Standard ways to make a variable low pass filter

Question

I tried out a basic low pass filter on one of my analog synthesizers:

^{simulate this circuit – Schematic created using CircuitLab}

and this seems to work. I was wondering how I can create a variable low pass filter. I first thought that I would replace the capacitor with a variable capacitor, but I have a hard time finding these at the range of about 470 nF. Then I did replace the resistor with a variable resistor and this seems to work somewhat. But I am wondering if it is just that simple. My question is therefore:

What is the/a standard way of making a variable low pass filter? (used for synthesizers.)

Answer 1

You can use a digi-pot to make the resistor variable. Then select multiple capacitors that have values that define the filter ranges that work with the resistance range of the digi-pot. Use an analogue mux chip to select which capacitor is switched into the filter circuit at any given time.

Deploy a microcontroller to control the pot and mux. The MCU can also support the user interface of your choice.

Answer 2

One method I have seen used in bass guitar add-on circuits is a PWM modulated R using a CMOS transmission gate.

If you choose the frequency such that it is not audible and vary duty cycle,d then the effect Reff=R/d.

This results in a reduction of f(-3dB)=1/{2piReffC}, but also a rise in output impedance so it must be buffered.

A simple vbl. PWM can be made by offsetting the DC bias input towards Vcc/ or Vcc*2/3 of a CMOS Schmitt Relaxation Osc.

Answer 3

Your varipot approach works "somewhat" because the simple filter you built (one-pole) doesn't have a very steep drop-off. For the fairly narrow band of the audio signal, what you're trying to do will yield mediocre results.

You're better off making a few higher order filters, and using a multi-position switch to switch between them.

Alternatively, you can buy reasonably-priced switched-capacitor filters (sort of what @TonyEERocketScientist was talking about). Such devices use the frequency of a square wave clock to set the cutoff. You would then need to generate a clock signal, maybe with a microcontroller or 555 timer. For an example part, there's the MAX7490