Introduction to designing crossovers without measurement

[sandro600]

Terrific tutorial. Thanks for taking the time to put it together.there is a lot of assumed information in forums like these, and a lot of it unfortunately goes over my head. Your tutorial sheds much light on what for me are obscure topics.

[Bahman]

Hi
Been trying for some years to adjust my speakers sound to own liking without any luck! Changed x-overs using tables and charts on web pages three times! Till I came upon this thread, really straight forward and step by step guide, thought give it a fourth try! Just one question though. May I come back for help in a new thread hoping to get non-sarcastic, non-insulting answers?!
Thanks for a really fine-tuned thread.

[tranhieu]

Originally Posted by AllenB

The capacitor
Take the value of impedance you wrote down two posts ago (for our example it was 3.75). Multiply it by 12.6 and then multiply it by the crossover frequency. For example, 3.75 x 12.6 x 2000 equals 94500.

Sorry for asking this but I don't really get how you got the figure 12.6 there. I assume you were working backward using the equation for the cutoff frequency:

but that works out roughly 6.3, not 12.6. I wonder if I'm missing something here?

[AllenB]

Hi tranhieu, yes there's a little more going on in this case and it's to do with the second order filter.

The formula you've shown gives a capacitance that equals the resistance at the given frequency. It produces a cut of 3dB in the resistance (only 3dB because the capacitor is slightly out of phase with resistance), and is used with first order filters.

With a second order filter, the capacitor and the inductor resonate at some frequency. Doubling one of them and halving the other doesn't change the resonance frequency, which is true in this case. Far above and far below the crossover they'd still behave the same, but at the crossover, the balance of capacitance to inductance controls damping (filter 'Q'). The Q I have used is 0.5, a 'Linkwitz-Riley' filter.

[tranhieu]

Thanks, so that means the schematic you used in that post is for a 2nd order filter? Could you please point out which one is 1st stage and which one is 2nd stage? It looks so much like a 1st order one to me.

[AllenB]

With the schematic in post 11, the capacitor is used to hold the low frequecies from the tweeter the same as in a first order filter. The inductor is added which returns further lows from ground so they won't be seen by the tweeter.

This capacitor and inductor interact in a unique way, and for this reason I'd be reluctant to call one of them the first or second. In typical use though, you wouldn't choose to run the inductor without the capacitor, although it has been done.

[tranhieu]

Then how about using the the capacitor without having an inductor? Will I run into any trouble with that configuration? Since due to some technical issue I cannot have an inductor in my network.

[AllenB]

It can be done (and often is), but try not to cross too close to the tweeter's resonance. This will be necessary to ensure reasonable power handling, and it will reduce the effect of the impedance peak interrupting the crossover...both to which a single capacitor will be more sensitive.

[danoz]

Thank you for this excellent series of tutorials. They contain a lot of important points that I only discovered through many years of research and experimentation. Just a couple of questions:

1. Why did you stop at first order filters? I have noticed substantial improvements with second order. My best speakers are fourth order, and I have noted big improvements in clarity with higher order crossovers, and there are excellent reasons to support this.

2. Does anyone have any mathematical models or ray diagrams for the step effect? People often make a big fuss about narrow speaker cabinets for improved imaging, which leads to a multitude of 6.5 inch designs with compromised low frequency performance. This configuration can lead to just as many headaches because of such effects.

[AllenB]

Originally Posted by danoz

1. Why did you stop at first order filters? I have noticed substantial improvements with second order.

Thanks for your comments, danoz. Using an inductor with a series RC on a woofer is not necessarily a first order filter. In the plot below I've shown a fourth order slope on a woofer that I've used, and it is crossed using only those three components. The acoustic response is the goal, the electrical components are just the tools we use to get there.

You'll notice I decided to follow a second order electrical filter for the tweeter so as not to introduce undue excursion issues, but moreover in this tutorial I wanted to use something that would typically function well in a reasonable cross section of designs to use as a basis. I also needed something that did not turn tweaking into a chore. Furthermore, without phase and delay information available, chasing higher orders can be relatively ineffective.

Seasons greetings to all.