Questions about a compressor/eq for upright bass

[Josh?]

Hi everyone!

I'm currently working on an always-on buffer/eq/compressor to use with an upright bass and some really cheap piezos. Here's a schematic of what I have so far, which I'm pretty happy with the sound of, but I'd like to learn how to improve it if anyone's willing to help me:

https://imgur.com/a/aCb1UPd

My signal chain goes:                                 |----------------------------------envelope generator---|
                                                                |                                                                            |
piezo---variable 2nd order HPF---buffered split---fixed frequency gyrator EQ (for low-mids)---compressor---amp

...with pieces cobbled together from all over the place. While I did some of the math for the HPF and EQ stages, nothing else here is really "mine" per se.

I have some questions about the envelope generator portion in particular: I decided to add a MOSFET as a buffered splitter for the envelope generator, does this actually help with anything? If so/if not, why? Second, especially on lower notes, I get a smooth decay until a point where the compressor turns off rather sharply. Why is this?

Also, If you have any other critiques about any part of the circuit for me, I'd love to hear them!

P.S. I just realized I should've added a 100k resistor to 4.5v on my schematic right after the 0.1 uF cap on the source of the MOSFET, sorry about that!

[PRR]

Why is the MOSFET there at all? and...

This is a feed-forward limiter. FF is very difficult to work predictably with a sloppy loose-spec opto.

[Rob Strand]

does this actually help with anything? If so/if not, why?

The filter opamp is already producing a low output impedance, so you don't need to buffer.
The MOSFET is actually a worst buffer than the opamp.   Also, it will add noise.

Also, If you have any other critiques about any part of the circuit for me, I'd love to hear them!

The "threshold" for compressor is after the gain stage so you will find there' no "knee" in the compressor characteristic.   There's a few feed forward designs around maybe check out how the side-chain is organized.
Two come to mind, but I can't think of the names.  Perhaps one by Johnny Reckless.

If you tuned the EQ by ear then leave-it.  If you did it via calculations you might find the response is off from what you expect since the feedback resistors are large and the gyrator resistors to ground are small.  Also, there's no need to emulate a fixed pot on the boost cut you can design either boost or cut without having two "pot" resistors.

EDIT:
I left the schematic window open in my browser, I went to close it and a few other things caught my eye,

- The 100nF tone cap will connect directly across the last opamp in the min position.  Better to add a 1k between the series 100nF from the opamp and the tone pot.   This will prevent capacitive loading of the opamp.

-  You might find 4.7uF is too small due to the current required driving the opto.   The opto will have it's own time-constants but it might not be enough to prevent modulation.

- The way the comp control is wired won't work with the feed-forward side chain.   You have to make it a feed-back compressor or make the comp control control the gain of the side-chain.