[sdiy] MS20 VCF clone troubles

[René Schmitz]

Hi Frederic and List,

Frederic Bourgeais wrote:
> Hello All,
> 
> I build René Schmitz's MS20 vcf which can be found here:
> http://www.uni-bonn.de/~uzs159/rs20.png
> 
> I have some troubles, all components value and connections has been checked
> and rechecked.. Can you help me for debbugging ?
> 
> When no audio input is connected, Cutoff and resonnance pots seems to work.
> I can heard the modulation on background noise, and the filter self
> oscillate if I play with resonnance pot.
> 
> When I input audio signal into O1 inv.input, the signal don't seems to go
> throw O1 output. I can heard again the modulation on background noise, and
> filter can still self oscillate, but he don't process my input source...
> 
> I swapped all 3080 with new ones, but no way.. If I put audio in directly on
> O1 output, or on A3 the filter seems to work..
> 
> Should I hear input signal at O1 output (I think ) ? 3080 are connected to
> -15/+15, but i didn't linked the bias amplifier input (pin 5).. Should I
> ground it ?

Nooo! Don't do that, or you will instantly fry the 3080!

Hmmm.... But there is your problem. The amplifier bias pin would go to 
the 10k resistor that comes from the current sources PNP transistor.

> All the schematics has been build except CV2 input, and "Pitch input".
> what's the difference between externals CV and pitch input ? What kind of
> signal should I put in, ( voltage I believe, but can I leave them unplugged
> ) ? 

"Pitch" would be the main frequency control voltage, either from a 
keyboard, Midi2CV or Sequencer (or whatever). The other CVs can be 
scaled and thus are not necessarily at the 1V/oct setting.

Cheers,
  René

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