LM13700 VCA controlled wah questions.

[william]

I have a Vax wah pcb that I am not currently using.   It was bought cheap and the shell used to build an expression pedal.  I'd like to repurpose the pcb to be controlled by an external control voltage.  Looking at similar inquiries a lot has been written about LDRs.  I'd like to avoid using one due to scarcity and price.  I have also thought about a digital pot but I am concerned about zipper  noise. It was mentioned that a lm13700 vca could be used in place of the pot but I haven't found any examples on how to build one.  I'm not looking to build a lm13700filter though.  I still want to use the standard wah circuit.

[PRR]

....wah questions.....lm13700 vca could be used in place of the pot but I haven't found any examples

"Please teach me about OTA all-pass/phase stages"
https://www.diystompboxes.com/smfforum/index.php?topic=92217.0

"Hagerman Technology - Starting work on an OTA-based PHASER. All-pass filter will use LM13700."
https://www.facebook.com/photo/?fbid=1877867395677542&set=a.194018910729074

"Simulation of Voltage Controlled Tunable All Pass Filter using LM13700 OTA"
https://publications.waset.org/8924/pdf

[Fender3D]

Or if you just wanna copy anything, search for DOD Fx 17 schematic...

https://0x4c.files.wordpress.com/2020/05/dod-fx17.png

[Eb7+9]

I still want to use the standard wah circuit.

This is easy, all you need to do is design the ota-vca circuit with a gain spanning 0-to-unity (roughly) and at the same time not load down the 22k collector (output load) resistor too much ...

[puretube]

See page 4, figures 13/14/15 for voltage-controlled resistor:
https://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf