Variable filter methods

Discussion in 'General Electronics Chat' started by Veracohr, Mar 2, 2012.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I'm working on designing an audio line mixer (well, it's been on the shelf for a few months but I'd like to pick it up again) and I'd like to put in variable 4-pole high pass filters that would be controllable from a single knob to two filters (a stereo signal).

    After I learned a bit about MOSFETs in a class last term, I tried simulating a filter with a MOSFET as a voltage controlled resistor, but I couldn't get it to work at a low enough frequency due to the limited resistance range. Since this is for audio applications, I'd like a filter that can go at least down to 30Hz, possibly as low as 20Hz. Upper limit isn't as critical, probably somewhere around 500Hz. I thought if I replaced the resistors in an active filter design with MOSFETs, and controlled those with a single source, it would accomplish it.

    What other options would I have for varying two 4-pole filters with one control, without a lot of deviation in frequency between the two? I could go with an OTA-based filter, but that requires one OTA for each stage. Plus, a lot of the OTA filter info I've seen is kind of confusing. Any other common methods that could maybe use fewer components?
     
  2. Audioguru

    Expert

    Dec 20, 2007
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    A Mosfet of one part number has a wide range of spec's so matching two or four to make an adjustable frequency filter is almost impossible.
    All IC tone controls filters have matched transistors. Most are single pole, a few are two poles but none are four poles.
     
  3. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    So no suggestions? I'd already ruled out the MOSFET idea.

    The only other idea is cascading 4 stages like this one from the NE5517 datasheet, with Vc of all 4 fed from one source:


    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I was under the impression switched-capacitor filters weren't generally used in audio applications because of noise.
     
  6. Audioguru

    Expert

    Dec 20, 2007
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    My very low distortion oscillator uses a switched capacitor filter IC and the noise level is not measureable. The clock is 100 times the cutoff frequency so a simple single RC removes its already low level.
     
  7. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    Interesting....thanks. I'll look into that.
     
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