Low pass filters, who makes them, and which ones are good?

Discussion in 'Effects [BG]' started by chunky, Nov 19, 2004.

  1. chunky


    Nov 3, 2004
    Portland Oregon
    So, I'm looking at setting up a dedicated sub for my bass rig, and I've read that for optimum preformance, I want to run a 100 hz low pass filter before the sub. Bag End has one for about $2,000 retail. I'd like to think that that isn't my only option. Any suggestions?
  2. Passive low-pass filters? In between the amp and the sub?

    I believe Parts Express sells a low-pass filter for about $6.....it's just an inductor and capacitor.


    Even if you want an active crossover, before the amp, you can find a decent crossover for a couple hundred bucks.
  3. chunky


    Nov 3, 2004
    Portland Oregon
    This is awkward, because I'm asking a question about a field I know very little about. Perhaps I could use an active crossover. What would an active crossover, or a $6 low pass filter fail to do that this Bag End thing does?

  4. I guess I'm not sure what you want to accomplish. What's your setup now--a head and a cab? And you want to add a sub? Or do you want to go with a bi-amped or tri-amped setup?

    Here's the difference. Let's look at a normal 2 way home stereo system, with a woofer and a tweeter, playing pre-recorded songs. The music contains frequencies from around 30 hertz, give or take a few, up to maybe 18,000 Hz. A normal woofer can reproduce frequencies from maybe 45 Hz, up to 2500 or maybe 3500 Hz. A typical tweeter handles frequencies between 2000 and 18,000 Hz. So inside the speaker cabinet, the speaker builder installs a passive crossover. The signal from the amp contains the full range of frequencies, but the crossover inside the speaker divides low frequencies to the woofer, and high frequencies to the tweeter. It's called passive because there's no external power source, the components in the crossover (inductors and capacitors) divide the frequencies.

    An active crossover is used to make a bi-amped (or tri-amped) system. In this case, the signal is split into low-and high-frequency signals before the signal reaches the power amp. Since there are now two (or three, if tri-amped) signals, there must be two (or 3) amps, each amp is dedicated to a particular range of frequencies, and the amp is dedicated to the particular speaker that is designed to handle that range of frequencies.

    For high-end home stereos, this is good, because passive crossovers in speakers can add some coloration to the signal. An active crossover before the amp can divide the frequencies more accurately.

    Subwoofers are designed to handle frequencies below 500 Hz, give or take a few. A low-pass filter is just half of a 2-way passive crossover, it only allows low-frequency signals to reach the subwoofer.

    Remember I said home stereos have a full range of frequencies. Your bass isn't producing a lot of frequencies above 1000 hz, there's some but not much.

    Now let's go to the bass amp setup. In my opinion, if you have a simple, single-channel head with a cab, and you want to add a separate sub (like a PA subwoofer cabinet) .... A cheapie $6 low-pass filter would be all you would ever really need, if it's even needed at all. (You're not going to be putting 10,000 Hz signals into the sub.) If you have a single channel amp, this is the route for you. You plug your regular cab into the speaker output jack head, then you also plug the sub into the head's other speaker output jack (most heads have two speaker output jacks).

    If you want to go bi-amped, you need (in order of signal flow, from the bass to) 1) a preamp, 2) an active crossover, and 3) a power amp with two channels. The preamp boost your bass signal to "line level". The crossover divides the signal into two parts and sends the two signals to the two channels of the power amp. One channel will be the upper frequencies, the other channel of the power amp will be the lower frequencies (the sub). The crossover frequency is set by a knob on the crossover.

    I've run bi-amped setups for bass. IMHO it's not really worth the effort, but it's kinda fun to try.

    Remember too, PA subs really don't go any lower than a typical bass cabinet, so you may not hear a difference!! Car subs or home theater subs do go lower, but they are usually much less efficient, which means a huge power amp.

    Finally, many club situations do not lend themselves to low frequencies. The acoustics of the room usuallymeans that excessive bass (especially below 40 Hz) creates a very muddy or boomy sound.

    Hope this makes things more clear.
  5. chunky


    Nov 3, 2004
    Portland Oregon
    Crystal clear. Thanks.