Low pass audio filter placed after amplifier

Discussion in 'The Projects Forum' started by Enigmatic Entity, Mar 4, 2009.

  1. Enigmatic Entity

    Thread Starter Member

    Feb 16, 2009
    I wish to make a simple (passive?) low pass filter that is placed after the amplifier, i.e. somewhere along the speaker out wires. At the moment there are problems with the resistor reducing the volume level too much. The capacitor seems to do nothing. I used the 1/2piRC formula to get values for 2.4K resistors and 470nF capacitors. From what I have discovered in my projects, passive low pass filters only work before the amplifier stage and high pass filters work only after the amplifier stage (just a capacitor in series seems to work). This is really quite annoying and somewhat confusing.

    Does a low pass filter work when placed after the amplifier stage, or am I doing something wrong?

    Any help greatly appreciated,

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You want to do such filtering before you put the amp to the trouble of making what you don't want bigger (and harder to remove). All filters work just fine before the power amp.

    I think the high pass function you are talking about is to selectively pass the treble to a tweeter. Possibly these filter functions are part of an audio crossover network?

    Having some idea of what it is you are trying to do would help a lot.
  3. Enigmatic Entity

    Thread Starter Member

    Feb 16, 2009
    I'm just trying to make a "cheap" (in both ways?!) low pass filter for some speakers so that they act like subwoofers. The amplifier unit is part of a car radio/tape player, which is why I was trying to make a filter to put in after the amplifier, since there are only two speaker outputs. Listening to just the bass line is not the most appealing thing! (which would happen if I tried a "before amp" filter.

    I haven't tried looking for when the "radio part" of the circuit turns into the "power amp" part. On a side note, I modified the input from the tape player (which doesn't work, not that I have many tapes) to take audio from a 3.5mm jack. At least this part works well, you just have to insert a blank (or any) tape to switch from the radio.

    The reason why I'm attempting all these modifications is that I currently don't want to spend a lot of money getting a new head unit, dedicated sub-amp and drivers.

    If you need any other info, please tell me,


    Actually, would it be possible to make a high pass filter and then invert the treble back into the original signal? I have a feeling that this might not work...(sorry, I'm having a bit of a mind blank at the moment).
    Last edited: Mar 4, 2009
  4. Audioguru


    Dec 20, 2007
    A woofer uses an inductor to make a very simple lowpass filter that has a very gradual slope. Adding a capacitor makes it a sharper LC lowpass filter. Using two inductors with one or two capacitors makes it sharper still. Look at Passive Crossover circuit in Google.
    But the low frequency for a sub-woofer requires huge inductors.