[sdiy] Anyone built the ELEKTOR VOCODER?

[jh.]

> Before anybody tries to build my design, let me mention this caveat! The
> circuit looks good on paper, but when I breadboarded a couple of stages
> I found the filters and WAY to sensitive to component tolerance! All
> those calculations I did to find the nearest 5% resistance just WILL NOT
> work. The multiple feedback bandpass filter design requires around 0.1%
> tolerance. All the parameters are interactive - Fc, Q, and gain, so if
> you want to use trimpots its a real pain to trim.
> I picked up Don Lancaster's Active Filter Cookbook to delve into the
> theory more, and came to the conclusion that I needed state variable
> filters.
> These can be tuned nicely, but the parts count increases dramatically (3
> opamps per filter stage). I was even looking into doing third order
> filters (3 cascaded state variables) and whereas it looks like it would
> work quite nicely (I've done some computer simulations), we're talking
> about 100's of opamps here.

Very, very true.
It's easy to build a small vocoder, but the difficulties are dramatically
increasing with size. There's a reason that the classic big vocoders were
so much more expensive than the simple ones.

JH.