To: "Mike Peake", "Synthgods",
From: "Paul Schreiber",
Subject: Re: [AH] MOTM MS-20 Filter Question
Date: Fri, 19 Mar 1999 07:43:26 -0600

This will sound strange (pun alert!) but I suppose you will have to accept
it on faith unless you
buy one.

We built the original MS-20 filter. We reversed the expoxy block (it had 6
transistors in it. Big deal)
and built a perfect, excact clone.

Next, we built basically the same filter topology using OTA gain cells
instead of the 6 transistor gain
cell Korg used.

<<Donning flame suit>>


The MS-20 "squeaky-ness" is due to non-constant Q with input amplitude. We
SPICE'd the hell out of
both circuits. We spent 2 weeks A/B ing them with a real MS-20.


We added HP/LP/Notch response, exponential, temperature compensted CV, and
variable +/- sweep response.
Also added variable input overdrive with LED indicator.

So: why do we think the OTA version is better?

Answer: reduced CV feedthrough. The MS-20 with not-so-matched transistors
has considerable CVF
at bass frequencies. This causes "muddiness" and DC offsets.

NO, I don't think this is "part of the unique MS-20 sound". This again is
due to non-constant Q with input
amplitude. This feature IS in the OTA version, EXACTLY as is the original.

We feel the MOTM-420 capture the MS-20 "whispy grit" with MUCH better bottom

Only 1 way to find out!

No price yet, but I expect it to be around $130 kit $180 assembled.

Also: in the MS-20 you have both a HP and a LP in series. The MOTM-420 is
switchable to HP/LP/Notch.
So, you are correct that to get the MS-20s exacy HP>>LP configuration you
need 2 MOTM-420s
set the same way. BUT: you then get HP>>HP, LP>>LP, LP>>HP, Notch>>LP, etc.

Paul Schreiber
Synthesis Technology

>Paul Schreiber wrote:
>>MOTM-420 MS-20 filter (with added Notch response) will start shipping in
>>about 3 weeks.
>Is this a genuine, part-for-part, lifted from the schematic, genuine Korg
>filter chip using copy (excepting the notch mixing components)? I'd buy
>two if so. Please fill us in.