[sdiy] Ladder filter analysis

[Colin f]

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Michael E Caloroso
> Sent: 08 July 2014 18:35
> Subject: Re: [sdiy] Ladder filter analysis
> 
> If you look at each stage of the ladder filter, it has 
> differential inputs *and* differential outputs.  The simple 
> cascaded architecture contributes to the common mode 
> mechanism.  Combined with control current and audio voltages, 
> those are the keys to the performance of the design.

Another key part of the ladder design is the quirk of any low frequency/DC
component in the audio input adding to the control current that sets the
dynamic resistance of the transistors, thereby modulating the cutoff.
It's quite an audible effect on the minimoog at high resonance, and an OTA
filter doesn't do it.
Neither do a fair number of 'virtual' ladders, it's the first thing I test
for.

Cheers,
Colin f