Low Pass RC Filter

Hi all!

I am making a surround sound system, and I am going to have a esp8266 relay setup to switch between speakers. But I still am stuck on a subwoofer filter. I think I need a low pass LC filter, with the resistor being 2.2kΩ and the capacitor being 1µF, creating a 72 hertz cutoff. But can I put this filter after the amplifier?

I have a 40W 4-channel audio amplifier that is powered by 12V.

Thanks in advance!

An LC filter uses a coil (L), not a resistor (R). Plenty of "wind your own" examples on the web.

Here is one chosen at random: DIY Build Subwoofer Passive Crossover Low Pass Filter using 2 components!!! - YouTube

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Most every speaker has a crossover in the speaker cabinet.

You state LC filter (inductor , capacitor) but you identify a resistor and a capacitor. You would not want to put a resistor in series with a speaker, especially a bass speaker. It will loose power and sound sloppy (i.e. not "tight").
With a 2.2k resistor and an 8 ohm speaker you will not get much sound anyway.

When you figure out what inductor you need, built it without any core. (aka air core). This will insure the sound peaks don't saturate the core (as air does not saturate, short of plasma)

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Thanks @jremington and @JohnRob

I meant RC filter, sorry. Could I still use an RC filter, with lower resistor and capacitor values, maybe between the audio in and amplifier? Basically a separate amplifier for the subwoofer.

Yes, you can use an RC filter in high impedance circuits, like the input to an amplifier.

One chooses values for R and C that match the impedances as well as have the appropriate rolloff characteristics.

Sorry, how would i do that?

Is 22ohm resistor and 100uF capacitor ok? That would make a cutoff of 70 hertz.

An unlimited number of other combinations of R and C also have a 70 Hz cutoff.

Impedance matching is an extremely important concept in circuit design, and if the concept is unfamiliar to you, it is time for some background research.

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I don't know your fidelity goal but an RC does not have as sharp a cutoff as an LC. The issue would be the possibility of putting too much higher frequency sound into the bass speaker causing unpleasant sounds in the mid-low frequencies.

However, folks have done this before.

No. An RC filter will have to be line-level. Typically, subwoofers have built-in amplifier and a built-in active low-pass filter (in front of the amplifier).

Active filters/crossovers also work at line-level. They are built with op-amps and they can have much sharper cut-off than an RC filter.

Sometimes the subwoofer has a full active crossover built-in with high-passed line outputs for left & right active speakers.

Regular 2-way & 3-way passive speakers have an LC crossover network.

Active 2 & 3-way speakers usually have an active crossover (in front of the amplifiers) and separate amplifiers for each driver.

Most home theater receivers have powered outputs for the main/surround speakers (so these speakers are passive) but only a line-level output for the subwoofer (so the sub needs to be active/powered or have it's own separate amplifier).

The receiver also has a low-pass filter for the subwoofer output (so you don't usually need one inside the sub). The receiver also has "bass management" to optionally mix all of the bass from the other channels with the "point one" LFE channel to send ALL of the bass to the the subwoofer.

Or, you can keep the regular bass in the other channels and just use the subwoofer for the LFE (low frequency effects) channel.

Ok, thanks everyone.

I’ll do some research on impedance matching @jremington

I’ll try a separate amplifier with an active low pass filter before it for one way, and I’ll try a LC filter directly on a speaker and see how they compare.

Also, would anyone know why an audio amplifier would suddenly die? I have made several audio amplifiers from the same circuit, and I have had two die (no audio output) after a couple weeks. I have even tried replacing the main IC on one, but still no audio out.

Here is the schematic:

Any ideas?

In the LC filter, is it OK if I don't use bare copper wire, or enameled copper wire, but instead use insulated copper wire?

It really should be obvious, but you cannot use bare copper wire to wind a coil. :astonished:

You use enamelled copper wire to wind a coil because (PVC or whatever) insulation is generally at least as thick as the wire itself on either side, so the insulation itself takes 2/3 of the width and therefore about 8/9 of the cross sectional area.

That is to say, with fully insulated wire, you can only wind about one sixth as many turns in a given space as using enamelled wire. That is totally ridiculous and also cannot dissipate the heat either.

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