[sdiy] FET VCA in Music Easel timbre generator

[Dave Manley]

Aaron Lanterman wrote:
>
> 2) We did the experiments of directly applying a voltage to the gate 
> of the JFET since when Luke built the rest of the CV circuitry, 
> consising of the 100K pot, R24, Q5, and R23, he couldn't get any 
> change as he changed the CV or the pot. There was probably a wiring 
> problem somewhere, or an understanding problem... in any case we 
> couldn't really debug it effective since we weren't sure how it worked 
> and what it was supposed to be doing. So... uhm... what ia all that 
> "Timber In" circuitry supposed to be doing? Why the BJT?
>
> 3) We assume that the base of Q5 is supposed to be grounded, but it's 
> drawn
> kind of oddly. Is our assumption right?
My 2 cents -

Yes, the base of Q5 is grounded.  So the question is what is Ic on Q5 as 
Timbre In varies, or what is the relationship between Vbe and Ic for a BJT?

Running a spice simulation (using a 3906 for Q5) I see as 'Timbre In' 
increases from 0V to 1V the voltage at the collector increases 
exponentially from around -6.5V to ~0.5V. It's a poor man's expo 
circuit.  I didn't have time to simulate the FET/VCA.

Now,  if you look at page 2080_14A_200, pins 1 and 2 of board 9  are 
labeled 't.p.1' and 't.p.2' (test point 1 and 2?) and look like they are 
floating.  The text 'Timbre In from pin 6 board 7' appears wrong, and I 
don't see a pin 6 on the schematic for board 7.  But...on 2080_14A_200 
the block labeled board 7 does have a pin 6 which is labeled 'TIMBRE'.  
So what is really connected?  Hard to say. It looks like 'Timbre IN' is 
a control voltage that is expo converted and then scaled between 0 and 
-6.5V by the 100K pot.  For lab debugging ignore 'Timbre IN' and just 
hook up the 100K pot and 130K to -15V to give a 0 to -6.5V manually 
controlled CV and see what happens.  Later try controlling 'Timber In' 
with an ADSR or VCO (scaled down appropriately).  Also, there's nothing 
magic about this VCA, a more modern VCA could be used in its place.

HTH,
Dave