My first VCF

[Brian Towles]

Paul Perry wrote:

> At 10:18 PM 14/05/98 -0400, you wrote:
>
>   Would a exponential cutoff control be more useful?
> ......yes.....
>
> Brian, with my LM13600 filters I have to use a fet follower
> instead of the LM13600 buffer or the low freq is f*cked..
> do you have this trouble with the LM13700?

What's the problem with the low-freq? Distortion or cutoff? I've haven't
built anything, all my numbers are from HSPICE simulations using
National's LM13700 model.  The standard OTA configurations I've seen all
have something like this:

   Out
   >--------o---------- Buffer
            |
            |
            Z
            |
            |
            v gnd

Basically, the output current of the OTA is forced through one passive
component ( either a cap or resistor ) and the voltage across that
component is buffered.  One problem could be if a cap was used as the
"charging" element:  at low-freqs its impedance could get close to the
input impedance of the darlington pair ( I would estimate 10M ).  This
would explain why a FET follower worked better.  I'm using tiny 300pF
caps and these affects are not visible in the simulation.  What values
are you using?  From my calculations, values more like 100nF would give
audio freq response -- if this is really the problem.  Maybe you could
put a _big_ resistor in parallel with the cap to limit the equiv
impedance of the pair at low-freqs ( like 2-5 MEG? ).  There's got to be
a good fix -- I'd hate not to be able to use the supplied buffers of the
LM13700.

Other ideas?

Brian Towles