[sdiy] Re: CV wah-wah with gyrator <- with corrected link

[René Schmitz]

Hi Harry, Jonathan, Aaron and all,

Harry Bissell Jr wrote:
> The resistor in series with the LED seems improbably
> high. Almost a bet that it will not work with the
> next vactrol you try.

Well if I do the math I get 125 *Ohms* not kOhms. But whats
3 orders of magnitude amongst friends. :-P

> I'd look at better ways to control the LED...

Yes. It won't start to do anything below 1.5V...

> look at some Buchla examples... The Lowpass filter /
> gate is a good example.  The Mutron III envelope
> filter is another.

I have two basic ways of doing that, for linear control: with an 
inverting opamp, have the LED sitting in the feedback path, and the R to 
the virtual GND. This gives more accuracy and "definition" at the lower 
end. But here the driving voltage source must be able to supply the 
current for the LED. (Might need a buffer.)

Or I often used the ARP-style NPN-PNP expo convertor like on the 
VCO4069, letting the output current drive the LEDs. This is for 
exponential control of course.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159