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Moog 904A clone CV inputs gain confusion in Dotcom system...
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jpdesroc



Joined: Dec 05, 2007
Posts: 29
Location: Canada

PostPosted: Thu Aug 07, 2014 9:47 am    Post subject: Moog 904A clone CV inputs gain confusion in Dotcom system... Reply with quote  Mark this post and the followings unread

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.

Applying ADSR 0-5v envelope to one of its CV input
seems to move its cutoff freq not as much I would have expected..
(50k 1% summing resistors instead of 100k 1% would be better in the CV summing part ???)

So if you agree, I'd need some clarifications about the original 904A CV input gains..

A kind DIYer on the web suggested the following:
> 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. You 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 CONTROL 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
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