[sdiy] Multiplexed Expo Converter?

[David G. Dixon]

I've built expo converters to run 4-pole filters, and simply used (N + 1)
transistors for N currents out.  I don't understand why one would have to
multiplex an EC.  Why not just take multiple currents out?  Of course, I
have no idea whether this method could give multiple currents which are
exact enough replicas of each other to drive VCOs at exactly the same
frequency.  Do well-matched transistors (like 5 on a chip) in a chain with
common bases and emitters give exactly the same currents?

> I heard that Korg used to multiplex the Expo Converters in their
> polysynths. I think it's a pretty good idea to keep things in sync, so
> that even if the EC floats, every part floats in the same way at the
> same time. This is also good because it simplifies the design.
> 
> There are several questions to this, though:
> 1. what could the performance be, at best, in samples per sec?
> 2. when are EC's used with input signals that are audio-rate? and can
> this stuff be re-designed so that it doesn't require an EC at all?
> We're talking about a closed-box synth, not something that is fully
> modular and requires the signals to be patchable out in some sort of
> standard format.