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I am trying to build a LFO circuit that produces a wobble bass effect.

My overall plan is using an Op Amp to create the LFO and another Op Amp to create the low pass filter. How should I connect these two stages? I know that the LFO part will output a square wave, but how do I use that wave form to change the cutoff frequency of the low pass filter?

I know that the easiest way to change the cutoff frequency is to change the resistance, but how do I achieve that with the out put from an Op Amp LFO?

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  • \$\begingroup\$ If your architecture is indeed feasable (need to think about that a little more), then it sounds like you mustn't want to use a square wave LFO, but rather a sine wave oscillator. The obvious answer here would be to solve it digitally (not that I know how to implement a 'wobble filter') \$\endgroup\$
    – jippie
    Feb 3, 2013 at 20:29
  • \$\begingroup\$ Hi, yes you are correct on the sine wave, since it needs to be a gradual change. I am trying to learn how to use Op Amps and I was able to build a LFO and LPF separately. I am just stumped on how to combine these two parts to achieve the wobble bass effect. I realized that connecting the output of the LFO to the neg input of the LPF Op Amp will only change the DC gain, so it will not achieve the target effect. Are there any chips or circuits out there that converts voltage into resistance? (digital potentiometer?) \$\endgroup\$
    – tincan
    Feb 4, 2013 at 1:52
  • \$\begingroup\$ There is such a thing as a digital potentiometer, but I think it would make a strange design. It would be useful if you could post at least a block diagram of your circuit or a description of how a wobble filter works? \$\endgroup\$
    – pjc50
    Feb 4, 2013 at 17:25

3 Answers 3

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What you are looking for is a Voltage Controlled Amplifier (VCA), or a Voltage Controlled Filter (VCF) (which is often implemented using a VCA). These are classic synthesiser circuits and you should have no trouble finding example circuits from these search terms.

A reasonably good inexpensive chip providing dual VCA circuits is the LM13700 whose history is described here

Circuit here may help you.

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You will need the above mentioned LM13700/13600 operational transconductance amplifier. Check out the datasheet, they have application examples such as VCF or VCO. The OTA converts voltage to current and allows you to control filter, oscillator, etc. with voltage. Unfortunately VCA, VCF and VCO all use current (resistance), hence the need for an OTA. You may try JFETs, they can convert voltage to current but have a non-linear response and no two JFETs are the same (due to the inherent nature of the material used, no manufacturer can guarantee the same makes and types will sound the same). Probably vactrols are the easiest option, check this link. Vactrols are easy to make, cheap but have non-linear response and can be slow.

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The 'wobble bass' effect is made by modulating(or fastly moving back and forth) the cutoff frequency of the VCF. This can be done manually with a potentiometer, using an LFO circuit, or using a control signal generated digitally (possibly the waveforms generated by the Mozzi library for the Arduino).

If you want a greater understanding of this systems dynamics, might I suggest buying a Korg Monotron. It features a sawtooth LFO that modulates the VCF cutoff frequency. If you set the synths pitch as low as it will go and then experiment with adjusting the LFO speed, you'll see it in action.

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