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Old 11-16-2011, 10:10 PM
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Default How are VARIABLE low pass filters with more than one pole achieved in practice?

I would like to make a 4-pole, 2-stage filter for audio, but I want it to be variable. As it stands, I would need a 4-gang potentiometer to handle this. I do not believe such a thing exists, or if it does, it would be very expensive.

How can I achieve a variable critical frequency? My other idea is to investigate digital potentiometers.
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Old 11-17-2011, 05:27 AM
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Variable multi-pole analog filters are complex by nature. There is no easy way to build one. One example is a Moog type filter used in synthesizers.

Certainly you might also be able to vary the several pots simultaneously by using digital controlled pots. But I believe most digital pots require one connection to ground so would not work where you need a floating pot.

A somewhat easier way to make a variable audio filter is to use a switched-capacitor design such as the LMF100 from National. The center frequency can be readily changed by altering the filter clock frequency.
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Old 11-17-2011, 07:53 AM
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Look at schematics for parametric equalizers.

2 double pots (for stereo). One tunes center frequency, other adjusts gain level. High grade op amps are used as the active part.
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Old 11-17-2011, 10:23 AM
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Thanks! I think a parametric EQ is what I was looking for, I just didn't know what it was called. I gather that a good Parametric EQ would have many bands and thus many knobs, I would settle for one, as this is more of an effect than for the purposes of tuning the sound quality.

What op-amps would you recommend, or alternately, what parameters am I looking for in a better one? For the moment I am using the LM324 quad op-amp. THey have been satisfactory as a comparator and signal amplifier, but so far my experiments with sound have been very tinny and distorted despite using low gain.
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Old 11-17-2011, 10:24 AM
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I should state my entire purpose for this project: I have a school presentation to do on op-amps, and I thought this would make the demonstration a little more lively. If anyone else has other ideas of things I can do with op-amps that make a good demonstration for non-electronics people, I AM ALL EARS
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Old 11-17-2011, 04:03 PM
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The LM324 is not a good op amp for audio. You should use a low-noise op amp such as the TL071 and OPA134 single, TL072 and OPA2134 dual and TL074 and OPA4134 quads.
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Old 11-18-2011, 03:55 AM
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4 band parametric EQ Schematic PDF

Each band has Q, Boost/Cut, and Frequency controls, so with some surgery, you could make a 1 or 2 band EQ from it.

Not the 18V Supply, total of 36V
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Old 11-22-2011, 06:14 PM
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Thanks for the help guys, I have completed a breadboard version of a mono, one-band parametric equalizer based on the schematic at the URL below. So far, it works, but I am having some issues with distortion, despite using much better op-amps (TL072). I will have to wait until I get some lab time to check things out with the function generator and scope, and see if I can't get some better performance out of it.

This is going to be a fun presentation!

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Old 11-22-2011, 06:52 PM
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You can't properly prototype audio gear on bread board. You've probably got several 'earth loops'
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Old 11-22-2011, 06:58 PM
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I used to have a thing for VERY loud music. I had a neighbour wanting to throw a chair through my window at one stage, and he lived a few miles down the street. This thing was so LOUD. Whole house shaking. BOOM, BOOM, BOOM.
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