would appreciate any help with solving this one.
Scott

Scott D:
Sorry, pics were meant to be clickable to larger view.

It's not an effects circuit, and must be inside the guitar for this application.

capt bullshot:
Your circuit has a quite low input impedance (1k / R20). Try adding a unity gain / high input impedance buffer - just another Op-Amp with input signal on "+" and "-" tied out its output node.

Is the 9V battery supplying this circuit only? So consider moving the reference ground to your AGND (virtual ground) node, you can get rid of a lot of AC coupling capacitors then.

Audioguru again:
Your filters pass only a very narrow band of low frequencies, like truck horns.
The 0.1uF input capacitor feeding the extremely low value of the 1k input resistor cuts frequencies below 1600Hz.
Here is a simulation of your Multiple-feedback-Bandpass filters:

Scott D:
Hey Capt Bullshot,

I did forget in my original post - thanks for reminding me. I did try a unity buffer as you suggested (LM358 - which may not be suitable). That being said, it was on a breadboard, but it didnt have any effect. I built the circuit originally with consideration that I may have to have this additional buffer in place and it would resolve all issues.  :palm:
9v is indeed for the circuit only.

Correct me if I am wrong, but if I was to make my ground reference to 50% of V+, then my AC can only swing within 25% of V+. I would get clipping?!
eg 9v input, ground reference at 4.5v, then the AC swing can be between 4.5v and 9v.

R20 @ 1k, and R16 @ 1M gives me a nice hefty gain which upon testing was ideal. What is the relationship with the 1k resistor and impedance? ie reducing the value/gain of the op-amp increase or decrease impedance? I do not know how to measure or calculate it. I had thought that the op-amp / circuit would be high impedance, or at least within similar to a guitar amp

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