Added PWM to one of my 901a's...

[noddyspuncture]

Been experimenting with Pulse Width Mod for my 901a's.
Today I built a simple, dedicated triangle LFO for the purpose and hooked it up (to one controller) to see what it sounded like:

https://soundcloud.com/q-uazimodo02/mod ... ulse-width

Cheerz,
TOM

[JohnLRice]

Sounds great, Tom!

[CZ Rider]

Sounds great! Adding a dedicated LFO is a nice solution to PWM.
There was a panel design for PWM input on a 901 found in Bob Moog's archives. Although they are numbered as 921, it looks to be descrete designed 901's circuits behind those panels.

Would be nice to have the official R.A.Moog designed panel custom made for such a PWM mod. I like the layout and the correct pulse width calibrations missing on the original 901A.
Anyone have any better pics of those driver/controller circuits or schematics? Always interesting to see how Bob would have implemented this with discrete circuits.

[JohnLRice]

Well hey! Lookie what just popped up on COTK's site! They are now offering an extended C901-A with the pulse width CV in like what CZ Rider posted:
http://www.cluboftheknobs.com/pro_c901A-C.html

[analogmonster]

Wouldn't it be better to control the PW for each oscillator separately? Controlling the PWM from an oscillator controller (901A, 921A) means all dependant / controlled oscillators create the same square pulse width, but I would say PWM is a feature of an oscillator itself, the unit which creates the square wave. And one would be more flexible. One can thicken the phasing sound of an oscillator (bank) in a more subtile way or in a more aggressive way by modulation multiples. So here is my proposal for discussion:

I made a spice simulation of a 901 with PWM in the oscillator section. See a Moog modular standard CV mixer section below which I copied and used for buffered CV Input for PWM: (I am sure it could be simplified somehow, but I'm not sure about buffering is necessary or not...)



The CV is fed to the base of Q3 (see 901B circuit diagram) via an R2k2. The "C" input is omitted. The resulting square wave simulation is the following:



The prototype is currently under construction.

[noddyspuncture]

That is kinda what I have done analogmonster (sorry don't know your name). I have two oscillators per controller - and two controllers in the system. I added the PWM per bank (901a)... so if I want to I can indeed have two separate oscillators with different rates of LFO. But I can also have two sets of two oscillators with different LFO rates. It's the best of both worlds. And when I eventually build my own 901a and 901b set... I'll probably add the same function to those as well.

Cheers,
TOM

[analogmonster]

Ah, ok, I see.

Regards

CARSTEN

That is kinda what I have done analogmonster (sorry don't know your name).

[eben1000uk]

hi there

i just added pwm to my original 901 - what a coincidence

the 901 does sound amazing and now it sounds even better

heres a link to a blogpost i did about it, with a video:

http://myblogitsfullofstars.blogspot.co ... beats.html

i am now going to add pwm to the 901a / b combos - like noddyspuncture i will put the cv input on the 901a to control the following 3 901b together

otherwise it would mean modifying the 901b panel and i don't want to do that!