# Low Pass Filter for subwoofer

Discussion in 'Electronic Basics' started by Andrew Howard, Mar 3, 2004.

1. ### Andrew HowardGuest

I'm thinking of making a low pass filter to use before an amp, so it can
drive a subwoofer. The most economic design seems to be the RC one. The
input is from a computer soundcard, the output is to a 10W amp driving a 4"
speaker. The questions are:

What values are good for R and C to get a good cut-off point for this
speaker?

How do you find this? (I think it is 1/2piRC, but I don't understand
this very well, and anything I put into it comes out completely off.)

Does it matter what wattage the resistor is rated at? (can I use a
standard pot?)

Thanks
Andrew Howard

2. ### Bob MastaGuest

Let's say you want a cutoff of 100 Hz (about right for subwoofers
in general, but with only a 4" driver this might be lower than it can
handle.) Start by picking a value for C, since the R is easier to
adjust. Try 0.1 uFd and rearrange the formula to

R = 1 / 2*pi*f*C = 15915.5 ohms

Note that C is in *Farads*, so you have to use 0.1*10^-6
in the formula. I'm guessing that's where you went wrong.
You can use whatever C values you have on hand and
compute the resistor, or use a pot as you suggest.

This will not be a very sharp filter. The output will only
fall at -6 dB/octave maximum. You may want to look into
"active filters" using op-amps for a steeper response
if this one isn't acceptable.

The other reason yo might want an active filter is that
the simple RC type is sensitive to the source and
load impedances, whereas the op-amp type
can be made immune. In the above example, the
output impedance of the source will be in series
with R, and the input impedance of the load will
be in parallel. So it will work fine driven by a low
source impedance and feeding a high amp impedance.
The sound card output impedance is probably
low enough to ignore here, but the amp input
impedance may be in the range of 50 K or so,
so you'd need to tweak the R value.
Wattage requirements are very low since this will go before the amp.
Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

3. ### Andrew HowardGuest

Thanks for the help so far, but I just have one more question.

The capacitor is bi-polar right?

Andrew Howard

4. ### Bob MastaGuest

Yes, and at the low capacitor values you will be using
that's probably the only type you will find. Once you
get to the 1.0 uFd range or above you will find polarized
types the norm. You don't want or need those here.

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com