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capacitor-resistor high pass/low pass filters

Discussion in 'Burnt Fingers DIY Effects' started by boozername, Aug 31, 2013.

  1. boozername

    boozername TDPRI Member

    20
    Jul 9, 2013
    west coast
    http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/highpass/hp.gif

    does anyone know if i could put 2 of these in series, or one after another? would it change my desired frequency cutoff/break away point by using 2 of the same value vs using one? i am wanting to make a high pass filter for an upright bass at about 40hz using a 22uf cap and 200ohm resistor.
     
  2. limbe

    limbe Tele-Afflicted

    Sep 4, 2009
    Stockholm,Sweden
    What is your intention?To roll off the low end below 41 Hz which is the frequency of the low E on a bass?
     
  3. luckett

    luckett Tele-Holic

    897
    Jun 14, 2011
    United States
    Your frequency will be the same, but it will be 12db/octave rolloff with two filters rather than 6db with one.
     
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  5. boozername

    boozername TDPRI Member

    20
    Jul 9, 2013
    west coast
    limy yes that is my intention. when slapping on an upright bass the thud on the fret board is way too loud.

    luckett thanks for the info.

    do you guys think i could make up for lost gain with the gain on the amp, or do you guys think i'd need to pre amp the signal after 2 or 3 stages of filter? i guess i could try it without any op amps first right?
     
  6. poiureza

    poiureza Tele-Holic

    989
    Apr 19, 2012
    Belgium
    Yes 200Ohms +22uF will give you a 6dB/octave roll off @ 37Hz.
    If you put 2 of these in series, the roll off would be 12dB/oct (i.e steeper), the corner frequency would not change.
    Each filter stage adds another 6dB to the roll off.
    (putting them one after another is called putting in series, there is no use trying to put them in parallel)

    I don't know your circuit but I would suggest a large resistor and a small cap, it is usually a better match for most audio circuits.
    I'd use resistors at least above 10K (e.g. 50K + 0.1uF would give 32Hz etc ...)

    Corner frequency is given by 1/(2*Pi*resistor*capa)
    If I wanted noticeable rolloff @41Hz I'd choose a corner freq of about 45-50Hz because the rolloff is only 3dBV @ the corner freq.

    I doubt you'd need to recover the voltage drop with an additional preamp.
    A slight twist on the gain knob will do
     
  7. poiureza

    poiureza Tele-Holic

    989
    Apr 19, 2012
    Belgium
  8. boozername

    boozername TDPRI Member

    20
    Jul 9, 2013
    west coast
    thank you poiureza very informative. i think i will take your advice and use a .1uf ceramic instead of the 22uf electrolytic. im guessing the electrolytic would introduce slight latency or alter the circuit due to different charge and recharge rates? i think i will set my frequency breakaway a little higher as you advised.
     
  9. poiureza

    poiureza Tele-Holic

    989
    Apr 19, 2012
    Belgium
    My impression is that you'll need to play with resistor and/or capa values until you like what you hear. Theory is one thing, reality is another.

    Tune it with an external circuit where you can easily subsitute values previously to soldering anything.
    You could maybe use a 50-100K pot for the resistor and sub capas in and out until it's OK.
    This "breadboard" would also be useful for checking how many stages you need : I'm not sure you'll hear much difference (if any) between 6dB/oct vs 12-18dB/oct.
    Keep the corner freq formulae in mind so you keep things in the right ball park.

    Alas I have no knowledge regarding latency due to cap values ...

    Good luck :cool:
     
  10. ctte2112

    ctte2112 Tele-Holic

    582
    Jun 19, 2011
    USA
    2 would give it a 12 db slope, 3 would be 18 db, etc, but the frequency would be the same. A low pass and high pass in series makes a band pass. I'd use a pot for the frequency, or at least a switch.
     
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