NON-Mutable Instruments Filter troubleshooting

#1

So, I’ve posted this up on the appropriate forum, but it’s kinda a ghost town over there, so I thought I’d try and pick someone’s brain over here.

I recently finished a Music From Outer Space Soundlab MKII. Everything is working great except for the SVF self oscillation. Resonance & Cutoff work fine. I’ve gone around the schematic and verified I have the right parts the right way, the right resistors and the wiring in the right places. Everything checks out. I’ve swapped the 13700 with a known good one and still no self oscillation.

I’ll admit, while I am pretty good at troubleshooting a build, I am not an electronics engineer, so when I get to looking at a schematic, I can follow it on the PCB but as far was whats doing what, I’m a bit cloudy. If I could get a finger point in the general area of where to look, that would be most excellent.

Here is the schematic. My only thought is Q5:

Any info would be greatly appreciated!

#2

Is that meant to self-oscillate? Could it be related to the tolerances/parts value being slightly off?

Try playing with the value of R20.

#3

I have the slight feeling Olivier reads and understands schematics at a speed i only can reach for Adverts or Comics.

#4

@olivier, Yes, it’s supposed to self oscillate in LP Mode, not in BP mode. R20 was my initial thought. It is possible the tolerance is off. I used 5% everywhere except the oscillators. I’ll yank it and measure the resistance. Thanks for the lickity split response

@frank, right?!

#5

@Frank,
I’d say it’s about the speed I can read the label on a bag of sugar.

Damnit, tried to find a pic of a bag of sugar, not a damn one online, must fix!

#6

#7

EDIT: photo removed due to ridiculous size

Olivier I could kiss you right now! I won’t. I’ll just buy whatever you put out next :slight_smile: It was R20. I dont have a 22k handy, but I have 19k (18k & 1k in series) and I have self oscillation! It’s distorted, but I’ll fix that with a 22k .1%! Shoulda trusted my gut. I had a feeling that’s where the issue was. Thanks again!

#8

You might need soft-clipping to get a smooth self oscillation - not sure if the JFET buffer output and/or the darlington output are the right places for that. Try a pair of back to back 5.6V Zeners to ground at U2 pin 8 or U2 pin 7. No guarantee it’ll work…

#9

thanks for the tip. I’ll add some next Mouser order and try it out.