[sdiy] Semi-Discrete (3046) Minimoog Oscillator Theory Of Operation?

[JR]

Robert & Tim,

Thanks for your help.  I got that second oscillator working finally.
It ended up being that the collector of Q8 had a cold solder (well,
technically Q19 I guess, since it was for oscillator 2 -- Q8, Q19, and
Q32 are all the same thing across each of the 3 oscillators) .  I
guess I was in a bit too much of a hurry when I was putting the board
together originally.  I think that's the only cold solder I've had on
this whole project... I thought I was past that kind of silly mistake.

So, now I've got all 3 oscillators making that warm Moogy sound, the
filter seems to work, and the dual contour generator sweeps the filter
and vca (well, more or less... something seems a bit out of adjustment
since the DCG seems a bit slow).  I've also got several of the panel
potentiometers wired backward (oops), but all in all it's sounding
really good compared to where I started.

Now I'm going to rig up something to interface to the keyboard circuit
(a MIDI2CV interface built on a MIDIBOX core).  I'm going to set it up
so if I ever find a cheap mini keybed I'll be able to use either the
midi interface or the keyboard).

Thanks again for your help.

--Joe

https://sites.google.com/site/minimoogwiki/home



On Mon, Aug 2, 2010 at 2:31 PM, Robert Hume <robert.hume at gmail.com> wrote:
> The oscillators in the dashed lines are getting inputs from the left. From
> top to bottom the inputs are GND, +10V, a current that determines pitch,
> -10V, -4V, -10V.
> The current that determines pitch is sunk by the 3046 collector on the left
> and goes through capacitor C6, which creates a ramp. The ramp gets reset by
> transistor Q21 (capacitor shorted) periodically to make a sawtooth.
> Q18 is a JFET differential input stage with Q22 setting the current. The
> output of the first stage goes to another differential amplifier stage made
> up of Q19 and Q20. The output of that stage goes to emitter follower Q15.
> The Q18/Q19/Q20/Q15 amplifier is really a discrete op amp with negative
> feedback. It has a closed-loop gain of 2 (1+R75/(R56//R76)) and also
> corrects the DC offset.  Q16 and Q17 are two grounded emitter stages
> that act as a comparator. The comparator monitors the saw output and sends
> the reset pulse back to Q21.
>
>
> On Sun, Aug 1, 2010 at 12:09 PM, JR <calphool at gmail.com> wrote:
>>
>> Hi everyone,
>>
>> Does anybody know where I can find some documentation that describes
>> precisely (down to the individual component ideally) how the middle
>> style semi-discrete Minimoog oscillators work (This design:
>>
>> http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-minimoog/002/903-osc-print-schem-below.gif)?
>>
>> I've been building a clone and I've got 2 out of the 3 oscillators
>> working, but I don't understand the schematics well enough to use them
>> to debug the middle oscillator (#2).  I have kind of a general idea
>> how an op amp works for example, but how this collection of
>> transistors and op amps works together sort of eludes me (especially
>> the main oscillator source -- the saw -- it seems to come from the
>> combination of what's going on outside of the dashed lines and what's
>> inside them -- you don't seem to get anything if you just connect the
>> circuits labelled "Oscillator 1, 2 or 3" [in the dashed lines] to
>> power -- instead they seem to be cooperating with the left half of the
>> diagram to create the main saw that drives the whole circuit).
>>
>> Thanks,
>>
>> Joe
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>