How are VARIABLE low pass filters with more than one pole achieved in practice?

Discussion in 'General Electronics Chat' started by lowprofile, Nov 16, 2011.

  1. lowprofile

    lowprofile Thread Starter Member

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    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.
  2. crutschow

    crutschow AAC Fanatic!

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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.
  3. thatoneguy

    thatoneguy AAC Fanatic!

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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.
  4. lowprofile

    lowprofile Thread Starter Member

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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.
  5. lowprofile

    lowprofile Thread Starter Member

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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 :)
  6. crutschow

    crutschow AAC Fanatic!

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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.
  7. thatoneguy

    thatoneguy AAC Fanatic!

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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
  8. lowprofile

    lowprofile Thread Starter Member

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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!

    [​IMG]
  9. T.Jackson

    T.Jackson New Member

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    You can't properly prototype audio gear on bread board. You've probably got several 'earth loops'
  10. T.Jackson

    T.Jackson New Member

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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.
  11. lowprofile

    lowprofile Thread Starter Member

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    I was wondering about that. I have heard that there is a lot of stray capacitance due to the nature of breadboards. I will have to do some research about "earth loops". I do have everything tied to the same ground, but could there still be earth loops?


    What is the preferred method for prototyping audio stuff?

    I want to lay this out on some perfboard as a more permanent solution. Are there any major layout considerations? In fact, any advice you have on this subject would be greatly appreciated.
  12. thatoneguy

    thatoneguy AAC Fanatic!

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    A Ground loop/Earth Loop is created when your amplifier becomes a path to ground from either the device feeding it, or the device after it in line. Think of it as the ground prong of an AC outlet not being present, but using the sheild/outside ring of all the RCA jacks to keep them at the same potential.

    This happens when outputs use floating grounds and another device uses earth ground. or a different voltage for floating ground. In earlier times, each component used a transformer between units/areas to get rid of any DC bias. today, we use capacitors. Assemble the circuit to see if it is a problem before changing stuff around to fix a non-existent problem. The filter above is just that, a filter, you WILL need a power amplifier after it to have any volume without distortion.

    Google will have a ton of information on the topic of audio ground loops, make sure you end up on circuit design pages and not "audiophile" pages, you can tell you've found an audiophile site when tubes, silver, gold, and oxygen free are mentioned a lot, as well as $500 10uF capacitors, and most signature lines will have a list of equipment they own. Usually ESP will have the article you need when it comes to amps, but I didn't find it in a glance. However, DO bookmark that site, as it is a cornucopia of information about amplifiers of every type, filters, power supplies, and a ton of other topics frequently discussed here.
  13. Audioguru

    Audioguru New Member

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    Most good quality audio opamps like the TL07x oscillate when driving the capacitance of a sheilded output cable. Use a 100 ohm resistor in series from the output to the cable to isolate it.

    I agree that a breadboard is no good for audio or for RF.

    If the output is too loud then simply turn down the volume control.
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