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

[Terry]

Easiest solution would be to perhaps change a resistor on the Dotcom
envelopes to match the output on the original Moog 911 envelope
generators. That Moog spec for the 911 output was taken from a sheet
printed in October 1965. (I have the original) They did not print the part
that reads "Specifications subject to change". All the 911's here are
circa 1969, and all three output at least 6.3 volts. Moog made many
revisions since 1965. 0 to 6 volts is the magic Moog number controlling
VCO, VCF. VCA.
     The Dotcom Q109 envelops here, two of them, put out a maximum 4.9  
volts.
Thats 1.4 volts less than the 911's and in filter response almost an
octave and a half less. When I patch in the Q109 to the Moog 904A they do
not open the filter as much as the 911's do. Same with the 902 VCA, the
Q109 envelopes will not open the VCA as much as the Moog 911. Easy to
figure out why when measuring the actual voltage outputs.
     Looking at the 911 schematics, resistor R27 is "Selected" for Esus
level.Nowhere on the schematic is that voltage spec listed. Mine are all  
set to
roughly 6.3 volts. Two are original with the system and a third circa 1969
was added. Can ask Roger at Dotcom if there is a similar resistor on the
Q109 to make them Moog compatable.

The "Fixed Control Voltage" on the 904A is just that, and front panel
calibrations are accurate. Regardless of the actual pot sending ground to
-6 volts through a 51K resistor, the effect is the same as sending -6
volts CCW to +6 volts CW through the 100K CV inputs. This can be easily be
verified by observing the response by patching in the same voltages. The
cutoff frequency is the same when the dial is set to zero as it would be
sending 3 volts and offsetting this by turining the fixed control voltage
to -3 volts. This is a voltage adder that adds the three front panel
inputs the input node and the fixed control voltage. So yes, adding in -6
volt offset from the fixed voltage, one would now need 12 volts at the CV
input to open the filter. There are many reasons why in an open patching
system these negative offsets would be needed. Depending on if using a
keyboard to control the filter tracking the CV at middle "C" could be 3
volts. This 3 volts added to the 6.3 volt 911 output may set the filter
higher than needed. So a negative offset would be dialed in via the fixed
control voltage to compensate. In a typical patch one might have both an
envelope and keyboard tracking CV all added together and a dial with a
negative offset is handy to bring all the voltage inputs in range. Also
not uncommon in a modular systen to send two 911 inputs for a more complex
envelope or other modulation sources that all add together where a
negative offset is needed.

     The Moog 901 oscillators are the same with the fixed control voltage
used as an offset to operate the 901's in the specified voltage range. The  
901
operate best in a -0.5 to +6.5 volt range spaning 7 octaves. The 901's
crap out if you send more than 6.5 volts to them added together. Easy to
do if you do not add together all the voltages and calculate correctly.
After the 6.5 volt limit is reached the 901 will stop responding. Have
seen this on a demo where the operator claimed the 901 only had a 3 octave
range, and it was clear it hit the 6.5 volt limit. If the offset was
dialed in to -2 volts, the 901's would have had the full 5 volt range of
the keyboard. Easy concept to grasp if you just stop and think about how
all these voltages are added together. In a fixed system like a Minimoog
this is all patched in for you. It's up to the operator in a modular
system to get everything in range. The Moog system mostly has all the
voltages you need for positive and negative offsets right on the front
panel. The +6 volt offset on the 901A might seem unnecessary when they do
not respond above 6.5 volts. Untill you try to patch in a 0-6 volt ribbon
controller that you flip over to play like a guitar. Since the pitch is
backward this way, you invert the voltage so it outputs 0 to -6 volts. Now
the +6 volt offset gets the 901's in perfect range.
The Moog system was very well thought out, but different from today's
systems in so many ways.

> 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
>
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