What type of cap for a low pass RC filter?

Discussion in 'General Electronics Chat' started by hrs, Nov 10, 2015.

  1. hrs

    Thread Starter Member

    Jun 13, 2014

    I'm working on ESP audio amplifier project 72 but with my own pcb. In the project desciption it says that it's the same as the application note "and with added RF protection at the input". My guess is that this is a low pass filter to keep out RF picked up from wherever. I also guessed a red square around the filter in the attached diagram. With R = 1 kohm and C = 220 pF this works out to a frequency of about 0.7 MHz. Any corrections on this guess work are appreciated.

    It seems that 220 pF limits my options in selecting a type of through hole capacitor with a lead spacing of 0.2 inch / 5.08 mm (the holes are already drilled). There are:
    - ceramic disc
    - mylar film

    In an oscillator project I did ceramic disc caps were not the way to go. It was pointed out to me that the capacitance changes with applied voltage(!). And when I measure the capacitance of some of the ceramic disc caps that I have, all of them are way off spec (100% or more). All of this leaves me with a mediocre impression of ceramic disc caps and I'm reluctant to use them.

    Since I can't find any "ordinary" 220 pF polyester film box caps, are they different from mylar film caps? 220 pF mylar caps are readily available.

    A filter frequency of 0.7 MHz seems rather high. With a 1 nF polyester film box cap (the smallest I can find) the filter frequency would be around 160 kHz. Is that too close to the audio range or would it be fine?

    What would you recommend?
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    You need to be a little careful with caps. It's not to unusual to see tolerances of +100/-20%.
    But having said that film will be better than ceramic for an audio application. Any Mylar cap would be ok.
    You could move it a little closer if you like.
    hrs likes this.
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    I would have used a ceramic cap. Is is true that we should avoid the ceramic capacitors in the signal path.
    But in this case the cap is smaller than 1nF so we can use a "good" ceramic cap from Class 1 ceramic.
    Because almost all capacitor smaller than 1nF are made from Class 1 ceramic (NP0/C0G).

    And, yes you can use 1nf (MKP) instead of 220pf capacitor.
    Last edited: Nov 11, 2015