Subwoofer low-pass filter question

tbolk

Member
2016-02-29 7:34 pm
I have a stereo class D amp; one of its channels I am currently using to drive a custom made subwoofer. Now I want to make a low-pass filter for it. Searching the web I have found many different Op-Amp filters (and a big passive filter is not really an option). The problem is that my power source has some very dirty EMI, so I can't really use anything but a good class D for amplification.

Now my question is: is it possible to use one of the channels of the amplifier to amplify the filtered signal? In other words, what if I use a small passive RLC-filter, amplify it with one of the amplifier channels, filter it again with another RLC filter, and then feed the signal back to the other channel? Hoping to get some 4-pole filtering @100hz going. Will it work? What are your ideas?

For context, the amp is a standard gain 50W+50W (about 30 really) TPA3116D2 class D amp.
 

TBTL

Member
2013-10-08 12:26 pm
It could work, but creates new problems. For example, filters must be driven from a low impedance source. Inductors with the right values for line level crossover are expensive and hard to find.

Why not use a regular opamp? Most opamps have a good PSRR and you could always use a cheap separate power supply.
 

tbolk

Member
2016-02-29 7:34 pm
Yeah after some more research turns out passive RLC are not as good as I thought. Besides, feeding one tpa channel into the other didn't really work in the end (I guess I'd need something like a transformer to fully decouple the output from the input, but that is a high-pass filter and way too complex).
So I've designed a 4-pole active Butterworth on two Sallen-Key filters. Haven't built it yet, so the question of a dirty power supply still stands. I guess if it will be that bad I'll either use a second smaller supply or just ditch the thing altogether and get me a good toroidal transformer
 
Actually, sounds like a pretty clever concept for whipping up that sub filter. But you are using a pretty fancy amp to drive the filter circuit. Why not! the near-zero Ohm driver allows you all kind of latitude in designing the filter and ignoring interference, etc.

I wouldn't lose any sleep over exactly which filter theory to espouse. In the great mess of sound getting to your ear, couldn't matter less. Anyway, results never match the theory.

Main thing is: try it now.

Ben