Low Cost PT2399 Depth Control?

[Ben Lyman]

Hey everybody! I'm back, finished school and trying to pick up right where I left off 3 years ago, that being Anchovie's minimal chorus. I have it on the bread right now and it sounds great, very thick and warbly. In fact, I would prefer to tame it a little bit if possible.

I notice the "Depth" control seems more like a "Mix" control to blend the dry signal in with the chorus signal. That's all good but what if I want to make the chorus signal less wonky, a little more musical?

btw, thanks everyone who helped me get started with that math class last fall, I aced the final and got straight A's in all my classes

[merlinb]

Try this

[duck_arse]

I don't think I've seen that circuit before. is the pin 13/14 opamp working as an ocillator? nice trick.


and welcome back, no-name Ben.

[Ben Lyman]

Try this

Thanks Merlin! Also, was this originally your creation? I thought it was yours but when I searched I found Anchovie's thread. so, sorry if it was yours, let me know and I'll edit my op to reflect. Thanks!

[anotherjim]

That spare op-amp is the LFO oscillator. It becomes a little clearer if you know that opamps' +input is internally connected to pin 2. So there's the positive feedback path and the main timing cap is on the -input pin 13.
To maintain oscillations, pin2 voltage must wobble.
It ought to be possible to reduce the LFO depth by increasing one of the 10k resistors in the path back to pin2. Though those caps on the pin2 path are meant to filter the square wave to a smoother saw/tri shape, they can influence the frequency too.

[Ben Lyman]

Try this

That spare op-amp is the LFO oscillator. It becomes a little clearer if you know that opamps' +input is internally connected to pin 2. So there's the positive feedback path and the main timing cap is on the -input pin 13.
To maintain oscillations, pin2 voltage must wobble.
It ought to be possible to reduce the LFO depth by increasing one of the 10k resistors in the path back to pin2. Though those caps on the pin2 path are meant to filter the square wave to a smoother saw/tri shape, they can influence the frequency too.

Been experimenting with those caps and resistors, added the 100K pot and everything I do in that area seems to change the rate but still no depth control. I was able to achieve a square wave, which was kinda cool, so Jim, you were on to something there.

Any other ideas? I'm looking for a way to slow it down (which I figured out with a toggle switch option) but I also want to get a really shallow, subtle wobble so I can blend it about 50/50 with the dry signal. I think, if possible, it will create nice 80's sounding doubling effect.

[anotherjim]

Yeh, I got a suspicion that the way it relies on pin2 being the +input of the opamp, means reducing depth there might not be the way to go.

This is the type oscillator. If you ignore the Low-cost circuits' filter capacitors, R2 is "kind of" the 10k series resistors and R3 is "kind of" the lower half of the internal reference at pin2 (and the +input of the opamp).
Normally, modulation depth means varying the LFO depth into whatever control input the delay time has, but in this case, that control input is part of the LFO and there's no means to interpose a depth control in between.
Because any changes to the reference voltage on pin2 change the delay time, there's no way to break the bond between the oscillator and delay time.

You might try another type of single opamp circuit, such as the phase shift oscillator. That means the opamp +input can be left at the internal pin 2 reference and that pin only has the decoupling cap as per standard PT2399 circuits. Then the output from pin 14 should be a sinewave and connected to vary the delay time at the pin6 resistor as per normal. The PSO won't have a huge tuning range and the delay time won't swing as short as pin 2 modulation can. It won't be much more parts and the modulation amount can be adjustable down to none.

Don't forget most examples out there are for dual supplies so some of the "grounds" might have to be pin2.

[Ben Lyman]

Thanks Jim, I suspected it might be something like that. I feel like if I could speed up the delay time the depth might mellow out a bit. Probably not possible to speed it up with the PT2399 anyway

[ElectricDruid]

I feel like if I could speed up the delay time the depth might mellow out a bit.

Unlikely. The pitch shift caused by a varying delay is related to the rate-of-change of the delay time, so faster changes cause *more* pitch shift and bigger warbles.
That means if you slow the LFO down, you'd get more subtle chorusing, if that helps.

[Ben Lyman]

Unlikely. The pitch shift caused by a varying delay is related to the rate-of-change of the delay time, so faster changes cause *more* pitch shift and bigger warbles.
That means if you slow the LFO down, you'd get more subtle chorusing, if that helps.

ok, thanks Tom. I made a Deep Blue Delay clone with an optical LFO wiggling the delay pin and it always seemed to get thicker warbles when I slowed the delay time down. Thats why I thought maybe the opposite of what you said here. It was a very different idea though, so I bet you're right about this one, probably because the LFO is coming from in the PT2399 instead of from a separate circuit? Maybe? I dunno