Crossover help 8 ohm tweeters & 4 ohm woofers

Hey guys, I'm kinda new to the speaker building arena and could use a hand in making a suitable crossover for my little pet project. Sorry if this is in the wrong forum and please point me in the right direction if so.

I have 2 4 inch 4 ohm woofers (WFR0400215) with Frequency Response of 90 to 10,000 Hz

I also have 2 1/2 inch 8 ohm tweeters (Tang Band 13-1761S) with a Frequency Response of 4,000 to 20,000 Hz.

From what I have read along the forums I believe they should be crossed at 5000 Hz. Am I correct?

I was looking at an off the shelf crossover (5 kHz High Pass 8 Ohm Crossover) from parts express. I am curious to know if this would be suitable for this setup?

Also how would I kill off any frequecy below 100 hz for the woofer in said setup?

Again I apologize for the noobness(?) and greatly appreciate any help!
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
As long as you understand that this crossover is generic in nature, it will be a starting point so further down the track you might find you've replaced some or all of the values.

One thing you'll need to do is adjust the levels. This usually means bringing the tweeter down (I haven't looked at your drivers).

Broadly speaking, a 4 ohm unit uses an inductor of half the value, capacity would also double and using the values you have shown above might lead to an unexpected droop in the response at the crossover knee.

Sometimes bass gets compensated due to the way it is produced on a small baffle. The simplest way this is often dealt with is by using a larger inductance and you might luck out in this case with the 8 ohm values, and bringing the tweeter to the right level.

If you just need to get started then that's one thing.
 
Juanito,
That generic 5k crossover from PE assumes both drivers are nominal 8 ohm impedance. Because your tweeter is 4 ohm, it requires different crossover parts values to make the crossover freq be 5k.

Here's a simple crossover calculator to enable you to make your own version of the generic PE crossover, but using 4 ohm tweeter and 8 ohm woofer.

Like AllenB said, generic is generic, so don't expect perfection when going this blindfolded route. The nominal driver impedance is rarely actual impedance, and the impedance of any driver changes with frequency! They don't make it simple. :)

To make your crossover circuit more precise with your drivers, you could measure the actual impedance curve of each driver thru the crossover band. Google "how to measure loudspeaker impedance." It is possible with simple tools. Or buy DATS from PE. You'll probably find that the impedance of both drivers is fairly constant up around 5kHz, so the generic xo calculator might be ok if it knows the actual impedances of the drivers at the xo freq.

Once you get the crossover parts selected, you'll have to adjust the SPL of one driver to make it match the SPL of the other. This is done with resistors, called attenuation, so it is always done to the louder driver, usually the tweeter is the loudest driver, or "most sensitive." Use a sound level meter to determine how much softer the tweeter must be, then use an online L-pad calculator to determine what resistors are needed to lower the volume of the tweeter. An "L-pad" (2 resistor network) is used instead of just a single resistor because it preserves the original impedance of the tweeter that you used to determine the crossover parts. If you use only a single series resistor it will change the impedance.

When you get to working on the bass crossover for the subwoofer, you will need to know the natural rolloff behavior of your 4" woofer, because that will affect how much more rolloff you must add with crossover filter to get the net rolloff you want. If you have a simple sound level meter you can graph the FR using test tones like those from realtraps. You could also use a test tone generator like WinISD. You'll also need to know the impedance curve of the mid-woofer at low frequencies so that your crossover filter will work correctly. Just like in the treble crossover, the function of any crossover filter is heavily dependent on the filter's load impedance. A filter prefers a nice flat impedance through the xo band, but in the bass register, drivers rarely provide flat impedance to their crossover filter.

All standard type speakers with voice coil, spring suspension, etc, have a mechanical resonant frequency toward the low end of their frequency band. Put them in a closed or ported box and that resonant frequency changes slightly. At the resonant frequency there is a big spike in impedance. If this impedance spike occurs in the middle of the range of your bass crossover band, then the crossover filter functions will be skewed by the varying load impedance. The signal that makes it through the filter will not be the smooth high pass filter you expect, it will have big bumps that might not sound too good. An additional electronic circuit must be added to the crossover to flatten the impedance of the driver so the crossover filter can function correctly. This impedance compensation circuit is called a series notch filter, or LCR filter.
Series Notch Filter Designer / Calculator Help

Your midrange to treble crossover of 5kHz is probably high enough to avoid the tweeter resonance interfeing with the treble crossover filters. But your woofer resonance will probably fall somewhere in the 80-200Hz range, and the crossover band will probably extend over that range also. Adding the LCR to flatten the woofer's impedance sounds like a charm, but it does affect the SQ, so you'll have to try it and see. Some people prefer to omit it, others pick top quality parts to minimize the damage, and/or fiddle with the LCR values to minimize the damage.

Because your woofer is small you may only need a single series capacitor on the woofer to steepen the rolloff enough to match the subwoofer's low pass filter. If the sub can be set to 12dB/oct LP that would be good. You'll probably have to substitute series caps until you find the value that blends best with your sub, in your room, to your taste.

The sub should be a pretty good one to be able to play up high enough to meet a 4" driver.

That's more than enough to confuse you. ;) It will get easier the more you do it and learn. have fun!