Crossover question

[gatemplin]

When designing a crossover, how do you know what impedance to use?

For example:

Say I have an 8 Ohm nominal woofer and an 8 Ohm nominal tweeter of identical efficiencies. Also, lets pretend I want to crossover at 2000 Hz.

At 2000 Hz, in my enclosure, the woofer's impedance is 10 Ohms and the tweeter's impedance is 14 Ohms.

So when using the crossover formulas, which impedance would I use? The nominal, or the specific at the crossover frequency, or is it something else entirely?

I am just trying to learn more about speaker design. I have read The Cookbook and other books, and searched, but have never found this answer. I just dont get it.

Thanks in advance,
Graham

 

[jerrymrc]

Graham, for the most part you use the specs. IE 8ohms. The drivers will change up and down as to the frequency.

Now let me try and tell you how the real world looks at it.

The drivers rating is for the range that it is intended for. This may be from 2 ohms to 12 ohms for this driver. The summed resistance
is 8 ohms.

Without testing the driver your in a crap shoot. Texbook crossovers are a almost kind of a joke.

Deal with one driver at a time. If you have a tweeter that you like and want to build your own find a design that uses that tweeter.

Then look at what woofer they used and what you want to use.

If your intended driver will be close then try the Xo that was given. It is easy to tweek a design that has been published.

Starting from scratch can drain alot of money. I know.

 

[gatemplin]

Thanks Jerry. I know that textbook crossovers are a joke and you really have to build, and test, and tweak, and lose money. That is why I stick to making single driver speakers. Normally I would never consider building a multi-way from scratch, but with the new Adire Audio drivers coming out, I think I might give it a try because I can push the crossover up into the 10 kHz range where my ears are MUCH less sensitive.

 

[House de Kris]

This is the classic problem with passive crossovers. As you approach the upper end of the passband of a driver, the inductance of the voice coil dominates the reative compontes of the speaker impedance. This will yield an ever increasing impedance with frequency. To make your crossover calculation correct, it is necessary to correct the impedance curve of the driver. A zobel network is applied to the woofer to make it equal to the nominal impedance at the desired crossover frequency, thus making the crossover work properly.

As for the tweeter, if it is really that high at 2kHz, then it would appear that you are very close to the resonant frequency of this particular driver and I would suggest raising the crossover frequency a bit to avoid the resonance. Try something like 3.5k to 4k for this tweeter, I suspect it would be much closer to the nominal impedance value.

 

[gatemplin]

"As for the tweeter, if it is really that high at 2kHz, then it would appear that you are very close to the resonant frequency of this particular driver and I would suggest raising the crossover frequency a bit to avoid the resonance. Try something like 3.5k to 4k for this tweeter, I suspect it would be much closer to the nominal impedance value."

Thanks Kris. I just made up those numbers, the drivers I am looking at dont have those problems. I would never try to cross over in that range anyway. If I do build a multiway, I would use a 4th order and cross over between 5 and 12 kHz.

 

[spkrman57]

Try a 1st order and a 3rd order...

1st order on the woofer(include a zoebel if necessary), then run a 3rd order hi-pass on the tweeter.

If you know the natural roll-off of the woofer, that will help you decide the crossover frequency better, unless the tweeter won't let you crossover that low.

Also, with the 3rd order on top, you can add HF compensation network to smooth out a peaky, or non-linear tweeter.

If you are running horns on top, this is especially a good circuit to use.


If you are running flea-power(less than 4 watts/chnl), try a 1st order for top and bottom.

A lot of questions need to be asked and answered to really design a successfull crossover.


Regards, Ron

 

[gatemplin]

Thanks for the input Ron,
The woofer wont roll off on axis until about 20kHz, I was thinking of using a tweeter for better detail and off axis response. I also thought about 1st order for both but it seems to me that at high frequencies (around 10 kHz), the distance between the acoustic centers and distance on the baffle can cause a strange freq. resp. problems.

I am thinking of using the new Adire designed WR125s full range XBL^2 driver and possibly the Seas 19taf d08 because it has a similar impedance, and efficiency so I can avoid an L-pad or Zobel. I will be using 40 Sansui watts per.

I will be building the WR125s as a near field speaker so off axis is not really a concern. I thought I might explore the use of a tweeter if I find the high end detail lacking.