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In Reply to: RE: Mixed crossover??? posted by Paul Joppa on November 05, 2015 at 11:34:36
Hey Paul
Sorry for the confusion. The example I gave with the 22.5 uf cap and 6 ohm resistor to ground was designed by a guy using soundeasy v5.0 10 years ago. He called it 2.5 way cross over. He said it was a 2nd order on low pass and 4th order on the high pass. The only thing that did not look like the normal text book was the 6 ohm resistor under the cap to ground. I follow you on the zobel network so could it be a 1st order with a zobel network and he did not realize it?
On the second cross over it was the stock one that came with the speakers. I just looked at the schematic and use crossover formulas. The only thing here was the numbers came up with 10.3 cap 1600 hz for high pass ribbon and 2.0 mh for 1300 hz high pass woofer. And the cap to ground on the woofer was 22uf the books say it should about 7uf.
So with the stock crossover the big number difference is the cap to ground on the woofers in parallel.(18w/8545-00) scanspeaks. Not sure what changes when you increase the size of this cap from 7uf to 22uf.
I am still confused let's say I wanted a 1st order on tweeter and a 2nd on woofer let's say at 1500hz. All the books and calculators are done with the same slope/order for woofer and tweeter. So if you use 1st order on tweeter it will be 1st order on woofer. So could you say use the values from calculator for a 2 way 2nd order crossover then just remove the grounded inductor on the tweeter.
I follow in the real world there are many variables which create less than a flat response curve. I am trying to understand how calculate a starting point for a 1500 hz crossover 1st order on tweeter and 2nd order on woofer.
Thanks Tom
Follow Ups:
OK, one more try :^)
For a first order ribbon tweeter crossover you start from the actual impedance of the tweeter in the vicinity of the crossover, assuming this to be fairly flat and free of reactive impedances. Then, given that resistance and the target crossover frequency, you can calculate the capacitor. In fact, given any two of those numbers, the third can be calculated. For example, if you have 1600Hz with 10.3uF, then the resistance you assumed must be 9.66 ohms. That seems an odd value to have chosen, but I have no data on the ribbon impedance.
The woofers are more tricky. For simple home-based calculations it is usual to first put a Zobel series RC network across the woofer to make it a constant resistive impedance, then put the calculated network in front of the [driver plus Zobel]. For commercial manufacture you could then use a computer optimization program to adjust the values for a simplified network, for example replacing the crossover cap plus Zobel with a single cap. It's not exact, but in combination with the driver's response you might easily get a good fit with the desired acoustic crossover function.
Finally, you said "I am still confused let's say I wanted a 1st order on tweeter and a 2nd on woofer let's say at 1500hz. All the books and calculators are done with the same slope/order for woofer and tweeter. So if you use 1st order on tweeter it will be 1st order on woofer. So could you say use the values from calculator for a 2 way 2nd order crossover then just remove the grounded inductor on the tweeter." No, the capacitor for a second order filter will depend on the Q of the filter, and will not in general be the same as the capacitor for a first order high pass filter. For second order filters, the Q determines the type of response - Butterworth, Linkwitz/Riley, Bessel, etc. You can instead use the low-pass portion of a second order design on the woofer, and the high-pass portion of a first order design on the tweeter.
Hi Paul, Can you tell me how to determine impedance for
a particular frequency when impedance vs frequency graphs
are not available ? I have EV SP15B's and find no such
info.Thanks for any help.
Clayton
Clayton
You can download Room Eq Wizard (REW) for free from the Home Theater Shack site (do a search), and it's available in both Mac and PC versions. With this, and a simple home made test box, you can generate your own impedance graphs. There's also a good instructional you-tube video on using REW. I have'nt tried an imp. graph with REW yet, but it has to be a lot less tedious than the old school way of using a test box, an audio oscillator, and a multimeter to make a paper graph.
And share the results here with the Electro-Voice Brotherhood!
One of the other Pauls
Thanks Paul, I will look into this.
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