MOTM

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. You 
wouldn't really notice too  much... until you listen to them side by side. 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 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.    
 
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?
 
-Karl.
 
In a message dated 6/22/2007 1:08:33 A.M. Eastern Daylight Time,  
djhohum@... writes:

>  This is hands down, my favorite filter. I wonder if the MOTM 440
>  really sounds like the real thing?

I don't know, it would be  interesting to hear from people who have
tried either the MOTM design and  or JHs design next to the real thing.
It's certainly a filter worth  building and if the discrete designs are
good, or perhaps even, perfect to  human ears, then that makes sourcing
a real 2040 less important for diy  efforts.







************************************** See what's free at http://www.aol.com.

>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.