MFOS VC LFO abcd

Hard to believe but it looks like there’s a rather serious design flaw in Ray Wilson’s VC LFO. It’s here:

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There’s a 100k pot with two 100k resistors connected to ±12 V; this gives the manual pulse width setting. There is also a PW control voltage, PWXV. Both go via 10k resistors into a comparator op amp. They serve as the reference voltage, so they determine the pulse width. There’s also a 1M to ground.

Okay, but what if the CV you want to use gives you too large a PW swing? You’d put it through an attenuator, right? (In fact my plan was to put an attenuator on the LFO front panel.) But now if you run the attenuator down to near minimum, the R37 wiper has a 20k path to ground, messing up the pulse width setting voltage divider. And: With PWXV floating, R58 and R34 form a voltage divider that reduces the PWC voltage a tiny bit… but if PWXV is grounded R34 is in parallel with R59 and V(PWC) gets cut in half!

What I find, then, is that if PWXV is floating, then the range of voltages going into the op amp is ±3.7 V, but if PWXV is grounded, that range drops to ±0.5 V!

(Incidentally, the schematic for SynthCube’s Eurorack version of this module has the PWXV jack tip normalled to ground. Wilson’s panel wiring doesn’t have that. If you do that then with no CV plugged in you have almost no manual pulse width range…)

The only reasonable fix I can come up with is to buffer PWXV and PWC to isolate each’s voltage divider from the other. The range would then be halved compared to if PWXV were floating, so maybe reduce R36 and R42 to say 20k to get some of that range back.

If I’m way off base on any of this please let me know…

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A video showing the bad behavior:

Also, the MFOS VCO has a similar setup. I didn’t notice similar problems in all the time I spent looking at its pulse width but I suppose they’re there. Need to check.

The YuSynth VCO has a similar setup but with smaller pots (25k, and 12k series resistors, instead of 100k and 100k) and larger mixing resistors (100k and 180k, instead of 10k). If I’ve calculated correctly, it should also have pulse widths that vary when you turn the PWM CV attenuator, but by a much smaller, arguably negligible, amount.

I see no reason why similarly smaller pots and series resistors, and similarly large mixing resistors, shouldn’t work in the MFOS LFO and make it much better behaved.

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I changed the pots from 100k to 25k, the fixed resistors on the PW pot from 100k to 22k, and the mixing resistors from 10k to 200k. Everything seems to work and changing the PWM attenuator (with no CV plugged in) doesn’t change the pulse width noticeably.

The width range isn’t 10% to 90%, though, it’s only about 35% to 65%. Which probably makes sense in hindsight, with the attenuator attached. I considered reducing those 22k resistors but decided not to mess with it further. If I need more extreme widths I can get them with a DC voltage on the PWM input.

The sync input mod doesn’t work due to a Dumbass® mistake on the auxiliary circuit board. I could kludge around it, but I’d rather just leave it as is for now and replace the auxiliary PCB after my next fab order.

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I just got and assembled the corrected auxiliary board, and the sync input now works! I also went ahead and changed those 22k resistors to 3.3k; I now get a pulse width range from the knob (with no CV in) of about 7% to 91%.

Gerbers, schematics, and writeups of the auxiliary board, Kosmo format front panel, and PW component changes may be found at GitHub - holmesrichards/MFOS_VC_LFO: Auxiliary board and Kosmo format front panel for MFOS VC LFO synth module. I put a couple of auxiliary board and front panel sets up on Tindie.

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nice , thanks for putting in the work . I will be interested for sure , I have 3 of the MFOS LFO’s it would be a good upgrade for them .

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Video about the problem and its solution:

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And video about the sync mod:

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Ive been lazy building an LFO since i have a half functioning VCO ive been tuning low and using for a free-running LFO. This mod convinces me that i shouldnt be so dang lazy.

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