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Old 12th July 2006, 10:50 PM   #1
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Default Crossover help needed

I am planning on building a set of stereo speakers. I have been trying to learn how to design crossovers for them but am having a very difficult time understanding how to calculate the final impedance of the assembled speaker. I would like to end up with an 8 ohm speaker, but the only setup I can come up with ends up as a 4 ohm design. Currently, I am planning on having each cabinet contain:

Two 8 ohm tweeters
Four 8 or 4 ohm mids
Two 8 ohm woofers

I looked at the designs on parts express and tried to make sense of how the impedance the builders claim adds up, but to no avail.

A general layout of how this would be possible would be greatly appreciated. Any input on the order/type of network to use would also be a great guide my understanding. Thanks.

Vic
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Old 13th July 2006, 12:05 AM   #2
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Hi Vic,

This will be basic but might help.

You will need two 4 ohm woofers to reach your objective.

The impedance from there on up can be what you want provided you don't drop it below 4 ohm.

For a basic XO, you can start with an XO calculator and perhaps consider a L.R. 2nd order.

http://www.mhsoft.nl/spk_calc.asp

You will need to know the actual impedance of the drivers at the XO point to calculate the values of the caps and coils.

This is just a starting point and hasn't addressed the sensitivity of the drivers so you either have to match them or add some padding (series and bypass resistors) to the circuit. This can get complicated.

If your drivers have non-linear frequency response, then you may also have to add notch (by-pass) filters if the humps in the FR fall within the XO points. Now we're getting into the fun stuff.

If you haven't done this before it can get frustrating. You really have to want to build speakers to stick with it sometimes.

Or on a rare occasion you get it right the first time and voila!

Good luck
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Old 13th July 2006, 02:06 AM   #3
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Default I understand completely...

I understand completely...

Just my two cents but...

It isn't just about your XO, it's about your drivers and how they add up (and how you wire them).

Recently I designed a pair of MTMWW floor standers that decked my amp. Bummer. I went to this site :

http://colomar.com/Shavano/spkr_wiring.html

and here I learned how to change the impedence profile of my drivers. Excellent site.

I was able to take a pair of absolutely failed loudspeakers and make them big, functional and sweet.

It's the basics man, there are rules that dictate how these things work. Once you understand them - everything is good.

Try this knowledge out. I guarantee you'll come off better...

Regards,
Tom
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Old 13th July 2006, 02:09 AM   #4
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Thanks for the input, that looks like a useful site. Here's a pic of what my initial design is:

Click the image to open in full size.

The two holes in the middle are where the tweeters would be placed. The box has a slight taper as it goes up.

I understand where you're going with the 4 ohm woofers. If I did that and dropped down to one 8 ohm tweet I may be in good shape. Will one tweeter be enough to keep up with the rest of the system? I am building these to be played loud. I listen to heavy metal/rock if that matters.
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Old 13th July 2006, 07:16 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

Your box looks impressive, and the array of drivers also would,
but it seems obvious you've no idea what you are doing regarding
the finer points of speaker design, consequently the very likely
result will be a large piece of scrap, multi-driver 3-ways are so
easy to get completely wrong.

So let get back to basics :
What amplifier do you have ?
Stereo or AV multichchannel ?
What power per channel does it have ?
How big is the room ?
(basically the bigger the room the bigger the speaker needed)
What is your budget for the speakers ?

(I'd be looking at splitting your cabinet into two. The top
a sealed MTM and on the bottom an active subwoofer.)

/sreten.
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Old 13th July 2006, 11:29 AM   #6
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I have not purchased an amplifier yet that will be able to play something of this magnitude. I have been looking at getting a Dynaco 400 or 410 ( 200 wpc @ 8 ohms and 300 wpc @ 4 ohms). This is strictly a stereo system as I am not particularly interested in home theater. The room sizes vary as I move a lot because of school. I would say an average size would be ~200 square feet. My budget is around $275 for the drivers.

I kind of like your idea of splitting the speakers up. This way I would be able to mess around with them and get the sound correct and it would be less costly. If I felt it necessary I could then build new cabinets and add the other drivers.
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Old 13th July 2006, 12:13 PM   #7
tinitus is offline tinitus  Europe
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Allow me a little comment on your design

Use only one tweeter - several new quality tweeter has high sensitivity
It seems you are using small midranges and they are often with low sesitivity
I dont think you will get above 92db - so I think you will be fine with one tweeter

I would also consider using only two widerange drivers for midrange

But offcourse you will end up with 4 ohm, not the best with tube amps, but should not be any problem with a "modern" amp

UPS! I see you are playing loud metal rock - you will need drivers that can take some power
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Old 13th July 2006, 12:51 PM   #8
tinitus is offline tinitus  Europe
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A budget with 275 USD - even if its pr speaker it will be cheap driver, and probably with low power handling

Maybe you should consider a more simple and suitable design

http://pispeakers.com/Prices.htm
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Old 13th July 2006, 04:56 PM   #9
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Yes, considering your budget, music and the room size, I would consider a PA 12" driver and horn in a two way system. Not as accurate as some of the more expensive stuff but loud and proud and a real workhorse. A lot easier for the XO as well.
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Old 13th July 2006, 06:47 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

I'd consider a good quality 100w amplifier that has a pre-amp
out, power amplifier in capability. This will allow you to use
active baffle step compensation - instantly giving you increased
head room - 6dB (equivalent to a 400W amp) above ~ 1kHz.

Then make a high efficiency sealed MTM that has no baffle step.
Add a bit of bass boost between pre and power and this would
probably work fine on its own in small rooms.

For larger rooms you then add two powered bass modules as
stands for the MTM's. With a bit of jiggery pokery in your pre-out
and poweramp-in you could make a system that goes really loud.

Another option would be the MTM's used with one bass module,
but the module is crossed much lower i.e. its used as subwoofer.

So you could have a system that adapts to 3 sizes of room.

/sreten.
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