Paul posted this to AH, has some good info on the Korg filter - not all of
us are on AH, right?
Paul sez:
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 epoxy block (it had 6
transistors in it. Big deal)
and built a perfect, exact 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>>
WE LIKE THE OTA VERSION BETTER.
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 LIKE THE OTA VERSION BETTER.
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
end.
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.
>
>Easier,
>
>Mike
>
Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.
daveb@...