MXR LFO in an OTA phaser?

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Jcknowles89
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Post by Jcknowles89 »

Probably a dumb question but will a phase 90 style LFO work with an OTA PHASER (specifically an LM13700) plugged into op amp bias pin?

Thanks,

Colton

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plush
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Post by plush »

Hi Colton

You can control OTAs via lfo. Since OTAs are current-controlled, you'll have to add a series resistor before the ABI.
Also you can use current mirrors if you want to control multiple OTA stages with single LFO.

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DrAlx
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Post by DrAlx »

One thing to bear in mind is that it won't sweep the same way if you simply convert the control voltage to a control current.

When the LFO gives you a triangular control voltage and you use that to make a FET act like a variable resistor, then the effective resistance varies (roughly) linearly on the upward and downward sweeps of the triangle wave.

Changing that control voltage into a control current for an OTA means you end up changing the transconductance of the OTA linearly, so the effective resistance varies in a hyperbolic way rather than a linear way. So with large sweeps you can end up with the phaser spending too much time at one end of the sweep.

Take a look at how the Tri-Vibe circuit http://www.runoffgroove.com/tri-vibe.html addresses that issue. You'll see a triangle LFO feeds into a stage with diodes. The first pair of diodes before the Depth pot turn the triangle wave into a (roughly) sinusoidal wave. The interesting bit is the stage after the Depth pot (with diodes in the feedback loop). The job of that stage is to warp the control voltage so that the OTA transconductance varies in a way that produces a roughly sinusoidal variation in effective resistance (rather than some warped sinusoid).

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Post by plush »

That's an great remark, DrAlx.

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