[sdiy] LM13700 VCA circuit

[Ryan Williams]

hi all,

A while back I thought of a VCA circuit that uses the LM13700. both 
sections are used and the first is in the feedback loop of an opamp. The 
idea is to compensate for the non-linearity. the output of the opamp 
inputs into the second OTA also. I noticed whole lot of high frequency 
oscillation, most of this was fixed with a 100pF cap also in the 
feedback loop.

so my question is: Is there any reason besides the high frequency 
oscillation that this circuit won't work as well as my calculations say 
it will? (if I'm doing the algebra correctly, it should cancel out the 
tanh and Vt from the OTA equations). It seems to sound ok but my ears 
aren't great anyway.

the circuit that I tested can be seen here:
http://www.sdiy.org/destrukto/schematics/linearized-lm13700-vca.pdf
the only difference is that I left off the second opamp (TL072B) and 
used the LM13700 buffer instead. this was because I don't care about DC 
offset in my application. I left off that offset trimmer also. those 
diodes that are just to protect the OTA from the opamp in case the opamp 
tries to swing to a rail (too much voltage at the input), that could 
also be done by increasing the input and output resistors.

credits: that current source is from Thomas Henry's CA3080 book.

-Ryan