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Old 19th August 2008, 06:36 AM   #1
focal7C is offline focal7C  Australia
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Default Focal 2-way crossover design options

Recently rebuilt a Focal DIY project. The woofers have been refoamed and tweeters replaced. Now considering changing an off the shelf 2nd order crossover with something better. Would really appreciate some advice.

I don't have any measuring capabilities and can't find RFD or ZMA files for the elements.

The sound is very clear and clean with fast precise response. What I think will improve these characterisitcs even further is:
- Reducing comparative responsiveness of tweeter with 2ohm resistor (currently 2dB higher than woofer)
- Promoting the base (possibly using a Zobel network on the woofer)
- Using better capacitors (eg. MCaps)
- Selecting a known/ideal crossover point

Options are:

1. Modify the existing XO (as above)
2. Use a Zalytron project crossover
2. Start from scratch


The tweeter are is a TC120 TD5 (6 ohms) and the woofer a 7C013 (6.8 ohms).

1b - The crossover had been bought at the J-Car electronics store. Don't know what the crossover point is but has . It is mounted externally near the amp. The caps are 10uF on woofer and 4.7u on tweeter. What is it's crossover point?. The two air coils don't have any markings (impedance unknown).

2b - Zalytron crossover. XO schematics for Budget 102 project. It has the same tweeter but two (4 ohm) woofers.: http://www.zalytron.com/Schematics/Budget102.jpg

The box is a ported enclosure of about 1 cubic foot. The inside dimensions of the box are 623mm high x 198mm wide x 241mm deep. The inside is lined with lined with 13mm bituminised panels on all surfaces. The steel port is 65mm wide 80 mm deep (made of very thin metal) with the lower edge located about 3-4 cm from the bottom. They are 1243mm tall made of thick 35mm particle board.
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Old 19th August 2008, 10:28 PM   #2
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Focal7C,

Forget an off the shelf xover, it will crossover, but with little regard to any of the characteristics of the drivers you're using.

If you have the manufacturers specs, Jay has a page on designing xovers using these without measuring:
http://www.geocities.com/woove99/Spk...esigningXO.htm

You could modify the project xover you posted; Using only one woofer, the component values for the Low pass filter will be x2 for the inductor, and half for the capacitor (assuming the baffle width is the same for both systems, otherwise you'll need to tweak the L). You will need to adjust the Tweeter attenuator to match the reduced output. I would do that by putting extra resistance before the 8uF cap, if you put it after, you will change the xover frequency.

Setting up measuring capability isn't that hard; You have a PC, all you need besides that is an electret mic and some freeware; try Synrta or ARTA.
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 20th August 2008, 02:06 AM   #3
focal7C is offline focal7C  Australia
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Thanks PeteMcK,

does not the two 3.85 Ohm woofers in series equate to 7.7 Ohms? My single woofer is 6.8 ohms according to the specs.

Thus the woofer inductor and cap will need to be decrease and increased respectively (by a factor of 6.8/7.7)? Or is it more complex than that.

How do I calculate the crossover point of the Zalytron XO design?

Looking at the Focal speakers frequency response / impedance graphs 2400 kHz seems like a good crossover point.

There is a peak in the TC120 TD5's response around 3500 kHz. Is the tweeter's XO parallel R2 15 Ohm resistor and C2 1uF cap is to tame this?

Where do I find out more about "baffle width"? I presume this relates to diffraction loss and box/room/placement influences? The tweeter and woofer are recessed (level with front panel). The tweeter on my box is offset (closer to one side). My front panel is narrower (+ deeper) than the Zalytron and has larger rounded (with a 45 degree step) corner.

/Roger
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Old 20th August 2008, 02:38 AM   #4
focal7C is offline focal7C  Australia
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I should mention that another diyAudio member has kindly sent me another Focal crossover design with similar elements (see attachment). What are the crossover frequencies here?

That design has a single Focal 7v013 woofer and TC120 TDX2. Maybe this is better starting point?

I don't know the type/dimensions of that enclosure.

According to some posts this tweeter is hard to get a flat frequency response but is fantastic once you do. Where can I get the frequency response plot for the TC120 to see how similiar it is to my TC120 TD5? The construction is the same and the sensitivity the same.
Attached Images
File Type: jpg focalcrossover.jpg (27.7 KB, 198 views)
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Old 20th August 2008, 02:56 AM   #5
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Re:"does not the two 3.85 Ohm woofers in series equate to 7.7 Ohms?" err, yep, didn't spot that; but unless you measure the actual impedance at your chosen xover freq., using the original xover values will be close enough.

Re: "baffle width", the answer is in the following, I've given you all the links because xover questions are common, but the answers highly variable in assumed ability...

Getting started in Crossover Design

The basics (You've got to understand the rules before you can break them :-)
http://sound.westhost.com/lr-passive.htm
choosing xover point:
http://www.speakerbuilder.net/web_fi...xpointmain.htm
Baffle Step Compensation: http://sound.westhost.com/bafflestep.htm

Getting it done:
Jay has a page on designing xovers using manufacturers specs without measuring:
http://www.geocities.com/woove99/Sp...DesigningXO.htm

Calculators:
http://ccs.exl.info/calc_cr.html#second
http://www.sengpielaudio.com/calculator-Lpad.htm

Measurement freeware:
Synrta - http://libinst.com/SynRTA.htm
ARTA http://www.fesb.hr/~mateljan/arta/download.htm
ARTA Jig - http://zobsky.blogspot.com/2008/01/s...t-jig-for.html

Examples from the designs of others can be quite instructive:
http://www.zaphaudio.com/
http://www.troelsgravesen.dk/Diy_Lou...r_Projects.htm
http://www.rjbaudio.com/projects.html
http://www.humblehomemadehifi.com/

Ignore the naysayers who will always tell you why you can't do something, yet never have a positive alternative to share(usually having done nothing themselves).
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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