Cool project!
I'm going to try to go as quiet as possible this time, using metal film resistors and good opamps and such.
How noisy was your last mixer (like, if all the volumes are turned down is the hiss unacceptable?)?
Metal film is good. Low noise opamps are good.
What are model numbers for good/quiet/headroom dual and quad opamps?
I usually just use the TL07* family for my pedals, cheap and good. If those weren't quiet enough, I think the design would need to be changed to a more sophisticated one, rather than turning to audiophile components.
I went with WIMA caps for the input caps before. Anything better there?
Whatever film caps are around will be fine. Not worth getting exotic.
Also, the following changes in values were suggested to me on another forum (along with including bypass caps at the power input and opamps).
R5-R8 = 22k
I don't like getting lower than 47K, but you can try it and see if the channel volumes get weird and interactive. If you must get lower, than use stiffer pots, like 5K.
R9-R10 = 10k
OK. Those can be whatever. 47K'd be fine. 470K is high, but fine. Remember the 1 uF cap is bypassing the voltage the opamp sees, so resistor noise is no issue.
D1 = 1n4004
Unnecessary. 1n914's are rated to 100mA. If you wanted to do something fancy there, use a Schottky diode (like the 1N5818). They have a lower voltage drop.
100uF at power input
That is the best suggestion your other forum made. Put it downstream of the diode.
.1uF across opamp power
That's a good suggestion too, for noise. If you bypass the opamps individually with .1 caps the idea is to put it physically as close to the pins as you can.
If this design isn't cutting it for you performance wise, I could imagine a more hi fi version with more parts, with discrete FET buffers, variable gain with an inverting opamp stage on each channel (rather than attenuation followed by fixed gain here), and an inverting, unity gain summing stage. Possibly a boosted power supply for more headroom.