PT 2399 delays with LFO modulation. How to make it usable in all the time range?

[Morocotopo]

I´m currently designing a 3 chip double delay with some bells and whistles, like for example time modulation with an LFO, made the usual way, using a transistor on pin 6 as the mod element. But there´s a problem: if I make it able to modulate the time at the shortest times, it gets way too much at the longest times. If I make it modulate less at the longest times, it doesn´t have any effect at the shortest times. The range of times I have is 30 - 600 ms. more or less. I tried most of the mod circuits: the one in Rebote3, Echobase, Abductor... none of them offer usable modulation over all the time range.

So I guess the problem is, the amount voltage variation that the LFO /control element offers versus the amount of voltage at the various time settings. Example: if the LFO swings 2V and the fixed voltage is 4V, the LFO swings 50% of the voltage. If the LFO swings the same amount but the fixed voltage is 20V, the LFO swings 10%. So when I change the time, the percentage of LFO voltage changes, that´s why it has different mod depths at different times.

Would the answer be, to have an LFO voltage swing that tracks the time? Or am I totally off track (not that I know how to do it)? But figuring out the problem is the first step to solving it.

What do you guys think?

[anotherjim]

I suppose if there was only delay time control, you could have a dual gang pot with the second control being mod depth (or acting as a modifier of an existing depth control). If multiple delays have voltage control of their delay time, then a dual pot could still work provided there is only one overall delay time control.

[Morocotopo]

Unfortunately I can´t use a double gang pot because I´m already using one to control the delay time of two chips in series... so no go.

[nocentelli]

If you use a single pot for the delay length with a current mirror (e.g. deadastromaut's abductor) you could use the other gang to adjust the modulation depth.

[Morocotopo]

If you use a single pot for the delay length with a current mirror (e.g. deadastromaut's abductor) you could use the other gang to adjust the modulation depth.

Yeah, I´ve already made a delay with the current mirror / single pot setup, but, that requires a 2M pot to get a good range of delay times, and I can´t find those here (Argentina). I had to add a switchable 1M R to a 1M pot to get the full range of times, but in this one I can´t spare the room for another switch in the box, already has 10 pots and 3 switches...



Might try to mod that to use a 1M pot...

[nocentelli]

I've actually got a 2 x PT2399 delay on the breadboard right now: it's just the deep blue delay/sea urchin delay, but two in series: I used the current mirror setup from the rebote 3, which uses a 10k pot as a voltage divider on the base of the control bits, and goes all the way down to about 2300ms out of two pt2399's

[Morocotopo]

I've actually got a 2 x PT2399 delay on the breadboard right now: it's just the deep blue delay/sea urchin delay, but two in series: I used the current mirror setup from the rebote 3, which uses a 10k pot as a voltage divider on the base of the control bits, and goes all the way down to about 2300ms out of two pt2399's

OK, I´m really really dumb.



I´m actually using that same configuration, but I never realized it´s a current mirror. Anyway, I modulate only one of the two series delays with the LFO because if I modulate both I will get "double modulation", so to speak, too much mod depth. But, I could attenuate the LFO amplitude to half and modulate both, that should give me the same result as modulating only one of them and it would free one half of a ganged 10K pot. Hmmm.