[sdiy] Circuit queries

[Yves Usson]

Hi Pat
Considering the VCA schematic, it expects positive CV input thus
there is a negative control at the bas of the transistor !

I built 5 VCAs based on this schematic (one built with BC109 and the 
other
built with BC547) and they are all working well. Normally, the gain of
this circuit is something like 1 for maximum positive CV. The gain 
dynamics
is something like 60dB.

If you wish, I will make some measurements at various points of my 
working
modules to help you troubleshoot your circuit.
But I wont be able to do it before Friday !

Regards

Yves

>
> And considering the following VCA schematic;
> http://www-timc.imag.fr/Yves.Usson/personnel/SDIY/images/VCA-US1.pdf
>
> Can anyone help me understand what I should be seeing at the CV summing
> stage.  I see it's inverting, so theoretically the output should be
> negative, or is this amp expecting negative control voltages?  I do 
> get a
> negative output, but both inputs are at ground.  I've check the input 
> and
> feedback resistors, and the soldering and continuity are solid, but 
> still no
> voltage is present.  The output only MINUTELY changes with change of 
> the
> input.  Is this anything like what should be happening?  I get no CV 
> control
> of gain by inserting voltage at the CV input.  One put changes the 
> overall
> gain a bit, but the other seems to do nothing at all.
>
> Thanks if anyone can help out!
> Pat
>
>
-------------------------------------------------------------
Yves Usson

Lab. TIMC UMR5525 CNRS
Institut d'Ingéniérie et de l'Information de Santé (IN3S)
Faculté de Médecine
Domaine de la Merci
38706 La Tronche cedex

email : yves.usson at imag.fr, yves.usson at ujf-grenoble.fr

tel.  (+33) (0)456 520 074
fax.  (+33) (0)456 520 022
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