It's relative.
Simple for me would be to buy a THCustom 1-band para-EQ board (fits 1590A) and stick it in with the SOFTii into a 1590B3 or 1590BB2 — highly doubtful I could squeeze it all into a 1590N1. Then I'd have sweepable mids, and it'd be simpler than trying to hack the SOFTii-circuit itself.
Depending on what type of mids circuit you employ, I think the easiest is to tack it on at the very end of the circuit, from the SOFTii's VOL-3 to your mids-in and then mids out to an off-board wired master Volume — though I have seen master-volume feeding an op-amp mids-control then on to bypass-control.
Looking at some other Mids EQ schtuff in other circuits, I'll suggest...
- Simply make R14 a 25k pot? Cowboy suggestion, no idea how that'll sound or how it'll mess up the BASS control, but definitely they'll be interactive.
- As you suggest, replace R23 with a B100k pot. Signal in to lug3 with a 10n cap going to a 10k resistor to ground. Then off the mid-pot's lug1 another 10n cap that leads to that same 10k resistor-to-ground. Mid-pot's lug 2 then goes to R23-out. Does that make sense? I can do up a schematic later, maybe, if I have time tonight.
You could also try sticking this one after the µ-amp of Q5/Q6. Use the C12 pads to do it (retaining C12 as output cap).
- Actually, looking at R21, that's pretty close to what I'm suggesting above, just gotta make R21 Variable, and C13 and C14 feeding that 10k resistor to ground; mids-lug2 feeding the Master-Vol Lug3.
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