RC Bass filter

Discussion in 'The Projects Forum' started by noingwhat, Sep 11, 2010.

  1. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Hi! I was going to build a RC bass filter for my speaker, and the one thing I forgot to pay attention to was the fact that I bought a polarized capacitor. Is it possible to use a polarized capacitor, and if so, which way do I wire it?
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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  3. sceadwian

    New Member

    Jun 1, 2009
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    You could use two polarized capacitors back to back, but this isn't ideal. If you're going to have to buy a new one anyways I'd get a proper non-polarized one.
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Problem is if you try and use an RC filter in the lower frequencies the value of the capacitor is huge.

    That article I linked explains how to hook them up back to back, you need two of double the value you're wanting to end up with. The link I posted before that is a supplier that caters specifically to speaker builders.

    Most low frequency crossovers just use a large inductor.
     
  5. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I do have two of the same capacitor, I had bought three because I was going to make 3 different filters, and put them on a switch so I can change it, but I got the wrong switch too, so what do I do to wire it back to back? Should I wire the two anodes or two cathodes together?
     
  6. sceadwian

    New Member

    Jun 1, 2009
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    It doesn't matter the connection needs to be isolated though and the effective capacitance will be half of the total of the two capacitance's, and the ESR will be double.
     
  7. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Sorry, I'm a noob, I got the capacitance will be cut in half, but do you mind explaining the rest?
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Put some tape around where you splice the two leads together since it won't be hooking up to anything.

    ESR = equivalent series resistance = a capacitor has a bit of DC resistance to it as well, probably pretty low on yours since you're probably using fairly high values = don't worry about it.
     
  9. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Ok, so I think I get it. But I still don't know which way to wire it. Which one of these should it be? (I put a little image as an attachment)

    Is it one of those, or do I have it all wrong?

    Thanks to all that have helped me so far!
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    An RC filter is very simple first-order so it makes a poor filter.
    A second-order or third-order Sallen and Key active filter using an opamp is a much better filter.
    Your RC filter cannot drive a speaker, it drives a power amplifier.
     
  11. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I am unsure of how to wire it, can someone please tell me?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    1) Look in Google for Second Order or Third Order Butterworth opamp Filter for your sub-woofer amplifier.
    2) Then look in Google for a Second Order or Third Order Butterworth highpass filter for the amplifier for your little higher frequency speaker amplifier.
     
  13. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I would, but I am looking to build a passive filter, at least for now. On my journey of learning circuitry, one of my goals is to understand why I am using each component in my circuit, that is why I am doing something as simple as an RC filter. I was just on my way to the electronics store (which I only go to every so often as it is quite a drive) and I had picked up the parts still not 100% sure what they did, I had an idea, but I was still reading up on it.

    So I just need to know how do I wire up two polarized caps so that it will work on this RC filter?
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    As it indicates in that article - to get a 500 uF non-polarized equivalent you'll need to wire two 1,000 uF capacitors in a back to back situation; meaning hooking both negative leads together, insulating them, then using the remaining two positive leads as your (now) 500 uF non-polarized electrolytic.

    Just realize this is only a band-aid solution, true non-polarized electrolytics are built differently so they'll last for quite a few years.
     
  15. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    So I should wire them like this, correct?

    [​IMG]
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Correct, that's how you do it.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    1000uF electrolytic capacitors are used as filters in power supplies. Then the actual value doesn't matter. So their tolerance (accuracy of the value) is very broad. Your 1000uf capacitors might be anywhere from 500uF to 2000uf so the accuracy of the frequency of your filter will be horrible.

    Where will you install the filter? It won't be a filter if it feeds a speaker because the low resistance speaker will be in parallel with the capacitor. Instead it will simply reduce the level of the speaker.

    It is a very simple filter that will not filter much. Here is an example of a single-order RC filter that feeds a power amplifier and is set to 100Hz:
    1) 100Hz is reduced to 0.707 times (-3dB) then it also reduces 20Hz a little.
    2) 200Hz is reduced to 0.5 times (-6dB).
    3) 400hz is reduced to 0.25 times (-12db).
    4) 800hz is reduced to 0.125 times (-18dB).
    5) 1600Hz is reduced to 0.063 times (-24dB).
    6) 3200Hz is reduced to 0.031 times (-30dB).
    7) 6400hz is reduced to 0.16 times (-36dB).
    8) 12800Hz is reduced to 0.008 times (-42dB). The simple "bass" filter is so poor that you can still hear this high frequency!
     
  18. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Thank you very much for all of your help everyone, and I know that this is not exactly the preferred choice for audiophiles, and is not the best as far as fidelity, it will work for a very- very small budget, (project could have been done for a quarter, try doing that with an amp)

    That is why I chose such a simple RC filter.
     
  19. bertus

    Administrator

    Apr 5, 2008
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