I am trying to make a low pass filter to take normal audio signals and filter out the low frequencies so I can amplify them for a subwoofer. My problem is that the amp and sub act like there is no filter there. The resistor is 30R and the 2 caps are 10uF in parallel

The resistor is 30R and the 2 caps are 10uF in parallel. The whole setup is (relatively) weak audio source --> low pass --> small amplifier --> subwoofer. Any help would be appreciated.

  • \$\begingroup\$ Show a circuit diagram. \$\endgroup\$
    – winny
    Commented Sep 18, 2016 at 7:07
  • \$\begingroup\$ Did you do any maths at all? \$\endgroup\$
    – Asmyldof
    Commented Sep 18, 2016 at 7:32
  • 1
    \$\begingroup\$ I get 265Hz cutoff frequency, which is fine. And for low frequencies the impedance seen from the input should be high enough so... I don't get your point @Asmyldof \$\endgroup\$ Commented Sep 18, 2016 at 7:36
  • \$\begingroup\$ Well for me even 900hz sines will go thru to the amp and sub \$\endgroup\$
    – p7810456
    Commented Sep 18, 2016 at 7:37
  • \$\begingroup\$ Not sure if it is a good idea to use electrolytic caps in an audio low-pass. They will be subjected to AC voltages. \$\endgroup\$
    – user57037
    Commented Sep 18, 2016 at 7:38

1 Answer 1


If your Low pass Filter is something like this:


simulate this circuit – Schematic created using CircuitLab

Your Fc is calculated like this :

enter image description here

Make sure about your input because I can't see your input wired correctly . One head should be connected to the resistor and the other one should be connected under the capacitors . Output will take from capacitors .


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