Pushing the Little Angel/Dimension P style LFO into audio range

[picohogshead]

I have the Dimension P chorus on breadboard and I'm trying to make it capable of ring modulation. I'm new to electronics and can't claim to really understand how different components interact here, but I've experimented with different values of C14 and R14 in the schematic at the top of the link. R14 seems to be the key here, but weirdness ensues. With values of R14 below 3k, at the fastest end of the modulation range the LFO stops modulating the signal and instead just adds its own pitch on top of it. Yet if I stick an audio probe (with a 100n cap) into pin 2, the LFO snaps out of it and modulates as it should. What's going on there?

[Kipper4]

Maybe try a bigger speed pot eg 500k or a smaller c15. Won't take long on the breadboard?

[duck_arse]

C14 is meant to slow-down the waveshape a little, so it won't tikk as much. R14 is the minimum stop, chosen becuse the osc don't run with a lower value .....

C15 - smaller equals faster, larger equals slower - done.

[picohogshead]

Forgot to mention I already have a 500k speed pot. C15 does change the oscillator range but doesn't help keep the modulation going when you go into audible range.

C14 is meant to slow-down the waveshape a little, so it won't tikk as much. R14 is the minimum stop, chosen becuse the osc don't run with a lower value .....

Weird—I had a ferrite bead at R14 at one point (since you can use a 10R resistor instead of a ferrite, you can also do the other way around, right?) and could sweep the oscillator to a whistle before it audibly farted out.

Oh, hang on a second—could it be that the 2399 can't handle such fast modulation speed through pin 2? I gather that using pin 2 to modulate is creative abuse of the chip.

[slacker]

The depth pot and C21 make a low pass filter that will cut off audio frequencies, and with the depth pot at minimum resistance C21 is parallel with C15 so even if you make C15 smaller it probably won't do audio frequencies because C21 will mostly determine the speed. You'll need to make C21 much smaller or remove it to get audio into pin 2.

You could be right and you're hitting the limit of how fast you can modulate the delay, the minimum delay time is something like 25ms which is 40Hz, so if you try and wobble it faster than that it might just mix the audio in.

[picohogshead]

I followed the v.2 schematic which does away with C21 entirely, hadn't noticed that difference, sorry. Anyhow, I determined that the given values of C15 and R14 (that also match those in the Little Angel) limit the top speed to just below the area where the modulation starts to drop off so their designers were probably aware of the issue. No warp speed switch for this one then, I guess.

OTOH the 2399 block diagram reveals that pin 2 is used as reference in the internal low pass filtering so there may be some unintended side-effects from modulating it but I'm nowhere near advanced enough yet to investigate that so I'll just build a regular one.

[anotherjim]

Modulating pin 2 at audio would add in the modulation as audio mixed in, since Pin 2 is the + input of all op-amps in the chip.

In the LA, the single amp LFO relies on pin2 wobbling to oscillate, and it happens to also be a way of modulating the VCO  and have pin6 resistor really low value for a proper chorus effect. It's a clever scheme.

If you have opted to keep the Pin6 resistor at the recommended min 1k and don't care about getting proper chorus, you probably can better perform FM of the VCO via pin6 instead?