# Simple speaker crossover 1st order filter design

Opened up an old speaker and am trying to figure out what is going on with this capacitor.

The gray cord leading to the larger 16cm speaker cone is an 8Ohm/6.3W capable load and then there is a 1uF capacitor in line with the audio-in line and leads to the smaller 8Ohm/4W speaker cone. So we should be able represent this like so:

simulate this circuit – Schematic created using CircuitLab

I assume this is supposed to be a crossover filter, so a first order high pass filter. However, if we say the -3db-point is 1/(2piRC), this would be around ~20kHz. Wouldn't a tweeter want to remove anything above 20kHz, not vice versa?

This also makes me wonder about the print on the outside of the speaker box that simply states that the entire speaker box can handle a maximum of 6.3W, even though the smaller cone is only rated at 4W. Does the capacitor and the other speaker "soak" up some of the current so it's fine for the entire enclosure to be rated at a higher power?

• Tweeters play the higher frequencies - larger speakers tend to play the lower ones. Commented Nov 14, 2020 at 5:12

## 1 Answer

It's a cheap crossover. You're right about the corner frequency, even if the speaker's impedance is not resistive, but not about the role of the tweeter: it's meant to play only the high frequencies, and nobody says they have to stop at 20 kHz. But the amplifier, in general, limits the total bandwidth.

As for the power, the "cross-over" leaves the high frequencies that have a much smaller energy than their low frequencies counterparts, so the handled power is lower for the tweeter than it is for the "woofer" (I'm slightly generous with the terms). Since the "woofer" gets all the frequencies and the tweeter only some, then it's the "woofer" that determines the total power, and in this case it's 6.3 W.

• I see. I had always thought tweeters would stop around 20kHz, but when I looked them up I see now that they do have higher ranges and I guess the roll-off of the corner is slow so it'll still let through some of the higher frequencies. Commented Nov 17, 2020 at 20:47
• @lemonlime One problem with electrodynamic tweeters like this one is that the low frequencies can affect their excursion and, if not kept in check, they can go off-center. That's why, at their minimum, they need a series capacitor (or a dedicated output from an active speaker amplifier). Otherwise, even if they do allow over 20 kHz frequencies, the mechanical properties will also contribute to the total attenuation, so you don't have to worry that your dog might respond to Beethoven's calls. :-) Commented Nov 17, 2020 at 21:18