[sdiy] VCA circuits?

[Tim Stinchcombe]

Hi Troy,

> The most basic thing I need to figure out is what the CV 
> voltage range into the first transistor (Q7) should be.  The 
> DAC puts out 0-5V, but there is a 
> trimming circuit before the input to the first transistor.  I 
> am only seeing 
> a few hundred millivolts of swing at the input of this 
> transistor, and that 
> does not seem right.

My quick calcs, ignoring effect of R75 voltage:

Iabc = Iref * e^(VCA_CV/(k*0.025))

where Iabc is current out of Q6 feeding the LM13700; Iref = 36uA = 12/330k,
current through R77; k is the scaling factor of R142+R71 to R73, say about
30= 150/4.7; e is base of natural logs, 2.718..; 0.025V = 'thermal voltage'.

Thus if VCA_CV is approx 3V, Iabc is about 2mA, about the max the LM13700
will take (I think, I've not checked datasheet). So max at Q7 base around
100mV is about right. The voltage at R75 will also be scaled (by about a
twentieth) and will subtract out at the base, so the allowable VCA_CV swing
will be greater (and in any case at least these calcs seem ball-park).

> circuit.  Does anyone know of any resources that might 
> explain this particular circuit?  Any good descriptions of 
> some basic VCAs out there?

Can't help there at the moment: if you want a more detailed breakdown it'll
have to wait until tomorrow, it's getting late (for me!) in the UK.

And again, off the top of my head without checking the datasheet, since
there is diode biasing current feeding the LM13700 (R82), there should be no
linearity problems with the inputs (and I add, 'I think'!).

Since you have R76 there, you can measure the voltage drop across it and
hence calculate the current through it, for the complete range of VCA_CV in
- if this doesn't go above 2mA (or whatever that figure is...), then at
least the expo converter part of the circuit is doing its job!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk