[sdiy] MS20 VCF clone troubles

[Scott Bernardi]

You say you didn't link the Bias input pin five of the CA3080 - that is 
probably your problem. If you look at the schematic, both of the CA3080 
pin 5's connect with 10k resistors to the pnp transistor that is the 
output of the exponential current source.
The output current of a CA3080 is multiplied by the Bias current. If you 
don't have any Bias current, you are in effect multiplying by 0 so you 
won't have any output current.  Do NOT ground pin 5 - you will fry the 
CA3080 for sure.  Pin 5 expects a current flowing into it, and is a 
couple of diode drops above the negative supply. You can connect it to a 
more positive voltage (such as ground) through a resistor to create a 
current, but you will usually see it connected to a current source (like 
the pnp transistor in this circuit).  Hook it up as shown.


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 ?
>
>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
>) ? 
>
>I didn't use any CV source or pitch input, because 47k lin pot connected to
>+15/-15 seems control the input voltage. Does this circuit can work like
>that or I missed something ?
>
>Thanks
>
>Fred 
>
>
>
>  
>

-- 
Scott Bernardi
sbernardi at comcast.net