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 Forum index » DIY Hardware and Software
Moog 901 DIY, or skip it and look elsewhere?
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rjd2



Joined: Sep 02, 2007
Posts: 235
Location: philly

PostPosted: Fri Jan 25, 2013 1:14 pm    Post subject: Moog 901 DIY, or skip it and look elsewhere?
Subject description: advice welcome
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hi folks, im wondering if you may have any suggestions. i can't tell if listening to too many records that used moog modulars has corrupted my brains, or if those VCO's are actually worth DIY-ing. after some googling and forum searching, i can only come up with non-labelled pcb layouts, not the actual BOM and placement of parts. i'd love to build one or three of these, but it seems like it may be an uphill battle. moog 901/921 users-is there another discrete build out there that would be more documented, that would sound similar? or should i go chase that unicorn? as always, thanks for the advice.
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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Fri Jan 25, 2013 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

There are the 901a driver and 901b oscillator. 901b requires 901a which can drive up to three 901b's. Not only do they use unobtanium components (IE CA3019) but their range is limited to less then five octaves and they drift like a mofo.

There is a 901 standalone oscillator, but no schematics are known to exist.

Same convention with 921/921a/921b set. They stay in tune better but they also contain unobtanium components.

I don't recommend either for a DIY project. Yes there are COTK and SSL, but they are panel replicas not circuit replicas.

Last edited by The Real MC on Thu Jan 31, 2013 7:55 pm; edited 1 time in total
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rjd2



Joined: Sep 02, 2007
Posts: 235
Location: philly

PostPosted: Fri Jan 25, 2013 6:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

got it. thanks for the info. so, if you were to suggest a DIY VCO that you think would be most suitable who was shooting for the closest SOUND to an original moog VCO, what would it be?

im tempted to throw out qualifiers like "discrete", but frankly, I dont know enough about the nature of a circuit to make causal claims like that.
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davemoog



Joined: Feb 23, 2008
Posts: 52
Location: Vancouver

PostPosted: Fri Jan 25, 2013 6:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

It was a tough build but works well, quite stable, but does have limitation of that just under 4.5 octave range useable on a keyboard- this range is greatly extended and this works well by changing overall freq with capacitors in the "Range Switch". Fixed CV switches are used to shift the sum of all the incoming CVs into this 5 octave sweet spot. IE this 4.5 octave range can be moved super low or super hi and tracks OK. It's just that if your keyboard is longer than this or the keyboard has octave switching built into the keyboard this will cause big problems in tuning.

http://minimalist.davidleith.com/DeitzModSynth4.html
http://minimalist.davidleith.com/DeitzModSynth6.html

pcb layout as it stands not useable as it required changes in power supply regulation, etc. Calibrating this VCO into the sweet range is a major hassle but once tuned is quite stable

Was it worth the difference in sound? - not entirely convinced, but yes it does sound good. The units I built combine the 901A and B into a single unit which is basically what the 901 is. The 901 does have output level pots for each output though.

Probably not a good project to start with, layout a bit critical regarding temperature, ( I moved the on board regulators) diode array is hard to find, and very difficult to calibrate. Other parts can be easily substituted. Also this oscillator outputs signals in the 1 to 2 volt range which needs to be boosted to cross modulate etc and is audio low compared to many other designs like Eurorack, ARP .....

Personally I like the sound of the standard ARP VCO and it's waveshapers and the Buchla 258 - both which have pcbs and info very available on the forums
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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Thu Jan 31, 2013 8:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you want the sound of the moog then you need the VCO, the 904A VCF, and the 902 VCA. The Moog 902 VCA is not high fidelity with its non-flat frequency response and subtle distortion. The resonant character of the 904A lowpass VCF is in a class all its own. Also the impedance mismatches between modules shapes the sound. The sum of these makes the moog sound.
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