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Old 31st March 2013, 12:49 AM   #1
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Default Can someone check this crossover design for me?

Hi guys,

As some of you may know, I'm using Fostex FE126s for midrange and treble duty.

I'll admit that I like loud music from time to time, including things like Marilyn Manson. Those of you that have heard these drivers will know that such music isn't within their "comfort zone". The rising response in the kHz range, coupled with (seemingly) higher distortion up there gives a sound that will rip your ears off quite quickly.

So, I thought about this for a while and came up with a couple of solutions...

- change the Fostex driver for something else
- stop them running past a few kHz, use tweeters.

The extra treble dispersion would be quite nice, so I've opted for the addition of a tweeter.

I started by checking out some of the smaller Dayton tweeters, but found they were all ~90dB sensitivity. I also need a tweeter small enough to fit on my current baffle, giving an absolute maximum of 80mm external diameter.
After a bit of looking around, I found this tweeter:
BC25SC-06/04 - Vifa 1 inch Neodymium textile tweeter - Europe Audio

Zaph seems to like them: Zaph|Audio (you'll need Ctrl+F) and their distortion checks out rather well, even down to around 1.5kHz.

Realistically, I'd like to cross over as low as possible, which would keep dispersion wide (I normally sit quite far off axis), and also stop the rough upper midrange of the Fostex coming through.

I think 2kHz fits the bill quite nicely: if the Fostex datasheet is to be believed, there's a small loss off-axis up there, but nothing much. That crossover point should also keep the tweeter happy.

Anyway.

I went over to Rod Elliott's articles and found one about designing a "proper" passive crossover. Here it is.

I followed this through, and arrived at the crossover below:

Click the image to open in full size.

In the datasheet, the tweeter shows a fairly sharp impedance spike at resonance. As this is so close to the crossover point, a correction circuit is used to flatten it - the article goes into the effect the peak will have on the crossover, and it ain't pretty.
There's a tweeter L-pad to bring it down to the sensitivity of the Fostex driver.
The rest is the usual caps and inductors for a 2nd order crossover.

However, I decided to take Rod's recommendation for the exact Q of the crossover:

Quote:
Again, this is a world of compromise. My preference is for a sub-Bessel alignment with a Q of 0.5, since it provides a close approximation to a Linkwitz-Riley alignment, and has zero peak or dip at the crossover frequency. Since the Q is lower, it is also marginally less sensitive to variations in loudspeaker driver impedance, but this is not something that should be relied upon.
I decided not to bother with a Zobel on the Fostex - the datasheet shows a very minor change in impedance in that frequency range, so I decided that it was safe.

So yes, I've a few days before I can order the parts, so would appreciate any input you guys might have - this is my first attempt at a proper crossover.

Cheers
Chris

PS - if its relevant, the amplifier will be ~120w/ch into 4ohm, with a line-level LR4 crossover at 500Hz, where some woofers take over.
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Old 31st March 2013, 06:19 AM   #2
system7 is offline system7  United Kingdom
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Brace yourself, Chris. Not much of this makes sense to me, really!

Your Fs resonance correction is at the right 1.4kHz frequency though the resistor is too big at 2.8R. But that little Vifa 4 ohm cheapie tweeter is never going to work at 2kHz crossover.

I found a Troels project using your FE126E+ Fostex driver:
Fostex FE126E +

Troels does a rather good job equalising the Fostex. Why reinvent the wheel?

Crosses over at 3.5kHz. That sort of frequency is just your usual 3.3uF and 0.4mH followed by an attenuator making about 8 ohms overall load on a tweeter filter, since I don't suppose you'll be using his magnaplanar tweeter. I'd prefer to do this sort of thing with an 8 ohm tweeter rather than a 4 ohm one, but I'd forsee a bit of fiddling around with resistors on the tweeter for level.
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Old 31st March 2013, 06:19 AM   #3
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Quote:
Originally Posted by chris661 View Post
I think 2kHz fits the bill quite nicely: if the Fostex datasheet is to be believed, there's a small loss off-axis up there, but nothing much. That crossover point should also keep the tweeter happy.
Quote:
With a 6dB/octave filter, I suggest an absolute minimum of about 1.5 octaves between the driver resonance and crossover frequency. A tweeter with a 900Hz resonance should therefore be crossed over at a minimum of 2,500Hz, but preferably higher. If you use the minimum possible frequency separation, there will be a small peak at tweeter resonance - this is a combination of the tweeter's resonance itself, and the fact that the crossover cannot maintain the correct rolloff if the load impedance changes.
Rod Elliott - 3.1.2 Midrange and Tweeter Drivers
Vifa BC25SC-06/04
fs=1390Hz, one and half octaves is 2780(1.octave)+1390 1/2 5560(2.octave) = 4170Hz
Octave bands | APMR
Crossover frequency is very low considering the resonant frequency of the tweeter. Look at the use of the Fostex by others like Troels.
Fostex FE126E +
If the Fostex is 8ohm why you are using a 4ohm tweeter.
It would help if you were using frd/zma's of the drivers in a simulator.
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Old 31st March 2013, 06:23 AM   #4
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I'd try to not use an electrolytic.
If you must, then be sure to bypass it with the largest value film cap you can muster - the total value being what you need/want.

Finally, you have to listen and measure... on paper it might be wonderful, but until you listen and look at the resulting curve there is no real way to know what you actually are getting.

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Old 31st March 2013, 12:30 PM   #5
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Quote:
Originally Posted by system7 View Post
Troels does a rather good job equalising the Fostex. Why reinvent the wheel?
I don't trust the Fostex driver with frequencies that high (he also uses tweeters that are 66 a pop).

The crossover for the tweeter is low, yes, but the thermal power handling test that Vifa put on the datasheet indicates a 1.3kHz 2nd order filter, and the driver will stand 25w doing that (which is more than enough). Zaph's measurements show low distortion to 2kHz, with a bit of a rise below that. I really don't see much of a problem, so long as the tweeter resonance is suitably suppressed.

Which resistor value should I use instead of 2R8?

Inductor,
Note that the article states a 1.5 octave rule for 1st order filters. I'm going 2nd order.

I see no problem with using a 4ohm tweeter - the amplifier will play into 4ohm happily.
I have, however, eliminated the parallel resistor of the L-pad, and re-done the crossover values for a 3.8ohm load. The parallel resistor was only going to waste power by pulling more current through the rest of the components.


I suppose I could try a 2.5kHz filter, but I'd definitely like to keep below 3kHz.

Thanks for the help so far

Chris
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Old 31st March 2013, 01:55 PM   #6
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Have you ever tried Passive Crossover Designer? It's not too difficult to learn, and can be quite helpful. Hopefully you have measurement tools, but if not you can use SPL tracer to get the measurement files from published measurements. SPL tracer isn't perfect, and you won't have any phase information. It's still a good learning experience, and can help you get a start...
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Old 31st March 2013, 02:21 PM   #7
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Quote:
Originally Posted by Josephjcole View Post
Have you ever tried Passive Crossover Designer? It's not too difficult to learn, and can be quite helpful. Hopefully you have measurement tools, but if not you can use SPL tracer to get the measurement files from published measurements. SPL tracer isn't perfect, and you won't have any phase information. It's still a good learning experience, and can help you get a start...
I've downloaded it, but there's a problem: my laptop came bundled with MS Office Starter. The Excel program included doesn't allow the use of macros, so I can't run the spreadsheet.

The SPL tracer link doesn't go to anything audio-related.

I also have no measurement tools. I have a 31-band graphic eq that I'm using to tame some room issues, but I'm reliant on my ears for this one.

One day I'll get myself some measurement equipment, but I'm on a student budget so the measurement stuff will probably have to wait for a while.

Chris
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Old 31st March 2013, 05:10 PM   #8
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Quote:
Originally Posted by chris661 View Post
Inductor,
Note that the article states a 1.5 octave rule for 1st order filters. I'm going 2nd order.
Don't take that to the letter.
Sorry to say, but your design approach doesn't look appropriate. I would be losing my time here.
If you are not using a xover simulator or a Spice software you have no idea of what you are doing. Impedance peaks can be very tricky. You want to stay away from those resonance frequencies at the crossover band. A RLC would be a last resource to fix a good (bad) design, not a design feature to start width i.e. handicap.
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Old 31st March 2013, 05:42 PM   #9
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by chris661 View Post
I found this tweeter:
BC25SC-06/04 - Vifa 1 inch Neodymium textile tweeter - Europe Audio

Zaph seems to like them
probably get the good scores because its cheap, which makes it a contender in the 'value group'

doesnt't really mean its good
only that it may have 'potential' for the very low cost designs
which happens to be one of his 'domains'
and I suppose Zaph could make it work ok, with a lot of work and care

but without skills/experience/measurements ....
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Old 31st March 2013, 05:48 PM   #10
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by chris661 View Post

I'll admit that I like loud music from time to time, including things like Marilyn Manson.

So, I thought about this for a while and came up with a couple of solutions...

- change the Fostex driver for something else
- stop them running past a few kHz, use tweeters.
ahh, I see

you need a tougher midrange with bigger voice coil if you want to play loud without distortion
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