A crossover question

[Nick A]

Parts Express has a generic crossover chart that shows what capacitor and inductor values to use depending on what frequency you want. The chart also shows different values to use for either 4-ohm or 8-ohm speakers. My question is: If you are designing a crossover for a MTM design that uses two "8-ohm" woofers wired in parallel...Do you design the crossover for 4-ohms or 8-ohms? Two "8-ohm" woofers wired in parallel would be 4-ohms. So do I use the "4-ohm" column on the crosover chart or the "8-ohm"? Thanks for any info.

http://www.parts-express.com/resourc...tion-guide.cfm

[Wolf]

First of all- those are textbook formulae derived answers, and won't get you an optimized design. It'd be about like buying off the shelf xovers.

Second- you would theoretically design for a 4 ohm impedance. You see- drivers do not have a flat resistance, as they are varying impedance with frequency.

It's a lot easier to model a xover when you know what the response of a certain driver does, so you can account for other problems or issues in the frequency response of the driver. Yes- frequency responses of drivers are not flat either.
There is no free lunch, and the equations only get you started.

If you want to build a pair that will be a success right off the bat, then I advise you to look at the 'Bible' thread at the top of the page, and maybe build a proven design first. You won't have any guesswork, and it should sound good.

If you have any other Q's, a lot of members are very thoroughly helpful here, and should be able to steer your right.
Later,
Wolf

[greywarden]

Yes! Definitely build a proven design first, you will not regret it at all! I got myself in deep and I was looking at the same chart, thinking I'd just buy the parts listed and solder them up

I bought two of the most difficult drivers to integrate, and the tweeter with absolutley no information available to me, not even from the manufacturer either!

So now I'm having to eventually purchase all of this equipment to measure the drivers and develop a proper crossover for it. I'm $300 into my parts list with no measurements, other than what other people on the forum provided me for the woofer.

[Nick A]

Thanks for the info. I'm not new to speaker building...I've built several of the projects posted in the project showcase over the years. I have no problem building the actual enclosures but when it comes to the crossovers, I just can't really seem to grasp how to go about designing one on my own. I don't build speakers every week (last pair I built was 6 years ago) so I can't justify buying a bunch of fancy testing equipment. But, it seems without any sort of measuring equipment building crossovers on your own is pointless. They should just get rid of that chart if it's pretty much worthless lol

[captainobvious99]

Jeffs PCD program will be a good aid toward finding what you need the crossover to look like for a desired result by modeling your drivers and the intended enclosure ahead of time.

[Herman Trivilino]

I don't build speakers every week (last pair I built was 6 years ago) so I can't justify buying a bunch of fancy testing equipment. But, it seems without any sort of measuring equipment building crossovers on your own is pointless.

I believe you can design a crossover for specific drivers in a specific box without doing any testing. At least, that's what I've been led to believe by looking at the "Speaker Building Bible" and talking to folks on this forum. You will need software, though, and the data for the drivers provided by the manufacturers.

They should just get rid of that chart if it's pretty much worthless lol

Well, you know the old saying, "Let the buyer beware". I'm sure, though, that there are satisfied customers out there who've used those charts. They're readily available, and they're based on some very basic and well-known physics.

[johnastockman]

Thanks for the info. I'm not new to speaker building...I've built several of the projects posted in the project showcase over the years. I have no problem building the actual enclosures but when it comes to the crossovers, I just can't really seem to grasp how to go about designing one on my own. I don't build speakers every week (last pair I built was 6 years ago) so I can't justify buying a bunch of fancy testing equipment. But, it seems without any sort of measuring equipment building crossovers on your own is pointless. They should just get rid of that chart if it's pretty much worthless lol

Yeah, any of the textbook formula-derived alignments or the pre-made XO's are for any practical purpose, useless. I tried for years before finding Tech Talk to get those avenues to give me even an acceptable SQ, but never got close. Even with countless tweaking and adjustments. I thought "why aren't the numbers working?" Because all the numbers I was working with are not fixed, so using fixed numbers for a variable parameter was never going to work. I think one of the guys here has an excellent write-up about designing w/o a measurement rig...maybe Paul Carmody? Check his site, or someone can point you to that info. Good luck and let us know how it's coming along.


http://sites.google.com/site/undefinition/diy


John A.

[mikejennens]

If you post what drivers you are going to use, what you listen to, the dimensions of the room in which you'll be listening, etc. (the more info you provide, the better...) you might get a lot of help. I've been through the same thing, and like you, can't quite get my mind wrapped around the xover theory, but the more I read and ask, the more it's wrapping around.
Welcome to Tech-Talk.
Mike

[Wolf]

I wondered but didn't know if you were that Nick A.

Fancy testing equipment?? Shoot, man, I hadn't even anything more than a WT2 till about 8 months ago. Point is- I didn't have that fancy stuff, and still cranked out some decent speakers. I have the blogs about using tracers and such to setup a decent design, even if you don't have the equipment otherwise, and it can work very well.

The point of the chart is to provide info for those looking for the easy fix or experience, and to sell the caps and coils that they distribute. The values do work, but are not optimal, so it's not really a horrendous endeavor to keep it listed. That doesn't mean the speakers will sound good without more information, so YMMV.

Later,
Wolf

[Æ]

Parts Express has a generic crossover chart that shows what capacitor and inductor values to use depending on what frequency you want. The chart also shows different values to use for either 4-ohm or 8-ohm speakers. My question is: If you are designing a crossover for a MTM design that uses two "8-ohm" woofers wired in parallel...Do you design the crossover for 4-ohms or 8-ohms? Two "8-ohm" woofers wired in parallel would be 4-ohms. So do I use the "4-ohm" column on the crosover chart or the "8-ohm"? Thanks for any info.

http://www.parts-express.com/resourc...tion-guide.cfm

For an MTM with two 8 ohm woofers in parallel, the impedance of the two woofers is half that of one woofer. But the impedance of the tweeter hasn't changed any, still remains the same.
You design the low pass section of the crossover using the impedance of the woofers. Then you design the high pass section of the crossover using the impedance of the tweeter.