Tau Systems 1010 Filter

[Martin Czech]

> 
> I am curious about all of this because I would very much like to know how to
> modify a standard Moog ladder to make it act like a Highpass filter.  Does
> anyone know how to do this? (and hopefully without rebuilding the filter with
> PNP transistors!)  As some of you may already know, the Moog modular system
> offered a Highpass module, but it does not appear to have differential ladder
> filter topology.  Rather, it looks more like four simple 6dB highpass filters
> in series.  Also, unlike their lowpass module, it did not have a resonance
> loop. (why not??)

Well, the Moog cascade is also four simple lowpass filters in series...
If you do an lowpass-highpass transformation on the circuit, you'll
get the Mogg Highpass. It is not really differential but some kind of
symmetry property has been preserved. But one needs matched pnp
AND npn.
> 
> The classic Minimoog lowpass ladder filter is a sound we all know and love, so
> it stands to reason that a highpass version of this filter would be equally
> elegant and musical.  It's curious that there aren't any examples of this
> already out there, at least to my knowledge.  Is there a technical reason why
> it can't be done?  I wonder if there is a way to somehow implement Tau's
> Highpass input arrangement into an existing Moog ladder.  Any ideas?
> 

The Moog cascade adds a lot of distortion, but this is not so relevant in
a lowpass application. The additional sidebands may get dominant in a 
highpass filter application, however. Perhaps this is not nice at all,
more aggressive.

Feedback: If you have audio hp characteristic, what will happen if you add feedback?
Depending on parasitics there could be trouble. HP filters don't offer a
good sine wave, when resonance is max.

m.c.