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Re: 921b clone ?

PostPosted: Thu Jul 19, 2012 7:47 am
by jpdesroc
>With 12 bits, you have 4096 different voltage values divided by 120 semitones (10 octaves), which yields only 34 possible different >voltage values per semitone. Even though you can get a precise 1 volt per octave out of this, some notes in between will suffer from >imprecise voltage for accurate pitch.

You will reach the 'in between' 34 voltages values per semitone only if you use the pitch wheel on your MIDI keyboard to go
lower or higher the played notes. I did that in my tests and the 'staircase' effect in the rising and lowering
of the notes are not noticable even with 34 steps max. Smooth action !
And most of the time the MIDI notes that are received generate between 0.000 and 5.000vdc
in fixed steps of 1/12vdc per semitones.
So again the semitone's 34 voltage steps are used only in pitch wheel demands or MIDI modulation.

Re: 921b clone ?

PostPosted: Thu Jul 19, 2012 11:43 am
by thealien666
I understand what you mean, jpdesroc.

But I've just realized that there was one important word missing in my last post about voltage resolution per semitone: scaling.

With only 34 possible voltage values per semitone, the scaling of the whole 10 octave range, although correct at each octave extremities, might be off on a few notes in between them. Yielding an approximate keyboard scaling, instead of perfect pitch for every single note.

I have the same problem with my Korg Mono/Poly and my MFB Kraftzwerg mini modular. They both use 12 bits D/A converters to produce pitch CV, and it's impossible to scale them perfectly for each semitone accross their many octaves range, even though they both can produce perfect 1 volt per octave when properly calibrated. Some notes in between octaves are not producing perfect pitch. Some notes are off by as much as +/- 6 cents !

That might not seem like much, and unless one has a " perfect pitch ear" it won't probably be noticeable on its own. But when played against other instruments that are better scaled, you can clearly hear beatings between notes on some, and not on others. It's much more noticeable, now that my Minimoog D has an internal optical key contact system (Kevin Lightner's Opto-Key) that is using 16 bits D/A to produce pitch CV across only 44 notes ! Allowing a spectacular 1489 different possible voltage values per semitone !
I know it seems like overkill for so few octaves (3.5), but you can be sure of absolute perfect pitch for each and every single semitone with that. No approximation here!

I simply don't like the idea of settling for something like 2.354 volts when I can have exactly the 2.358 volts required, that's all.
Especially in 2012 with all those powerful and precise digital chips at our disposal for cheap.

Re: 921b clone ?

PostPosted: Fri Jul 20, 2012 7:43 am
by rarecomponent
I attempted to faithfully clone the 921b a few months ago..I spent many many hours re-creating the track pcb (double sided)
from whatever photos I could get..the most difficult Moog module Ive ever tried..tracks run under components , one has to deduct
routes etc. But I think I just about did contrast the cloning of the 921a was quite easy.

But so far the 921b "rests in peace" stone cold nothing from the saw wave. After many late nights testings I think I'm coming to the conclusion that the 2N 3954 I got from flea bay may not be an actual 2N3954..nothing on the casing..I could be wrong but isolating the saw wave part of the circuit, its still dead. I even transplanted the 2N3954 into a copy Mini Moog osc I made earlier.. dead.

I tried 2 J102's in the 921b...dead. But experimenting with my working 901bs Ive found that unless parts are identical to Moog's you get bad upsets in results...2N3954's are unobtainable..thats the problem..where-ever Mos lab gets them from who knows?

Re: 921b clone ?

PostPosted: Fri Jul 20, 2012 12:58 pm
by Kevin Lightner
I have 2N3954s. Even some 412s and other dual fets.

Re: 921b clone ?

PostPosted: Sun Jul 29, 2012 6:02 am
by forbin
Hi Analogmonster

The Moog sync is really quite different from the "others" in that it implements what is usually called a phase locked loop. It sort of measures the difference between the frequency of the "sync to" source and it's own frequency and then generates an extra voltage that will drag its oscillator to the same frequency as the "sync" source. It is considerably more sophisticated approach than just forcing a reset of a sawtooth as per usual hard sync circuits but comes at the cost of a multiplier. For a more detailed explantation with the maths google "phase locked loop".

Re: 921b clone ?

PostPosted: Sat Jun 30, 2018 1:29 pm
by jpdesroc
EricK wrote:So, your Moog© 921b clone is really an exact copy of MOTM© Ultra Vco©, but for copyright reasons you won't post pics of the pcb? :wink:
I still have PCB's of the 921A,B clones which are labeled M103A,B available for sale
but you must PM me to buy them.
By the way, did you have a look at my Moog 904A,B,C filter clones ?? ... 04A_clone/ ... 04B_clone/ ... 04C_clone/
These are the original Moog circuits that had their PCB layouts redrawn
to use new available components..
I still have PCB's of them for sale. These filter boards are the most popular I sold so far.
Still selling..
The front panels look are EXACTLY like the original ones.

The panels look good!

Thank You !!