[sdiy] My Music Easel timbre PCB works! (mostly) - and a FET VCA question...

[Aaron Lanterman]

Well, after fixing a wrong pot value and a wrong resistor value a had  
on the schematic, and adding a capacitor I had forgotten about, it  
works. ;)

You might recall me talking about accidentally swapping the inputs on  
some op amps - this is one of the PCBs I'd done that on. I used Ian's  
(some others suggested this too) trick of bending up the offending  
pins and swapping them using wire wrap. This was worth it, since it  
let me find these other errors.

Now I need to do a revision of the PCB, and hopefully not introduce  
new errors in the process. ;)

------

Anyway, my main reason for writing is the control range on my VCA  
doesn't seem quite right. Check out the schematic here:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_9_200.jpg

The FET Q6 is used as a variable resistor.

Even after tweaking the 100K trimpot (to the lower left of Q6 on the  
schematic), if I put in a 4v p.p. sinewave, I don't seem to get a full  
control range. When I turn the timbre offset pot (not on this  
schematic) all the way down, I still get enough gain that it activates  
the first first and a little bit of the second fold in the  
nonlinearity, so I'm getting too much gain on the front end. If I put  
in a lower amplitude sine wave, then I don't get the full range at the  
top end (i.e. I can't hit all the teeth in the nonlinearity.)

I'm thinking it might have to do with my choice of FET. The original  
specified is a 2N4341, which appears to be way out of production. I  
used a J201 since I happen to have a little drawer full of them.

Any suggestions? I'm not sure what data sheet specs are important to  
try to match up.

Another thing I thought to try was to increase R21, but I want to see  
if maybe I can pick a better FET first.

- Aaron

P.S. I used 10 microfarad instead of 15 microfarad for C2, but I can't  
imagine that would make much difference...

P.P.S. I also used a 2N3906 instead of a 2N4248 for Q5... not sure if  
that would make a significant difference either.