Moog Music Forum

Share Ideas and Pass the word

901 vs. 921

System 55 • System 35 • Model 15

901 vs. 921

Postby analogmonster » Fri Oct 06, 2017 9:29 am

Hi owners,

I read several times that the 901 sounds "better" or "smoother" than the 921. As a square is a square and a saw is a saw and so on: are there any measurable differences in the waveforms? An audible difference should be visible in osc pictures.
I noticed some waveform anomalies when I cloned the 901, but I attributed this to component differences, but perhaps owners of the original saw similar things or other differences in their osc pictures?
See also waveform pics at http://www.analog-monster.de/mmt901_en.html

Thanks
User avatar
analogmonster
 
Posts: 108
Joined: Wed Nov 24, 2010 9:50 am
Location: Germany

Re: 901 vs. 921

Postby ualslosar » Fri Oct 06, 2017 2:51 pm

The 901's synch better together than the 921's, not that the waveforms are any different. I believe this has to do with the internal architecture of the 901s vs the 921s, from what i understand.
ualslosar
 
Posts: 6
Joined: Mon Oct 13, 2014 2:32 pm

Re: 901 vs. 921

Postby MC » Fri Oct 06, 2017 3:08 pm

The waveforms on the 901 are more distorted. The rising/falling edges of the square waves are slower, the slope of the ramp is more curved, and the triangle has a slight discontinuity in the peaks due to the UJT used in the waveshaping circuit. The audible differences are subtle but they are there. In fact, the Voyager simulates the curved ramp and the triangle discontinuity.

Whether they sound "better" is subjective. The vintage 901A driver wasn't stable or accurate throughout the five octave keyboard range (due to the limited temperature control available with the transition knees of the CA3019 diode bridge), but I don't know if the reissue 901A has resolved that.
Gear list: '04 Saturn Ion, John Deere X300 tractor, ganged set of seven reel mowers for 3 acres of lawn, herd of sheep for backup lawn mowers, two tiger cats for mouse population control Oh you meant MUSIC gear Oops I hit the 255 character limi
User avatar
MC
 
Posts: 2825
Joined: Tue Aug 12, 2003 2:20 pm
Location: Secluded Tranquil Country

Re: 901 vs. 921

Postby robotmakers » Fri Oct 27, 2017 11:32 am

The magic of the 901 modular VCO set is that the 901Bs share one exponential converter in the 901A. This means that when the 901Bs are set to detune slightly from each other, the RATE of beating (i.e. the offset in Hz) remains constant throughout the range of the keyboard. Technically, each 901B is a linear response VCO.

On a 921 VCO set (and indeed with virtually all other VCOs), each 921B has its own exponential converter. This means that when one 921B is set to detune against another, the RATIO of the frequencies remains constant. This means that the rate of beating doubles with each octave you go up the keyboard. Technically, each 921B is an exponential response VCO.

So, with the 901s, you can set the detune beating to remain constant (and pleasantly slow) across the keyboard range. This gives the impression of "fatness". With other VCOs like the 921, the beating gets faster as you go up the keyboard. Less "fat".
User avatar
robotmakers
 
Posts: 18
Joined: Thu Apr 18, 2013 4:51 pm

Re: 901 vs. 921

Postby till » Mon Dec 11, 2017 5:26 pm

robotmakers wrote:The magic of the 901 modular VCO set is that the 901Bs share one exponential converter in the 901A. This means that when the 901Bs are set to detune slightly from each other, the RATE of beating (i.e. the offset in Hz) remains constant throughout the range of the keyboard. ...

You nailed it! This is something I had discovered after I tried the 901 clone oscillator bank be Synth-Werk from Munich/Germany.
keep on turning these Moog knobs

Sequence:
Prodigy * Minimoog (built 1979) * Voyager * MF102 * MF103 * MF104z * MP201 * Taurus 3 * Minitaur * Sub Phatty * MF105 * Minimoog reissues with MUSEs
User avatar
till
 
Posts: 1021
Joined: Tue Nov 22, 2005 6:17 pm
Location: south-west Germany

Re: 901 vs. 921

Postby thedangore » Fri Jan 19, 2018 10:42 pm

MC wrote:The waveforms on the 901 are more distorted. The rising/falling edges of the square waves are slower, the slope of the ramp is more curved, and the triangle has a slight discontinuity in the peaks due to the UJT used in the waveshaping circuit. The audible differences are subtle but they are there. In fact, the Voyager simulates the curved ramp and the triangle discontinuity.

Whether they sound "better" is subjective. The vintage 901A driver wasn't stable or accurate throughout the five octave keyboard range (due to the limited temperature control available with the transition knees of the CA3019 diode bridge), but I don't know if the reissue 901A has resolved that.


I can say with confidence that the issue has not been resolved in the new reissue units. Still the same old quirky tuning problems over the octave range, though certain parameters with the seem to be more workable than others for keeping tune. 901s most definitely have a different texture than 921s. And it is definitely better for certain things though I still find myself using 921s for other things just as much (like using the 921a for its pwm capabilities).
thedangore
 
Posts: 23
Joined: Mon Feb 13, 2017 3:21 pm

Re: 901 vs. 921

Postby noddyspuncture » Sat Jan 20, 2018 7:45 am

thedangore wrote:I can say with confidence that the issue has not been resolved in the new reissue units.


Are there reissued 901's...?
I thought all the reissue's contain the 921's...!?
User avatar
noddyspuncture
 
Posts: 414
Joined: Wed Sep 16, 2009 7:57 pm

Re: 901 vs. 921

Postby thedangore » Sat Jan 20, 2018 12:30 pm

noddyspuncture wrote:
thedangore wrote:I can say with confidence that the issue has not been resolved in the new reissue units.


Are there reissued 901's...?
I thought all the reissue's contain the 921's...!?



Hey Noddy,

Definitely yes! Recently (Aprilish 2017?) Moog reissued the IIIC which contains all the 901 oscillators - nine 901bs and one 901 oscillator! And Beautiful they are! HUGE sound. But all that same quirky stuff is still there as I'm sure you're well aware of. I was fortunate enough to grab #10/25
thedangore
 
Posts: 23
Joined: Mon Feb 13, 2017 3:21 pm

Re: 901 vs. 921

Postby EMwhite » Sat Jan 20, 2018 1:01 pm

thedangore wrote:I was fortunate enough to grab #10/25


I would love to see some pics and videos of it in action. Such a shortage (obviously) of this. Despite Moog's introductory video (which was excellent), haven't seen much.
'76 Minimoog, Taurus 3, Oberheim FVS + Son of 2-voice; Sequential ProOne; Juno 106; Moog Model 15; Kurzweil 250; Hammond M3; and a handful of Fender Basses Flickr!
EMwhite
 
Posts: 1651
Joined: Sun Jun 21, 2009 12:22 pm
Location: Middlesex

Re: 901 vs. 921

Postby noddyspuncture » Sat Jan 20, 2018 7:56 pm

EMwhite wrote:
thedangore wrote:I was fortunate enough to grab #10/25


I would love to see some pics and videos of it in action. Such a shortage (obviously) of this. Despite Moog's introductory video (which was excellent), haven't seen much.


Same here... yes I'd like to see that also please. :D
User avatar
noddyspuncture
 
Posts: 414
Joined: Wed Sep 16, 2009 7:57 pm


Return to Modular Forum

Who is online

Users browsing this forum: CommonCrawl [Bot] and 0 guests