How do you delay an LFO?

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Silesius
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How do you delay an LFO?

Post by Silesius » Sat Dec 14, 2019 7:03 pm

Hi, I’m building a monosynth 70’s style, with keyboard etc, and I want to have the classic delayed vibrato function. Specifically, I want to have a “Delay” knob for the LFO, as in many Roland Sh synths or in their 140 module.
I’ve been trying with an AD generator and a vca and it more or less works, but I need two pots instead of one and there isn’t a real delay. You can play with the attack time, but it’s not exactly want I wanted.
So, what is going on in that Roland circuits? I’ve looked at the schematics, but I’m not an engineer and they are quite difficult to understand. Are they using a envelope generator+vca to control the lfo? If so, what are you controlling with the “delay” control, the attack of an envelope?
In my synth I’m using the Electric Druid LFO’s. They have a very convenient level cv input that you can use as an vca. So what I really want to know is how to generate the needed envelope for the delayed vibrato with just one control. What kind of envelope would that be? Any ideas?

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devinw1
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Post by devinw1 » Sat Dec 14, 2019 8:07 pm

I haven't looked at the a schematic yet, but seems to me you could just have an envelope generator feed into a comparator at say the 90% voltage of the envelope and have that go to a VCA. That way a longer attack time would cause a longer time delay before the comparator fires and snaps the vibrato on.

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authorless
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Post by authorless » Sat Dec 14, 2019 8:25 pm

So you want an abrupt on after a fixed time, not a gradual fade in? Gate Delay into a VCA. An LFO with a reset input would be a nice addition.
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Post by KSS » Sat Dec 14, 2019 10:06 pm

The Roland 140 LFO delay is based on the charging of a capacitor (C7) to provide CV for a BA662 based VCA which controls the selected LFO waveform output.

In the mid lower right of the schematic in the service manual, find pin 37 which is the connection to the panel delay pot. Below this see point "A" which is the gate input to the delay circuit. Notice that the default condition is for C7 to be fully charged by the positive voltage from the Delay panel pot. Timing cap C7 is hung between positive and negative rails with the Delay pot between it and the positive rail, and R113 between it and the negative rail. FET Q30 is a high impedance follower which feeds the voltage level to the CV input of the VCA, which is Q31. VR13 is a trim for this CV input. Q31 is a simple uncompensated expo converter for the current going into the BA662. The selected LFO waveform comes into the VCA at pin 34.

The gate comes in at point "A", is differentiated into a pulse by C6 and D21 clamps the negative portion of this transient. Q29 briefly turns on and shorts out timing cap C7 to begin the delay. VCA will be off at this instance.

The delay panel pot sets the speed at which C7 charges and thus the voltage level on the gate of follower Q30. Which CV then opens the VCA to effect the LFO delay.

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fitzgreyve
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Post by fitzgreyve » Sun Dec 15, 2019 6:14 am

see schematic here:

100m-ish LFO

You want the circuit around Q1 going in to pin 10 of the PIC (the level control input).

This is the same circuit as the 100m uses, but adapted to PIC voltage levels (i.e. +5V supply)
Under developement:
- "Muse"
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https://fitzgreyve2.blogspot.com/

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Silesius
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Post by Silesius » Sun Dec 15, 2019 6:42 am

Thanks for all the interesting information. I didn't found fitzgreyve's LFO when researching, but that's exactly what I need. It even uses the Druid chip, that's fantastic :)

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