What gives the tri-vibe its bell like tone?

[Ksander]

Sure!

The pot is at 50%, and the voltage out of the LFO is 4.16v

This is the circuit (hope it is intelligible

[Rob Strand]

Sure!

The pot is at 50%, and the voltage out of the LFO is 4.16v

This is the circuit (hope it is intelligible

OK cool thanks.  I'll check it a different way.

When you built the filter circuit did you put a buffer at the input?   If you drive the guitar into the filter it's going to mess up the filtering for sure.

[Ksander]

yes, it is in the top left; S_{buf} is the output.

I saw I forgot to draw that the 'LFO' inputs to the LM13700 go in via 10k resistors

[Rob Strand]

yes, it is in the top left; S_{buf} is the output.

I saw I forgot to draw that the 'LFO' inputs to the LM13700 go in via 10k resistors

I checked my previous simulation and it seemed OK. However, I noticed some value differences between your new circuit and the runoff groove schematic:
- 5n6 instead of the 4n7 cap on the treble boost.
  That's going to make a subtle difference to the treble boost.
- 2n2 instead of 1n5 on the second OTA.
  That drops the notch frequency from 600Hz to 500Hz.
- There's also a small amount HF roll-off from the 330pF cap.

Another significant difference which will affect A/B comparisons is my filter was set to 0dB at low frequencies.   However, your panning circuit has about 6.8dB boost at low frequencies.    The gain difference doesn't change tone but it will make A/B comparisons difficult.    There will be a "correct" gain which sounds equal volume with the dry signal.  I really suspect that gain difference is making the biggest difference to your previous A/B comparison.

The notch depth on the OTA circuit is quite deep, so I'm not expecting that to be a difference.   Pot settings are never accurate but it should be close with a linear pot set to the middle.

Anyway,  if I incorporate the value changes then I get the following filters (~500Hz notch):



The reason there's four circuits is I've let you choose if you want to use new cap values or keep some of the old values.   I've also given two new circuits which have the 6dB gain to match the existing circuit.   That will help with the A/B comparison.

EDIT: just to be clear the output is take at the output of the filter circuit.   You don't need to pass it through the mixer.  Passing through the mixer would make the filters different from the original circuit.

[Ksander]

 

[Ksander]

I just built the second to the left circuit (had those values on hand), and compared the sound of this circuit to the OTA circuit. It sounds exactly the same to me, so seems like you absolutely nailed it! Thank you!

[Rob Strand]

I just built the second to the left circuit (had those values on hand), and compared the sound of this circuit to the OTA circuit. It sounds exactly the same to me, so seems like you absolutely nailed it! Thank you

Very cool.   Good thing there's no obscure differences left to explain.

A practical single supply circuit will need a cap between the opamp and filter to stop DC getting out.