MOTM

Folks,

I *finally* finished my 940/904A comparison.  I won't give you any excuses 
for the time it took.  The material isn't very exciting, in fact it is down 
right clinical. Please go to:

http://www.alt-mode.com/~motm

There are 4 files, 2 are .wav, and 2 are .mp3 (128K encoded).  One test has 
the 940 on the left and the 904A Right (if you don't have your left and 
right straight, it will be hard to tell which filter is which).  The second 
test is the same sequence but in mono alternating between the 940 and the 
904A (940 first).  The sequence is repeated with a low resonance, medium 
resonance, and high (but not oscillating) resonance.  I tried to tune the 
filters to each other as closely as possible by getting the filters to 
oscillate and using a tuner to match them.

The test setup was a single MOTM-300 SAW out into an MOTM-830 to attenuate 
the signal.  The 830 was feed into both filters and then into two Moog VCAs 
driven by Moog EGs.  The 904A was run in the middle response mode (2). An 
Encore Expressionist provided MIDI->CV conversion and CC#7 was used to 
sweep the filters after going through a MOTM 820 to smooth out the steps.

The output of the VCAs went into an Avalon AD2022 preamp, mostly to even 
out the levels, and then into an Apogee Rosetta converter into a MOTU 2408 
into Logic Audio.

Interesting observations:
- The filters sound pretty close to each other in this test
- I didn't quite get the resonance even between the two filters on the 
medium resonance test
- The resonance on the 940 is very touchy, going into full oscillation by 
about the 12 o'clock position
- The phase from the two filters is reversed - causing some phasing in the 
stereo test
- The output of the Moog filter was actually louder than the 940!
- The DC offsets are interesting between the different filters and tests...

In this test, the filters sound *very* similar.  In playing with the 940, I 
found it had a neat character but didn't quite feel the same as the 
904A.  It wasn't bad, just different.  I think a lot has to do with the 
range of the knobs.  There is a lot of range in a short span on the 940, 
particularly in the resonance!

Fire any questions you may have.  I thought about expanding this test to 
other filters but decided I didn't want to spend the time.  I could be 
persuaded if folks think there is a benefit.  I'm not sure if this simple 
sweep test really gets much of the character of a filter.  Lots of 
character comes from how it reacts to an EG or fast changing CV.  If 
someone has an idea for a good filter test, I might be interested in giving 
it a go.

	Eric