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

[Terry]

Those 904A responses sound about right to me. The 0-5v envelopes from  
Dotcom are less that the 0-6.3v swing the R.A.Moog 911 envelopes have and  
was designed to use. Could always use multiple inputs from the EG or use  
an amplifier to get up to R.A.Moog specs.
  The frequency range is just that and not the oscillation range. My stock  
1969 904A did not even self oscillate by design. It was a few months  
latter that R.A.Moog made the revision to decrease the feedback resistance  
to allow self oscillation in the 904A. This required a mod to the stock  
1.8K feedback resistor by adding a parallel 2.2K resistor piggy-backed.  
This gave a summed resistance of about 990 ohms (1K) and would begin to  
oscillate around the 7.5 mark on the regeneration dial.
Pic of the mod before soldering here:
http://mysite.verizon.net/vze6s1hi/sitebuildercontent/sitebuilderpictures/904afeedbackresistor.jpg

The lowest sine oscillation on this now modified 904A is around 140Hz. The  
response is not that linear but I can get about two octaves in tune.  
Requires a patch with adjustable intonation to vary the v/oct.response  
input. It really was not initally designed to be a sound source, so... The  
904B is the same way and when forced to oscillate is not very linear. But  
an octave and maybe two can be coaxed out of it.
The lowest range setting on the 904A is the interesting one. The filter in  
this setting will not go much above a certain point, no matter how much  
voltage is applied to the CV inputs. Can hear it using a white noise  
source. There is a soft wall the 904A hits and nothing above that  
frequency passes.

The R.A.Moog I have here will soon turn 45 years old with a build date of  
Aug.11,1969. Still plays fantastic, and I play it almost every day! Just  
completed a few DIY modules for the Moog.
Pics here:
http://i58.tinypic.com/20a6ziv.jpg
http://mysite.verizon.net/vze6s1hi/sitebuildercontent/sitebuilderpictures/setup2013.jpg


> 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
>
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