need help with car tweeter connection

[svizoman]

Hi,

need help with connectin a tweeter to my car stereo without switch.
Here is a diagram, is it ok?

equation for capacitor value should be:
C=1/(2Pi*f*Z)

but this does not corelate with online chart

also it says contrary to the chart most tweeter capacitors are 500uF to cut bass freq from 100Hz.
Also does it have to be bipolar or can it be regular electolytic?

 

[betwixt]

Must be bipolar I'm afraid.

The value is quite in order. The formula isn't strictly correct because it doesn't take into account exactly how much the higher frequencies will be affected. The capacitor has 'reactance', think of it as resistance but in a series circuit the value drops as the frequency increases. This means the lower the frequency, the smaller the conduction to the tweeter and conversely, the higher the frequency the less it is attenuated so more reaches it. It isn't a 'brick wall' filter, the amount of signal passing through the capacitor gradually increases as the frequencies get higher.

To get accurate figures you have to know the impedance of the tweeter at different frequencies and how efficient it is at those frequencies. When it mentions 8 Ohms I assume it means the tweeter has 8 Ohms nominal impedance but it isn't constant across the audio range. There is no exact science but the value of 3.9uF is a reasonable one for most situations.

Brian.

 

[svizoman]

the diagram is just to show wiring, in my case I have car speakers, both are 4 ohm. So what value capacitor should I choose?
My tweeter freq says 3-15kHz. They say you should double the freq. Should I gove then with round 20 Uf? In my store bipolar comes as 22 uF. Will that be fine?

I also read somewhere that mostly they choose bigger cap as 500 uF.

thx!

 

[betwixt]

500uF is nonsense, possibly a statement from someone who thinks bigger numbers mean better performance.

There is no 'correct' value, the bigger it is the more low frequencies arrive at the tweeter, the lower it is, the fewer low frequencies arrive. The rate at which the volume at frequencies changes is gradual so when your tweeter says 3-15KHz it really means "don't overdrive me with low frequencies below 3KHz". In physical terms it means if you push the small speaker diaphragm too slowly and too hard it will break. The capacitor causes a gradual reduction in low frequencies that pass through it so your real objective is to pick a value that prevents too many low tones reaching the tweeter.

As I said, there is no abrupt stop in the frequencies passing through the capacitor, it is gradual so you have to pick a value that compromises stopping low frequencies but still lets enough high frequencies through. For example, if you use 3KHz as your calculation frequency and assume half level should pass at that frequency, then you want a capacitor that has a reactance equal to the tweeter impedance. Using the formula, the value that equals 4 Ohms at 3KHz is 13.26uF.

In a better filtering arrangement, the main 'woofer' speaker also has a filter, this time an inductor in series with its connection. it has the opposite effect of reducing the high frequencies and with carefully chosen values you can get a 'see-saw' effect where only the approriate frequencies reach each speaker. Going from low to high frequencies, the bigger speaker gets gradually less while the tweeter gets gradually more. Its called a 'crossover' network.

Note that 4 Ohms is the 'nominal' impedance of the loudspeaker, it will almost certainly vary wildly between 3KHz and 15KHz. The other thing you should check is the capability of your cars amplifiers to drive impedances lower than 4 Ohms. When loudspeakers are wired across each other their combined impedance becomes lower so check for example that two 4 Ohms speakers making 2 Ohms total is within the amplifiers capability. Too low an impedance can damage the amplifier.

Brian.

 

[crutschow]

I also read somewhere that mostly they choose bigger cap as 500 uF.

"They" are blowing smoke.

How big is the other speaker?

I would select a (non-polar film) capacitor to give a crossover around 3-5kHz at 4 ohms.
You may need to test several values to get the best sound.

 

[BradtheRad]

To make a stand-in for non-polarized type, there's the trick of placing two electrolytics in series. Face negative-to-negative, or positive-to-positive.

Example, say you want 10uF, then use two 20uF.

When my multimeter reads voltage at a speaker's terminals, it's rarely more than 1 or 2 VAC.

Test various farad values. The aim is to hear equal volume from woofer and tweeter.

 

[crutschow]

The aim is to hear equal volume from woofer and tweeter.

More precisely, the aim is to have the response flat for the crossover frequencies from the woofer frequencies to the tweeter frequencies.

 

[svizoman]

"They" are blowing smoke.

How big is the other speaker?

I would select a (non-polar film) capacitor to give a crossover around 3-5kHz at 4 ohms.
You may need to test several values to get the best sound.

both speakers are from a kit with 5" main speaker, 2" tweeter and switch which is corroded. both are 4ohm, 5" is rated 75w rms, tweeter doesnt have that only 3-15khz.
in my local estore only 10 and 22 and 47 uf bipolars are awailable.

 

[crutschow]

in my local estore only 10 and 22 and 47 uf bipolars are awailable.

You could try the 22uF, or if you want to experiment, also buy a 10uF cap and, try each of them.
If the 22uF still doesn't seem to give enough high response, put the two in parallel to give 32uF.