Passive bandstop filter for ~ <220Hz

Discussion in 'The Projects Forum' started by -Excalibur-, Sep 23, 2008.

  1. -Excalibur-

    Thread Starter Member

    Jul 20, 2008
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    I have a working TDA2822M portable amp, thanks to some of you guys around here, and I was wondering if there is a simple way to eliminate the bass (or reduce it) using just a capacitor or the like. Preferably just a "to ground" filter before the amp, if not then perhaps integrated into the actual amp. I know that some capacitors have the opposite effect - letting only bass through to the amp, since the alternating high frequencies go straight to ground.

    The reason is to reduce distortion and get more volume for the same power voltage, due to the small size of the speakers.

    Any help appreciated,

    Excalibur.
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
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  3. Audioguru

    New Member

    Dec 20, 2007
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    Externet posted a notch filter, not a high-pass filter that you want.
    How small are your speakers that distort with an input power of only 1W?

    An opamp can make a good high-pass filter but low noise ones needs a supply of at least 10V.
    An MC34071 single or MC34072 dual opamp (MC34074 is a quad) works from a supply as low as 3V but they are not low noise.
     
  4. -Excalibur-

    Thread Starter Member

    Jul 20, 2008
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    27mm from DSE. The speakers don't actually distort, but the amp does if there is a lot of bass. The system is supposed to be small, 220mm by 42mm by 60mm. And they are just as loud as the 57mm 8Ω speakers. They just don't play below 200Hz, so I thought, why not reduce those frequencies since the amp doesn't like them as much.

    The other reason I want a passive filter is so I can switch it on/off since there is a headphone output, and so I don't have to add anything else to the circuit, since it is crowded inside the box.
     
  5. Externet

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    Nov 29, 2005
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    Last edited: Sep 25, 2008
  6. -Excalibur-

    Thread Starter Member

    Jul 20, 2008
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    Excalibur detects confusion which is leading to conflict and revises his request.

    I would like to reduce all frequencies below around 220Hz. I think I got mixed up with my words since Goldwave uses the term "bandstop" which acts like a high pass when you have the low value at 20Hz and the high value at ~ whatever (220Hz).

    Sorry about the confusion. :)
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Lots easier to come up with a high pass filter that has a corner at about 200 Hz. Might as well put it down a bit in frequency, as passive rolloff ain't too steep. part of the problem is that your speakers are a low impedance, so an RC network is going to involve a small resistor and a big cap.

    Now, an active filter before the amp might be more interesting....
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I ran a quick simulation using AADE Filter Design for a Butterworth high-pass filter in a "T" configuration.
    Basically, it calls for a 15uF cap on either side of the T (in series), and a 17mH inductor to ground between the two caps. 15uF caps are no problem. The 17mH inductor is the kicker. I don't have an inductance meter around here (yet) but an inductor of that size would likely be something like a small power transformer. You wouldn't want to try winding an air-core transformer that size, that's for sure.

    But if you DID want to try winding your own air-core inductor, you could take a spool that was 2" outer diameter with flanges spaced 1/2" apart, and wind on 475 turns of #30 magnet wire (about 264 feet); that would be nearly 10.1 layers.

    But if you have a couple of spools of similar dimensions, some similarly-gauged wire, and feel like trying to wind your own, give me your numbers and I'll give you an estimate of what you can do with it.
     
  9. SgtWookie

    Expert

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    Beenthere, I tried an active high-pass filter using TI's FilterPro V2.0.
    Another Butterworth design, really not too complex, just small caps and a few resistors.

    However, the OP wanted a passive filter, so... guess what he got? :)
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Take a look at these calculators.
    http://www.lautsprechershop.de/tools/index_en.htm?/tools/main_en.htm

    You can choose between the different db/octave types.
    You just have to make the highpass part.
    You can place the filter between amplifier and speaker.
    When you take a plug with a switch for your headphone, you can switch the filter off and pass the signal to the headphone.

    Greetings,
    Bertus
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    We should not always take newbies literally. What they say they want and what they actually need are sometimes different. In this case, if you look at the context of his first post, it is clear that he needs a high pass filter. It would make little sense to notch out 220 Hz, and leave the lower frequencies to pass through his amp.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The objective is to eleminate low frequencies into the amplifier, not into the speaker.
    Then the amplifier won't be over driven with low frequencies so it can be turned up louder for the frequencies that the tiny speaker can play.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to try your hand at an active filter, see the attached.

    It's a 2-pole Bessel filter. I chose that over a Butterworth design due to the flatter phase characteristics through the passband. This would be used on the input side of your amp.

    You would need two copies of this circuit; one for each channel. A TL072 opamp would be a good choice. Use poly caps for good frequency response. Use precision metal film resistors for low noise and stability over temperature.

    Not shown are bypass capacitors across the supply pins of the opamp, the supply pins nor the supply.

    If you wanted to increase the cutoff frequency to 250Hz, decrease C1 and C2 to 82nF (0.082uF)
     
  14. -Excalibur-

    Thread Starter Member

    Jul 20, 2008
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    [qoute=Audioguru;90153]The objective is to eleminate low frequencies into the amplifier, not into the speaker.
    Then the amplifier won't be over driven with low frequencies so it can be turned up louder for the frequencies that the tiny speaker can play.
    [/qoute]

    That is what I mean,

    but thanks for the link anyway, those calculators look interesting.

    As for that Bessel filter, I may try it if it fits within the enclosure, which is allready quite full.
     
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