Looks pretty straightforward; I haven’t checked the component values in detail, but here’s a few things that caught my eye:
- The LM13700 outputs are connected to +12V.
- The volume potentiometer is pretty low; consider using an A100k instead, to get a higher input impedance.
- Is the 10× input gain intentional? What signals do you plan to feed into this thing?
- (the ideal OTA input is 30-100 mV and here you start by multiplying by 10, only to divide by 40 in the next stage)
- You’re using polarized symbols for the small capacitors (47p, 100n). Not sure you can even find polarized capacitors that small but pretty sure that’s not what you meant to use here.
- Do you need the 22u NP output capacitors? You filter out the DC on the way in (C1) and there’s nothing in the signal path that adds it back (they won’t hurt, though, and may help with CV bleedthru).
- Don’t leave the inputs of the unused amplifiers hanging; see here for the TL074, and for the LM13700 buffers you can just connect the inputs (pins 7, 10) to ground to keep the darlingtons turned off.
- Add decoupling capacitors for all IC supply rails (100n ceramic to GND near the supply pins).
EDIT: A few more observations:
- Add 1k resistors between the − inputs on the LM13700 and ground (same as you already have on the + inputs). This makes things more symmetrical and avoids some input offset issues
- (if you check the datasheet, they often use a potentiometer between the inputs, with the wiper to ground, to make the balance adjustable, but pretty sure that’s overkill here).
- The 15k bias resistors are a bit on the high side; the datasheet recommends 1 mA bias and there’s a diode drop on the way to ground, so the ideal value would be (12−0.7)/1mA = 11.3k. Consider replacing the 15k with 12k (E12) or 11k (E24).
- Unless you plan to primarily use this for square waves (the input and output caps kind of implies you don’t ), consider leaving out the 47p capacitors from all opamps except possibly the output ones, or at least leave them out when you build the first version. You can add them back if you notice more ringing than you’re happy with.