Introduction: Build Your Perfect Subwoofer Filter / Low Pass Filter

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About: Maker ,Observer ,Reverse Engineer .Everything but theoretical.

Audiophiles like me can't get the full experience with stereo speakers ,we want the deep heart throbing bass. Hooking up the subwoofer box to a simple amplifier wont help .To filter out all the high frequencies and allow only low frequencies we need a filter called a low pass filter. This filter can be designed using minimum 2 components .Maximum count is not defined as you will notice below.

People who are doing this for the first and last time can continue with the instructable ,while people who want to know more about filters and its applications can take reference from this video :RC low pass filter

Step 1: Finding the Sweet Spot by Simulating the Filter

  1. Open Audacity. (download from here Audacity)
  2. Import a music that matches your taste.
  3. Connect your amplifier to your PC and subwoofer to the amplifer. Remove all the crossovers and pre amps ,they aren't required for this step.
  4. Select the full song (or Ctrl+A).
  5. Go to Effect > Low pass filter.
  6. Play your audio and experiment with different values of cutoff frequency and Roll off till you get the right amount of bass.
  • Keep cut off frequency between 30-100Hz
  • Roll off refers to orders of filter .It basically means how sharp is the bass response. Higher order filter will have more punchy bass but lower gain .The transition curve for high and low frequency is more steep in higher order filter as can be seen in the Bode's Plot.For example ,a 3rd order filter will provide more bass but will require more amplification to reach the same volume level as a 2nd order filter.
  • Depending on my subwoofer enclosure and type ,I will use cutoff freq of 80Hz and filter of order 3 i.e 24dB rolloff.
  • After applying the 3rd filter you will notice that the sound is considerably reduced.So at this point we need to amplify it.Go to Effect > Amplify and experiment with new values or better set the New Peak Amplitude to 0.0dB (Any more than this and the sound will start clipping).
  • So what we have learned from the software simulation is that we need a filter and a pre amplifer ,then we can feed this signal to our amplifer and get the required bass.
  • After deciding these two values (cuoff freq and rolloff) test it with subwoofer write them down somewhere and head to Low pass filter calculator.
  • On the website enter the values of desired cutoff frequency and any one of the other two values .( I would suggest enter the resistance value of whatever resistor you have lying around and then calculate the capacitance required).
    • On using resistance of 1 Kohm I got capacitance = 1.99uF ~ 2uF.

Step 2: Making the Filter

  • Find a capacitor and a resistor of above mentioned value and place them on the breadboard as shown in the figure.
  • Connect your sound source and amplifer with subwoofer and you will notice that the bass is more dominating over the normal frequencies but there is still some normal sound that you dont want your subwoofer to handle.This is because the filter you just made is a 1st order filter and as we saw in Audacity we need a 3rd order filter.
  • To get a third order filter we need to cascade our filter three times.
  • Another form of a 3rd order filter is a Sallen Key Low Pass Filter which is more complex and requires an op-amp but is more efficient and doesnt require a pre amp as the gain can be set as required. Sallen Key filter
  • Finally after assembling everything, tuck and hide all the wires, put on some good music and relax and forgive me for that intro picture and awful demo video.. XD

Step 3: Result