[sdiy] diode ladder filter design

[Magnus Danielson]

From: harrybissell <harrybissell at prodigy.net>
Subject: Re: [sdiy] diode ladder filter design
Date: Sun, 30 Mar 2003 18:22:24 -0500

Harry,

> Arrh ????
> 
> Steiner-Parker is a Sallen Key filter tuned with diodes as the variable
> impedance means... but it is not a ladder filter.
> The Korg MS-50 is a similar design using diode "rings" as the variable
> impedance means... but also a S-K not a ladder.
> 
> The EMS is a "ladder" filter... as is the TB-303 filter.
> 
> I'd say to qualify as a ladder filter, the unit has to have two
> sets of parallel diodes (or transistors)... with the capacitors
> as the horizontal 'rungs'.
> 
> note: This is NOT to say that the Steiner-Parker is not a hip filter,
> just that it is not a  ladder.

I could not agree more!

The Steiner-Parker doesn't really fall into the Ladder family. However, I do not
really agree with how the distinction is done.

The Moog Ladder, EMS Ladder and many followers all feed the signal diffrentially
into the the ladder. You could say that the ladder has a diffrential mode
signal and common mode modulation. The diodes in this type of ladder all
operate in the same direction, but each leg of the ladder operates on either of
the polarities.

If you look at the Steiner-Parker schematic:

http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html

you see that this is actually not how it operates. Instead is the signal
inserted in single-mode and with the diodes operating in different directions
of the signal. You realize this even more clearly when you see the bandpass
input which is inserted into the two diode-chains using capacitors.

However, just because it's not a ladder-filter doesn't make it un-hip, it may
be hip, usefull and lovely, but then on it's own merrits, and that my friends
isn't bad.

I have however not had the chance to fool around with one of these...

PS. Harry, you're out of the woods on the "old fart grunting displeasing" stuff
for this time, since I agree with you on this one... ;O)

Cheers,
Magnus