The OEM tweeters in my car have a resistor on the back, which (from what i've read) acts as a high pass filter.

The resistor says:

RB
3.3 µF
50 v

85° C
E C L

Can any clever person work out what the crossover point is?

No idea what the resistance of the speaker is - tried measuring it with a multimeter, but set to "2000k" it just slowly climbs to 2000, and as it gets there it then just goes to 1 and stays there. Going to geuss that it's over 2000k, tho!

D'oh, not resistor, capacitor!

are you sure that's a resistor and not a diode or a capacitor?

uF usually refers to capacitance, and i don't think resistors care about that.

edit: bah, you got to it first.

Are there any other markings on the tweeter? Manufacturer's name, part number etc? This link suggests you're looking at 6, 12 or 24kHz depending on the impedance of the speaker.

Quote from Jakg :

No idea what the resistance of the speaker is - tried measuring it with a multimeter, but set to "2000k" it just slowly climbs to 2000, and as it gets there it then just goes to 1 and stays there. Going to geuss that it's over 2000k, tho!

You probably measured through the capacitor, the DC resistance of a normal speaker should be quite low (somewhere between 2-8 ohm usually).
I would guess it's an 8 ohm speaker.
One thing you have to keep in mind though is that you're actually looking for the AC resistance, better known as impedance. That is difficult to measure and varies with the frequency of the supplied signal. If you want to change the crossover frequency, you might want to buy several capacitors and see which one sounds best.

Standard car speakers are 4ohm the vast majority of the time, it's not a rule though...

Try this mate!

http://www.carstereo.com/help/Articles.cfm?id=1

Assuming we are talking of a typical car speaker with a 4 ohm impedance, the cutoff is 12000 hertz.

Just for a bit of learning, can someone tell me what the crossover point of a resistor is? :/

Quote from amp88 :

Are there any other markings on the tweeter? Manufacturer's name, part number etc? This link suggests you're looking at 6, 12 or 24kHz depending on the impedance of the speaker.

Nothing that might narrow it down - it's made by Harmon Kardon, but as it's OEM supplied theres a MG Rover part number and little else, and HK are less than helpful.

Quote from kingfag :

If you want to change the crossover frequency, you might want to buy several capacitors and see which one sounds best.

I dont want to change it, I want to work out what the frequency is so I can set the crossover of the speaker to the inverse.

Quote from J@tko :

Just for a bit of learning, can someone tell me what the crossover point of a resistor is? :/

In car audio you can get a device called a crossover - this sends different frequencys to different speakers - i.e. you might want to send low frequency's to the sub, but higher ones to mids or tweeters. The cap on the back of the tweeter I have already does this.

Borrowed my Dad's multimeter, tuns out mine just wasn't man enough... 4Ω, which makes a crossover frequency of ~12,000 Hz.

I said that!

Quote from danthebangerboy :

I said that!

I said it first, technically