[sdiy] Newbie question regarding the 4069 VCO

[René Schmitz]

Hi Harry and all,

harrybissell wrote:
> One caveat of the summing design... (which I mentioned to James offlist) ...
> is that it is
> "almost" a current source (as Rene says) .   If you change the input impedance
> of one of the nodes (such as by changing it from an open circuit, to lets say
> an opamp feeding from an LFO with zero volts...
> you will change the pitch noticably !   In a modular this might not be a
> problem, in a stand-alone
> synth this must be designed around.   If you hold the modulation inputs at all
> times (with voltage
> follower opamp outputs) you will not have an issue.

Thats true, and the reason why there are switching jacks shown on the 
layout. The unused input is switched to GND to keep its input impedance 
more or less constant. (One of the design goals was "no opamps"...)

Cheers,
  René

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