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I'm trying to build something to change the gain of a line- or instrument-level signal very steeply using a microcontroller, to use as a guitar tremolo pedal. I've been looking at digipots but can't shake off the zipper noise associated with those.

One circuit I looked at places a vactrol between the two stages of an op amp. But I don't want to use a component with RoHS problems. So could I theoretically substitute an optoFET?

tremolo schematic

I understand how a potentiometer works, but I don't understand how the vactrol in the schematic works. (I only have high school physics from 30-odd years ago). Is it working as a current limiter?

There are a few sub-questions there, but the main one is: what other ways (besides a potentiometer) should I be exploring to vary the volume of my guitar?

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The oldschool Vactrol uses an LDR which is a light dependant resister .The light source can be a led but in the old days it was incandescant like 6.3Volt 0.3 amp .The LDR is very linear and can take rather high voltages .Old large ones would take 100 Volts .This means that noise pickup was not a problem .Unfortunately these LDRs were made of Cadmium Sulphide which is not ROHS .The newer linear fet optocouplers like say H11F1 can only run at low audio voltages like less than 300mV if reasonably low distortion is needed.Sure you could series connect the optos but performance will not equal the Vactrol.

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  • \$\begingroup\$ I think my microcontroller gives me a controlling signal between 0 and 5V. If I wanted to use its full range, could I cut that down with a trimpot to the H11F1's acceptable range? \$\endgroup\$
    – OutstandingBill
    Feb 20, 2020 at 4:15
  • \$\begingroup\$ @Outstanding Bill .The opto will run at the low voltage but there could be noise pickup . \$\endgroup\$
    – Autistic
    Feb 20, 2020 at 4:36
  • \$\begingroup\$ Oh, did I misunderstand? You mean 300mV on the audio side, not the control side. So because the audio signal is low, any artefacts the opto introduces will be significant wrt the signal. \$\endgroup\$
    – OutstandingBill
    Feb 20, 2020 at 5:08
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    \$\begingroup\$ I'm accepting this answer because my experiments with the h11f3 have so far been much better than with digipots. Thanks! \$\endgroup\$
    – OutstandingBill
    Mar 9, 2020 at 7:07
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The vactrol works as a gain-setting element for the op-amp. Gain will be 1+R15/R(vactrol). The more the LED light, the lower R(vactrol), and the higher the gain.

Another way to do this is to use a voltage-controlled FET as an attennuator. Here is an EDN article describing how to do that: https://www.edn.com/a-guide-to-using-fets-for-voltage-controlled-circuits-part-2/

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