Delay Pedal Modulation Cancelling Out???

[Scruffie]

Hey all, got a problem I feel must have an obvious explanation but I can't wrap my head around it...

So, I'm trying to add modulation and C.V. to an old MN3005 based delay, using a CD4046 to do the C.V. & modulation and feeding its output in to pin 7 of the MN3101 and for the most part, it works nicely but at certain combinations of modulation rate and delay time settings, the modulation just cancels out... the delay still works and I can hear the clock wiggling in the background but I'm getting no audible modulation happening.

It's not a voltage conflict causing the op-amp to hit the rails or anything (I drew the schematic quickly for illustrative purposes so take the values with a pinch of salt) and I've tried modulating the 4046 clock cap and I even tried a quadrature LFO @ 90 degrees shift to see if it was a phase issue and it still happens.

Anyone got any theories as to what's happening?
Thanks in advance.

[ElectricDruid]

Yes, I have.

That's actually normal. If the LFO period and the delay *match* or are *multiples* of each other, then the clock frequency when the signal comes in and the clock frequency when the signal goes out are both the same, despite that frequency going up and down over time. And since the input and output sample rates are the same, there's no overall modulation.

It doesn't happen often because usually the LFO rate is slow compared to the delay time. In a chorus, you might have a LFO rate of 1Hz to 10Hz, say, which is 1 second to 100milliseconds, but the delay time is probably 30 milliseconds or less, so it never occurs. Flangers push this even further, with slower LFOs (sometimes as much as ten seconds, 0.1Hz) and delay times that are even shorter, only a few milliseconds.

But if you try modulating a nice long delay with a reasonably fast LFO so the two periods are the same, then you can get this effect. It is *literally* what you said - the modulation just cancels out.

[Scruffie]

Yes, I have.

That's actually normal. If the LFO period and the delay *match* or are *multiples* of each other, then the clock frequency when the signal comes in and the clock frequency when the signal goes out are both the same, despite that frequency going up and down over time. And since the input and output sample rates are the same, there's no overall modulation.

It doesn't happen often because usually the LFO rate is slow compared to the delay time. In a chorus, you might have a LFO rate of 1Hz to 10Hz, say, which is 1 second to 100milliseconds, but the delay time is probably 30 milliseconds or less, so it never occurs. Flangers push this even further, with slower LFOs (sometimes as much as ten seconds, 0.1Hz) and delay times that are even shorter, only a few milliseconds.

But if you try modulating a nice long delay with a reasonably fast LFO so the two periods are the same, then you can get this effect. It is *literally* what you said - the modulation just cancels out.

Well, male chickens.
But thank you for confirming I'm not going mad.

Any ideas to get around it? It seems to happen at too many points or at least enough of the rate/delay combinations to be tolerable as before it cancels out, the depth starts to disappear, although it might seem worse because I've fixated on it.

I did consider using a 4013 for the division in place of the MN3101 and using the spare half to create a second LFO so it had 2 modulation sources but I'm open to suggestions as I'd prefer just the one modulation source.

[StephenGiles]

Morning all - I'm trying to get my head around this, when you say that the modulation cancels out, how does this sound?

[Scruffie]

Morning all - I'm trying to get my head around this, when you say that the modulation cancels out, how does this sound?

So, imagine you have your triangle LFO modulating the delay at a rate of 10Hz, as you gradually turn your delay control to around 200mS, the modulation depth starts to decrease until it's suddenly as if you've turned the depth completely down...

But you never touched the modulation depth pot and deep in the background you can hear the clock whine still wiggling up and down, but all you get it straight unmodulated delay signal.

[StephenGiles]

How odd, I never experienced that!

[Scruffie]

How odd, I never experienced that!

Same here! It's been very perplexing.

I had assumed it was the 4046 being its usual pain in the arse.