Need help with op amp control circuitry for resoelectric guitar

[crazydoc]

The cheap preamp ($9 USD) matches the piezo impedance and EQ's the signal +/-12dB at 60, 600, 2.2k and 8k Hz, so there is plenty of variation to play with from boomy to tinny.

 

[crazydoc]

Here are the electronics waiting for installation after applying the finish (except for the piezo and changing to an end pin jack).

 

[crazydoc]

To bring this finally to a close:
I decided to phase shift the piezo against itself, since the magnetic PU and the piezo didn't seem to have any audible phase relationship. I added another op amp to sum the allpass filter output with the unaltered piezo signal. I can now hear a difference as I rotate the allpass pot, but it almost sounds like an eq control and is not very striking.

But it was an interesting build, even if not useful, and I had fun and learned a bit. Attached is a schemo of the final circuit.

 

[abbey road d enfer]

I decided to phase shift the piezo against itself, since the magnetic PU and the piezo didn't seem to have any audible phase relationship. I added another op amp to sum the allpass filter output with the unaltered piezo signal. I can now hear a difference as I rotate the allpass pot, but it almost sounds like an eq control and is not very striking.

No wonder! It results in a mere 1st-order low-pass filter.

 

[crazydoc]

No wonder! It results in a mere 1st-order low-pass filter.

I can see how it looks like the caps connected to the 3 way toggle would shunt the high frequencies to ground, forming a low pass filter, but in a prior post you said:

Capacitor C1 must be terminated to ground. The half-voltage node must be connected to junction of R1 & R3.

So what is the solution to make this into an all pass rather than a low pass filter?

 

[abbey road d enfer]

I can see how it looks like the caps connected to the 3 way toggle would shunt the high frequencies to ground, forming a low pass filter, but in a prior post you said:
So what is the solution to make this into an all pass rather than a low pass filter?

The all-pass filter is the circuit built around the SouthWest opamp. It receives two signals; one that is indeed a low-passed signal on its non-inverting input, and a wide-band signal on its inverting input. The result at the output is an all-pass. You can write it as Low-Pass-WideBand
What the OP did is mix this output with the input signal. So the output signal at the SouthEast opamp is what?
(Low-Pass-WideBand)+WideBand=LowPass

 

[crazydoc]

Sorry about my electronic ignorance. I thought that if I summed the original signal with the phase shifted one (from the allpass filter) that the output would be the phase difference (as amplitude) at the various frequencies, as in a voltage controlled "phaser",  but controlled by manually rotating the pot.

"The resistor can be replaced with a FET in its ohmic mode to implement a voltage-controlled phase shifter; the voltage on the gate adjusts the phase shift. In electronic music, a phaser typically consists of two, four or six of these phase-shifting sections connected in tandem and summed with the original. A low-frequency oscillator (LFO) ramps the control voltage to produce the characteristic swooshing sound. "

Wikipedia

But apparently summing the signals just turns the whole circuit into a low pass filter?

 

[abbey road d enfer]

Sorry about my electronic ignorance. I thought that if I summed the original signal with the phase shifted one (from the allpass filter) that the output would be the phase difference (as amplitude) at the various frequencies, as in a voltage controlled "phaser",  but controlled by manually rotating the pot.

"The resistor can be replaced with a FET in its ohmic mode to implement a voltage-controlled phase shifter; the voltage on the gate adjusts the phase shift. In electronic music, a phaser typically consists of two, four or six of these phase-shifting sections connected in tandem and summed with the original. A low-frequency oscillator (LFO) ramps the control voltage to produce the characteristic swooshing sound. "

Wikipedia

But apparently summing the signals just turns the whole circuit into a low pass filter?

When you sum several all-pass filters with the original signal, you get notches.

 

[crazydoc]

Thanks - I'm starting to barely get a handle on this, but will never get all the way there.

https://ccrma.stanford.edu/realsimple/DelayVar/Phasing_First_Order_Allpass_Filters.html

"I see" said the blind man to his deaf dog as he picked up his hammer and saw.

 

[abbey road d enfer]

Thanks - I'm starting to barely get a handle on this, but will never get all the way there.

https://ccrma.stanford.edu/realsimple/DelayVar/Phasing_First_Order_Allpass_Filters.html

"I see" said the blind man to his deaf dog as he picked up his hammer and saw.

Although mathematically true, this article lacks simplicity.
The "phaser" effect tries to emulate a sweeping comb filter, i.e. a series of notches that are harmonically related.
A comb filter is created in nature by combined a sound and a delayed image of same sound. It happens naturally whenever there's a strong reflection. Notches appear at frequencies where the delay equals an odd number of halfwavelengths, and a correlated boost happens at frequencies corresponding to an even number of wavelengths.
Since delays are not easily produced in analog electronics, designers have devised circuits that create the same series of notches and boosts. Using variable notch filters proved too complex, expensive and hard to control.
Then came the idea of using the pedestrian 1st-order APF which, combined with its input, creates a notch. Using several sections of APF allowed creating several notches, thus approaching the comb filter response.
Note that psychoacoustically, the boosts are more audible than the notches.