LC Low Pass Filter Design

Question

I am attempting to design a low-pass filter for a subwoofer, as the audio amplifier that is connected to it does not filter any of the sound coming in, and it is getting pumped full of high frequency sound. I have settled on a Butterworth LC filter, and want a crossover frequency of around 200Hz. I have determined that I want a capacitor somewhere in the range of 140-160 uF and an inductor in the range of 4.5-6.5 mF to hit that frequency, and from a circuit standpoint, it seems to make sense. Source: this website

Here is where I hit a difficulty. I am aware that I don't want an inductor with very much DC resistance and is rated for a high current throughput (possibly in excess of 5A). I don't know which to choose.

I also am at a loss for the capacitor to choose, as I need one which is two-way (able to handle AC), but I am not sure for the other requirements of it.

If anyone can clarify these points for me, and give suggestions as to the actual parts to use, I would be forever grateful. It is also worth noting that I am on somewhat of a budget, so please try to recommend affordable products, even if they have lower performance.

Answer 1

To directly answer your question: The website you referenced can sell you the parts you want. Since there are some decisions to be made with their parts list and you are on a budget, I would make these suggestions:

1. For the capacitor, use their "Non-Polar Electrolytic Capacitors". It is their least expensive model and it will likely be "good enough" for you.

2. For the inductor, choose one of the "Air Core" products, whichever is least expensive. Air core inductors have the nice property that they will not saturate. (They might overheat and burn up if you put too much power through them but they will not distort!) I do not see amp ratings with those units, so I would suggest choosing something with perhaps a 14 gage or thicker wire. The thicker the wire, the lower the resistance and so the lower the heat build up. However, since budget is a concern, maybe start out using their 20 gage model and see if that works for you. Turn up the power slowly of course!

3. Lower resistance in all of the components is better, less heat build up and more power goes to the speaker. But it is not worth going crazy over.

Now, having said all that... There are plenty of other places to buy these components (Mouser, Digi-Key, Newark, and Allied Electronics come to mind quickly) and all of them should make it pretty easy to spec out what you need. And the specification sheets are easily available also.

One last suggestion... Filter the signal before it goes into the amplifier. Lower voltage, less expensive parts, safer. And if the voltages are at sane levels (say, +/-10 volts or so) there are much better filter options such as op-amps, UAF42 IC chip, etc.