homemalow pass filter ??'s

Hey guys, im building my own subwoofer amp. Most subwoofers do frequencies from like 80-200hz. I know in order to achieve this, i must create a low pass filter. Would this work? Is this really all that a low pass filter is? http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/lowpass/lpf.html

Just a simple resistor and capacitor? Also where does the low pass filter go? Before the amp, or after it?
Subs typically do 20-100Hz.

That is a low pass filter, but it's not suitable for you because the rate of roll off is too shallow - it is a 1st-order filter. You need at least 2nd-order. It should go before the power amp as an active filter.

Crap, dont know what happened to the title, but what do you mean by 1st-order and 2nd-order? Could you elaborate a bit more?

Do you mean, those are the wrong cap and resistor values for my filter?

Thanks in advance!
Last edited:


2010-09-14 2:30 am
It couldn't be more wrong. Well actually that is a bit harsh I suppose.

Read up about active filters, ESP site is a good one. I also have a sub-woofer controller project on my website.

Wow, thats exaactly what im looking for, thanks! Ill look at that, and come back here if i have any questions.

Also where does the low filter go, before or after the amp? Is this the kind of filter i need?



Last edited:


2007-03-19 8:41 pm
You're probably looking for a Sallen-Key filter if you're trying to do it with just one opamp. You'll probably want a Butterworth response for a smooth transition.
Sallen?Key topology - Wikipedia, the free encyclopedia

If you need it, there are chips that offer as many as 8 orders of filtering tunable just by changing the clock frequency!
MAX7480 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter - Overview

Do not confuse the crossover filter with the output filter. The crossover filter is what you're designing to filter the signal at line level. The output filter is used to block the high frequency carrier in hybrid and pure digital amplifiers and operates at the full output voltage. You'll probably want a 50kHz or so carrier in a subwoofer amplifier to reduce switching losses. (Interestingly, the common LC filter used for an output filter is a second order filter.)
Thanks guys for your nice comments! Anyway I read that article(Thanks for that) and i found out a lot of useful infomation. Basically what i need to build is a 2nd order sallen-key active low pass filter. I believe that should get the job done, and i have found plenty of schematics and calculators to calculate resistor and capacitor values.