[sdiy] Moog 904A clone finished and working..

[Jean-Pierre Desrochers]

Hi list,
I just finished the assembly of 2 new modules for my dotcom synth.
They are both Moog 904A low pass filter clones.
I built them with the same circuitry as the original.
I'm in the testing phase now.

Reading the Moog archive original specs:

http://www.moogarchives.com/m904a.htm

It says that the 3 freq ranges are as follow:

Cutoffs:
Position 1: 1hz to 5khz
Position 2: 4hz to 20khz
Position 3: 16hz to 80khz

Now when I put the filter to max resonance, no waves at the input
and I move the control voltage knob I read on the scope:

Sine oscillation from:
Position 1: 200hz to 5khz (32mv p.p.)
Position 2: 250hz to 18khz (32mv p.p.)
Position 3: 350hz to 75khz (40mv p.p.)

It shows the real min oscillation freqs for the 3 ranges are
higher than the Moog archive notes specs cutoff..

Also the CV inputs seem to be calculated for 1v/oct right ?
Well 2 things appears:
Applying ADSR 0-5v envelope to this filter
seems to move its cutoff freq not as much I would have expected..
(50k 1% instead of 100k 1% would be better in the CV summing part ???)
and the linearity of 1v/oct is questionable too..
At full resonance, I can tune the 1v/oct CV range trimpot for some of 
the filter span regions
but others are not linear and out of tune..

The LTSpice tests I made with this original schematics
showed these behaviours too at low resonance freq.
The 2 modules I assembled behave exactly the same.

Is there anybody who worked with that filter
that could point out any 'normal' caracteristics
for that specific filter ?

Finaly, is it behaving like it should with some 'normal irregularities' 
?

Thanks

JP