[sdiy] TB303 / Polysix VCO current mirror thingy.

[Gordonjcp]

If you look at the VCO in a TB303 or in a Korg Polysix they appear to
use a kind of current mirror arrangement with two NPN trannies, and then
a PNP tranny with its base wired to the collector of one and its
collector to the collector of the mirror through a diode.

There's a capacitor - to the "negative" rail in the 303 (really ground
but that's referenced to the +5.33V suppy) or to the "positive" rail in
the Polysix - which charges up and is discharged through the expo
converter.

At some point for a reason I can't entirely figure out, as the capacitor
discharges something in the current mirror conducts and it charges again
briefly, but I don't get why.

Can anyone shed any light?

-- 
Gordonjcp