>I have an Octave Cat SRM with the SSM2040 filter chip, and a MOTM 440
>filter. I can vouch for the fact that the 440 filter in the 'normal' mode
>sounds
>(timbral response) just like the SSM2040 LPF in the Cat. As I use it, the
>440
>has a quieter background, although I don't know if that is purely due to
>the
>filters themselves, or possibly the VCA and other signal paths in the Cat.
The original SSM2040 chips are very quiet by themselves, so I guess the
total noise depends on the surrounding circuitry a lot.
>I also notice when the Cat breaks into resonance at high Q, the
>oscillation is
>louder (but the same tone), and is a bit less controllable at the
>breakpoint
>than with the 440. The control aspect could be the age of the synth and
>the
>use of an old slider for resonance on the Cat vs. the better quality pot
>on
>the 440...I don't know. I haven't really explored if the two react the
>same to
>overdrive.
The 2040 has no internal resonance path, so every designer had to add this
externally, and would do it in different ways. The application in the 2040
data sheet uses a simple potentiometer and a BiFet opamp. Once you want to
get voltage controlled resonance, you have plenty of options to implement
that function (basically a specialized VCA).
The MOTM-440 uses a discrete VCA for this function, with as few transistors
in the signal path as possible.
>The 440 has also has a Bass switch you can flip to 'Enhance'. This makes
>the
>low end stay beefed up at higher cutoff and Q (compared to some loss of
>bass
>on the 'normal' 2040). Nice improvement, and you can always switch back to
>the true original response if you want. It's sort of like the loudness
>button
>on a stereo receiver.
This is built upon a special feature of the 2040 (and it's discrete clone's)
topology, which allows rather strong overdrive without too unpleasant
distortion - thus adding the bass frequencies can happen inside the input /
feedback section, rather than after the filter.
>John Blacet did a module back in the '70's called the 'Phase Filter'. I
>believe it configured the SSM2040 chip into different filter modes. I
>wonder if
>the MOTM 440 could be modified to do the same?
The SSM2040 was a very versatile filter building block, that allows for many
different applications. LPF, HPF, BPF, Phaser, you name it. There have been
a lot of publications in Electronotes and elsewhere when this chip was
popular. (And I bet it was popular as long as it has been _available_!)
The MOTM-440 is a complete filter, not a mere copy of a filter _chip_. It's
the equivalent of a SSM2040 _plus_ and all the surrounding components in a
certain (low pass filter) application. So you can't use a MOTM-440 in other
modes. But it'd possible to build all the other SSM2040 applications with
the same discrete technique as the MOTM-440. Hey, I even think you could
modify an existing MOTM-440 to perform HPF or Phasing tasks, but these would
be quite involved modifications.
JH.