Subwoofer Passive Low Pass Filter

Discussion in 'The Projects Forum' started by DeathSurfer, Dec 6, 2015.

  1. DeathSurfer

    Thread Starter New Member

    Dec 6, 2015
    Hi ...

    I'm building a subwoofer for my car, a Jeep which has space issues. I have solved the physical and audio (driver) part of the design. I could just allow the subwoofer amplifier do the filtering, but I'm considering adding more sound shaping to my subwoofer box with a low pass filter. When you research these, the general format is this: positive lead --> coil (or resistor) --> (fork) capacitor shorts to ground in parallel with the speaker driver.

    1)why is there the need for the coil BEFORE the parallel fork in the circuit? Aren't you just choking off the HFs that you are right after that going to bleed off (by pass) with the capacitor?

    2)why have the coil there at all? Won't the capacitor, sitting in line with the speaker, always present a load to the audio signal? For frequencies at 'x' or below (where the capacitor resistance becomes greater than the speaker's resistance), the LF signals will go through the speaker and create a load for the amp. The highs will pass through the cap with virtually no resistance, but the LF will still present a load to the amplifier.

    3)is the issue protecting the amp from a below 1 ohm load? I'm going to have 2 4 ohm drivers in parallel creating a 2 ohm subwoofer. If I add a bypass cap across the positive to ground (just before the positive driver lead), will this bring the total resistance to an unstable value? Is this why you see the coils and or resistors in series before the capacitor bypass line to ground?

    4)given the state of subwoofer amplifiers for cars, is the low pass circuit in the sub box overkill, and not worth the trouble and possible stupid factor in adding it?

  2. Veracohr

    Distinguished Member

    Jan 3, 2011
    An inductor and capacitor make a second order filter, which has a steeper rolloff than the first order circuit made by a resistor and capacitor.
  3. DeathSurfer

    Thread Starter New Member

    Dec 6, 2015
    But why is the coil put in line before the fork of cap & speaker (which are togerther in parallel)? Why isn't the coil simply put in parallel with the cap? Seems to me this config would "push" more highs through the cap bypass.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    OK, lets look at it from a different point of view.
    An inductor can be viewed as a frequency variable resistor. As the frequency go down the resistance goes down. So, for your low pass filter this is a no brainer.
    A capacitor can also be viewed as frequency variable resistor but as the frequency goes up the resistance goes down. (Opposite of the inductor.) This is a good part to prevent highs from getting to the sub-woofer.

    The reason that you can not have just the cap across the sub (without the inductor) is that the cap will look like a short when the frequency gets high enough. Not good for an amplifier.

    So, the inductor and the capacitor work together to send only the low frequencies to the sub and not short the amplifier at high frequencies.

    Make sense?
  5. Veracohr

    Distinguished Member

    Jan 3, 2011
    If I understand what you're saying, you're talking about the bottom circuit here right?

    (8 ohm resistor represents a speaker load)

    Screen Shot 2015-12-07 at 2.36.43 AM.png

    As you can see by the V(2) trace, it's a bandpass filter. This is because the capacitor is shunting high frequencies to ground while the inductor is shunting low frequencies to ground.

    Like Lestraveled said, think of them as frequency-dependent resistors. In the top circuit, the inductor at high frequencies is a high impedance while the capacitor is a low impedance, so in this arrangement they make a voltage divider with significant attenuation. At lower frequencies the impedance of the inductor goes down and the impedance of the capacitor goes up, so the attentuation goes down.

    Again I assume your description is as in the bottom circuit here:

    Screen Shot 2015-12-07 at 2.52.07 AM.png

    In this case the low-pass filter is formed by the output impedance of the driver (which I have entered 8 ohms for in the signal source), and the capacitor. You can see in the plot that V(2) (no inductor) has a more gradual slope than V(1). Whether this is acceptable is up to you, but a 6dB/octave rolloff usually isn't enough for most people in audio crossover designs.

    I don't know anything about subwoofer amplifiers for cars, but I'm going to assume they're designed to take a full range signal and do the filtering internally before sending a signal to the speakers. In that case it wouldn't make much sense to pre-filter it.
    Last edited: Dec 7, 2015