[sdiy] Simple VCA schematic, anyone?

[René Schmitz]

Andre Majorel wrote:

> I put the resistor in front of the inverting input so as to set
> the gain to 100. I don't know how to compute the gain of an
> inverting amplifier when there is no resistor between the source
> and the inverting input. 

The "gain" is a current to voltage relation if you have no input 
resistor. In this case Uout = -Iin*Rfeedback.

> I'll experiment. In the PS-3200, they
> appear to use 10 k 
> (I think "R21~32" in the schematic should be
> "R25~32"). 

Rather R29~32. The values for R25 to 28 are in the table bottom right.
So the feedback resistor is something between 68k and 120k there, and 
your 100k seems to be a good starting point.

> Oh, one more stupid question : the BC550 data sheet gives the
> following maximum values :
> 
>   V_CBO : 50 V
>   V_CEO : 45 V
>   V_EBO :  5 V
> 
> If the control voltage drops down to -15 V, will it damage the
> transistor ?

Well the current would be limited by that 1Meg resistor anyway, so no 
damage can occur. What just happens when you back-bias that junction is 
that the BE-Diode gets into avalanche, i.e. it acts as a Z-diode.

Cheers,
  René

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