[sdiy] LM13700/LM13600-based filters

[David G. Dixon]

> There are some major issues. One is that if you use
> the LM13700, the bias of the buffers is preset and
> may limit performance. In the LM13600 the bias is variable with
> the Gm input, and might feed through to the output.
> 
> In all cases, there will be an offset voltage because of the
> voltage drop of the darlington pair. This may not be a problem
> (especailly for YOU, JH... in your allpass stages :^)
> but if you run a lowpass filter into total cutoff, the output
> offset volatge will drift and it will thump when you come out
> of cutoff. A fatal flaw if you are going that low (I do) imho...

This was why I used opamps in my 13700-based 4P LPF design.  I couldn't
figure out how to un-bias the output of the darlingtons.  Indeed, that's why
I typically ignore those buffers in most designs.