[sdiy] Moog 904A clone CV inputs gain confusion... test 2..

[Jean-Pierre Desrochers]

Sorry, my first email was in HTML with too much spaces between lines..
This one will be easier to read..


Hi 'Daytona' and all members,

Replying to your last email about my Moog 904A clone
freq range questions on the DIY list (installed in my Dotcom system),
I need some clarifications about the original 904A CV input gains..

You stated:
> The 0-5v envelopes from
> Dotcom are less than 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.

Well I'm a bit confused here..
Looking at Moog's archives ADSR module 911
http://www.moogarchives.com/m911.htm

They say the peak DC output on Sustain level Esus is 5.5volts +/-10%
(Not that far from Dotcom 0-5vdc max)
Starting from there I heard a lot of 904A VCF
being modulated by their Moog 911 ADSR with quite
satisfactory freq headroom.
So I should be able enough to modulate the VCF cutoff freq
when applying my Dotcom 0-5vdc ADSR voltage
to one of the VCFs CV input right ?
Well that's not what is happening.

Looking at the original 904A schematic:
http://www.freeinfosociety.com/electronics/schemview.php?id=943
The overall VCF spectrum can be spreaded using
the front panel pot called FIXED CONTOL VOLTAGE
from 0v (filter wide open) to -6vdc (filter shut off). OK.
The pot's wiper goes to a 51k resistor (R12).
But all the other CV input resistors (R1,2,3) are 100k values...
To me, that means that if I shut the front panel pot off
(-6vdc) and I try to 'full open' the VCF from one of the CV inputs
I'll need to feed 2 X +6vdc (+12vdc) at one of the 100k input resistor
to get the same CV gain right ?
Well from my knowledge Moog's CV voltages are not +12vdc.
How did the Moog modulars use their 5.5v CV voltages in this case ?

So far I built my 904A clone with all the same parts
including the 100k CV resistors.
The results: The front panel works very well has it should,
but there is a lack of gain in my modulations.
That's understandable from what I stated above.

So... to get the CV levels working, 3 solutions:
- Use external CV summer modules with gain.
All my external CV summer should have more than
'one' has a gain they have now (maybe a gain of 3 instead)
so the 904A VCF CV inputs will be driven with more current
for a better modulation.

- VCF's CV input resistors R1,2,3 could be lowered in value (40k..51k)
so a 0-5vdc swing will more 'drive' the 904A CV inputs.
But doing so I'll loose the 1v/oct range adjusted with trimmer R8.
So maybe I could only change resistors R2,3 for lower values
and keep R1 at 100k for 1v/oct purpose..

- Could always use multiple CV inputs at the same time for VCF 
modulation.

These solutions are not pleasant ones...

In fact what is it going on these levels
in the real Moog modules world ?

J-Pierre

****************************************************************************************

> 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/904afeed
> 
> 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/setup201
> 
> 
>> 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