Passive low-pass filter

Discussion in 'General Electronics Chat' started by ElectricPotato, Feb 17, 2017 at 12:24 AM.

  1. ElectricPotato

    ElectricPotato

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    Hello, I just started learning about electronics so don't be too harsh on me if my question is stupid.
    I wanna drive a subwoofer and since I'm a beginner I decided to use an pre-made amplifying board. That works wonderfully and at a decent volume but I figured out it would sound a lot better if I used a lowpass filter to cut off anything above 150Hz from the input signal first. I researched how to build a passive one and it turns out it's quite easy.
    The filter itself works but it reduces my volume tremendously to the point I can barely hear it at max volume. I tried different combinations of resistors/capacitors that would get me the same cutoff frequency but it was still really quiet.
    My attempts were 1kOhm resistor & 1uF capacitor, then I tried 1Ohm & 1000uF and some other values that have a cutoff frequency of roughly 150Hz.

    Did I do something wrong and is there anything I can do to increase the volume while filtering?
     
    ElectricPotato, Feb 17, 2017 at 12:24 AM
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  2. ElectricPotato

    BobK VIP Member

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    First, you need to show us a circuit. We cannot assume what you did.

    If you are using a 1K resistor in series with the speaker, or course it will reduce the volume to nothing. With a 10 ohm, it will still reduce it to less than half. The speaker itself would be the R in a first order RC filter, there would be no other resistor.

    Bob
     
    BobK, Feb 17, 2017 at 12:45 AM
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  3. ElectricPotato

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And it may be easier to place the filter before the amplifier, assuming the amplifier is for the subwoofer only.
     
    (*steve*), Feb 17, 2017 at 1:59 AM
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  4. ElectricPotato

    Audioguru VIP Member

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    A filter is connected to the input of the amplifier, not the output. 1k ohms in series with a 1uF capacitor to ground reduces high frequencies but is too simple to cutoff anything. 160Hz will be reduced a little (-3dB), 320Hz will be reduced a little more, then more gradual reduction of higher frequencies where 640Hz (a low midrange frequency) will sound at half volume (-12dB). Even frequencies for tweeters will still be heard. An active filter with two or three RC networks is usually used.
     
    Audioguru, Feb 17, 2017 at 2:06 AM
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  5. ElectricPotato

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Like crossover filters? ;-)
     
    (*steve*), Feb 17, 2017 at 3:03 AM
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  6. ElectricPotato

    ElectricPotato

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    I connected the filter before the amplifier from the beginning because I didn't want to put too much of a load on the amplifier output. Also Bob, thanks for pointing out the speaker itself could be that R in my filter, putting an according capacitor in parallel with it filters out some of the low frequencies.

    Currently my raw signal from an audio jack goes into the amplifying board, then the output goes to the speaker and the capacitor and this works to some extent. I might work on that RC network Audioguru recommended but I have to learn how to make them first. By active filter you mean using an opamp?

    Also is it okay to just leave a capacitor at the output of my amplifier, that won't hurt the circuit, right?
    And one last question, for this purpose does it matter that my capacitors are polarized?

    Thanks for your time guys, I really appreciate it!
     
    ElectricPotato, Feb 17, 2017 at 3:11 AM
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  7. ElectricPotato

    Audioguru VIP Member

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    An active crossover filter avoids the problems of passive filters at the speakers. The resistance of the inductor in a lowpass passive filter ruins the damping of resonance from the amplifier and the result is a bongo drum at one frequency.
     
    Audioguru, Feb 17, 2017 at 3:12 AM
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  8. ElectricPotato

    Audioguru VIP Member

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    A capacitor at the output of an amplifier (across the speaker) is a short circuit to high frequencies and many amplifiers blow up when driving a short circuit.

    Audio is AC. A polarized capacitor is used only with DC and it, the amplifier or both might blow up when it is fed the AC from an amplifier.

    More than one passive filter in series results in a droopy frequency response affecting the frequencies you want to pass. An active filter uses an opamp to keep the frequencies you want but creates a sharp cutoff.
     
    Audioguru, Feb 17, 2017 at 3:27 AM
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  9. ElectricPotato

    ElectricPotato

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    I wish I read that a bit earlier because I manged to fry my amplifier after a bit. Good thing it wasn't that expensive. I'll try again with a new one and an opamp active filter, thanks for helping!
     
    ElectricPotato, Feb 20, 2017 at 1:43 AM
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