[sdiy] MOOG 901D schematic?

Terje Winther terje.winther at wintherstormer.no
Fri Aug 24 13:41:20 CEST 2012

Hi Terry,
Thank a lot for this thourough mail and analysis.
Really appreciated.
It is good to know that we have people in here with this kind of  


Den 24. aug.. 2012 kl. 10.40 skrev Terry:

>> Does anyone anything about it or  know where to find a schematic  
>> for this module?
> About two weeks late, but always willing to discuss old Moog modules!
> I think the schematic of the 901-C may give a clue as to what might  
> have been used in the 901-D.
> Looking at an early Moog catalog, the descriptions of the 901-C  
> the same, with the 901-D being the super deluxe variable model with  
> a few more functions. Both modules quote "In conjunction with the  
> 901-B oscillator, this instrument produces two outputs of equal  
> magnitude but opposite sign." Then goes on to read "Outputs are used  
> as control voltages".  So it would seem the intended or designed use  
> was for it's CV modulation output. The 901-D goes further with  "All  
> of the 901-B waveforms are available and may be mixed in any  
> proportion. In addition, variable clipping is provided to shape the  
> triangular wave." (Note: this reads "triangular" wave,and may  
> reference the shape of the newly formed wave, and not neccesseraly  
> the "Triangle" wave.)
> Here is the 901-C schematic:
> http://tinyurl.com/944erwf
> A photo of the left side:
> http://tinyurl.com/9jw7ek4
> And photo of the right side:
> http://tinyurl.com/9kkta5c
> Looking at the smaller version 901-C.  The 901-C is a 1U module with  
> a 3-ganged, 6-positon rotary on top with the first three selections  
> -- Sine, Sawtooth, Triangular. Then in a boxed section labled Pulse,  
> with  the last three  selections -- Center, Positive and Negative.  
> Below the rotary selector, a potentiometer for the Amplitude of the  
> selected waveform, and at the bottom two jacks labled Complementry  
> Outputs (+/-). Looking at the schematic, the four waveforms from the  
> 901, after being selected via the rotary switch, goes to what is an  
> almost identical circuit as the CP3 mixer. So looks to be boosting/ 
> doubling the 1.2 volt P-P 901 signal and giving both a positive and  
> negative output .
> The later CP3 module has the effect of doubling a single input,  
> where unity gain is around 7 on the mixer dial with the master gain  
> at 10. And a mixer input setting at 10 would double the incomming  
> signal. Here is the CP-3 schematic to see the similaraties:
> http://tinyurl.com/9v7fknw
> In use the simultaneous inverted outputs of a sine or triangle wave  
> could be patched into two seperate VCAs for stereo panning, or the  
> now available reverse sawtooth for a modulation source. But the  
> circuitry in the Pulse section is the interesting part and would  
> seem to include both a positive and negative wave clipper. Just two  
> simple transistor +/- clippers that are mirrored on both output  
> sections A and B.
> My guess as to why these special pulse settings are needed, is if  
> you were to modulate your 901 oscillator(s) with a pulse output from  
> another 901, the waveform is non-symetrical and will go in both  
> positive and negative directions. So the classic modulation trill  
> sound will be difficult to tune as a musical interval since as the  
> modulation amount increases, both sides go out of tune. This  
> modulation effect on say, the later designed Minimoog, is easy to  
> tune as you hold a C you push the mod wheel till you get to the  
> interval you want, because the pulse only goes positive leaving the  
> base note C in tune. Not so with the 901 and would require constant  
> retuning to get a  C to F trill, and every time you change the  
> interval would need retuning of the oscillator. So I would imagine  
> this would be the reason there is a Center being that standard  
> output. And the Positive and Negative setting, clipping the other  
> half, to aid in tuning as a modulation source. Remember this was the  
> 60's when these were designed and needed. Plus there was the added  
> boost for the 901 to modulate another signal with more than the  
> stock 0.6 volt swing. The 901-C module does have the 4 countersunk  
> screws on the panel, indicating there is a C frame behind the panel  
> along with a larger  circuit board. (Some of the simpler, smaller  
> circuits had an L bracket that mounted through a 1/4" jack to hold  
> the tiny circuit board. Such as the CP-3 mixer, reversable  
> attenuator.)
> Seeing that Moog went through the trouble of giving me a Pulse wave  
> with either the positive or negative side of the waveform on the  
> "fixed" output stage 901-C. I would venture a guess that that would  
> be the function of the super deluxe version of the "variable" output  
> stage 901-D.  I'm just speculating here. But if the function was a  
> pot that would blend  from  positive clipped to center to negative  
> clipped waveform, for each of the four waveforms, with variable  
> amplitude for each. That would be a somewhat interesting module,  
> both as a modulation source or an audio source.
> Here is a somewhat blurry photo of the 901-D.
> http://tinyurl.com/8hn2o3s
> Tough to make out exactly what is labeled on that panel, but my eyes  
> went 10 years ago, so I have a good bit of practice reading blurry  
> lettering. Looks like across the top "901-D Variable Waveform Output  
> Stage". Below are four knobs each with three lines of text, with the  
> first two lines the same on all four. Looks like "Variable Clipper  
> Sine".  Then Variable Clipper Sawtooth, Pulse, and Triangle.   
> Suprisingly none of the knobs on these have any calibration marks,  
> that would aid in guessing the function range. Below that, in the  
> middle reads "Input Mixer" with four more un-calibrated knobs labled  
> "1, 2, 3, and  4".  Further down in the middle left reads "Inputs"  
> with four jacks below labled 1,2,3,4. To the right of that, a knob  
> that  reads  "Master Gain". To the far right reads "Outputs"  with  
> two jacks below with what looks to be labled with arrows , one  up  
> and one down.
> So, the bottom half of that 901-D closely resembles the later  
> introduced CP-3 mixer with four inputs, four attenuators, a master  
> gain with positive and negative outputs. The only thing missing in  
> the 901-C schematic would be the four attenuators and resistors like  
> in the CP-3. My guess would be the four variable clipped waveform  
> outputs are normalized to the four switching jacks below. Giving the  
> added option to override and introduce a CV from another 901 or  
> envelope generator for more complex/interesting waveforms on any one  
> of the four mixer inputs.
> The question though would be where the clipping circuits are  
> implemented?. On the 901-C the pulse is clipped after it is  
> amplified. But I doubt that on the 901-D there would be four  
> seperate amp circuits before the clippers, then another mixer amp  
> circuit. So might be clipped directly from the 901 individual  
> waveform outputs, then mixed amplified. Probably the same or similar  
> transistor clipper circuit on the 901-C times four, one to each  
> waveform. Not sure if there would be a special circuit to mix  
> together the positive/negative portions of the clipped wave. Could  
> be as simple as the Minimoog modulation mix knob, where the noise  
> and osc 3 are on each end of a pot , with the output taken from the  
> wiper? Any guesses from the "Synth Gurus", if that would yeild a non- 
> distorted sine/triangle/saw at the half-way point?  I could just  
> breadboard my ideas to find out. And I'll bet those early 901-D's  
> are on breadboards too.
> Anyhow, if my guesses are right, most of the 901-D circuit is lifted  
> from the 901C, and would be a blueprint to try to make one. This  
> could be an interesting module if you were able dial in the lower  
> half of a sine wave with the upper half a saw with variable  
> amplitude. Since the four 901 waves are locked together, might be  
> fun to assemble an odd "Triangular" modulation waveform that way. Or  
> add in some other mod sources like a rising envelope into the mixer  
> or a faster moving sine from another 901. The module was probably  
> seen as obsolete in the later modulars. With the 1968 model I, II,  
> III there were several of the newly introduced CP-3 mixers on all of  
> them. Those many mixers were absent on the early systems. And the  
> CP-3 could do most of the waveform mixing/inverting for more complex  
> modulations. The only thing missing is the clipper. But the CP-3 was  
> more of a modular approach and could be configured many different  
> ways from audio mixing to boosting/amplifing, to modulation mixing.
> Here is a photo of Bob with his hand just below one of the 8 or so  
> made 901-D modules. That early modular really did not have any extra  
> mixers, for modulation mixing. So in an old circa 1967 system like  
> that, the 901-D would be very usefull. By the time the 1970 Moog  
> modular price list was issued, both the 901-C and 901-D had been  
> dropped.
> Bob and 901-D:
> http://tinyurl.com/8fdn9kh
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Terje Winther
terje.winther at wintherstormer.no

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