Cross over help needed

-- Background. Very experienced electronics DIY (and some I was payed for). I did not intend to become a speaker DIY again, but I have been forced into it. I hope that some of the gurus on here can help out and maybe I can return the favour on the electronics side.


At this point I am only looking for hand waving answers that lead me on the path. I think I can get to the exact specs....

I recently purchased what were "first" quality speakers at a clearance sale. Well, the drivers in them may be, but the cross-over, frankly, was the most screwed up thing you can imagine. The system consists of DUAL 8" woofers and a 1" soft dome.

I am guessing that the design was not supposed to be this way, but essentially the upper woofer crosses over somewhere around 5 KHz. I say around, because it has a massive resonance peak at 4KHz because the values are screwed. Ditto all around and nuff said. The lower woofer is maybe in the 3KHz range. Tweeter crosses somewhere in the 2-3KHz range. Again all the values are screwed so the responses are screwed.

So... I want to put in a proper cross-over. I am aiming for a 2.3KHz cross over frequency from the lower to upper. I will probably go LinkWitz Riley 2nd order for upper and lower.

?? My question. What should I do with the lower base unit. Should I run it exactly the same as the upper base unit? I think this may give me too much output and then I will need to put a resistor in the tweeter. Not to mention so much for pin-point mid-range. I am thinking about crossing over the lower base in the 400-500Hz range. I would probably go simple first order cross-over to minimize phase change. What are peoples thoughts?

The upper and lower base unit are in the same cabinet. It is a ported cabinet, but heavily damped with good quality insulation. If I cross-over the lower unit at 400-500Hz, will it not still behave like a passive radiator to some degree? Would this happen at over 400-500Hz? Is it a bad thing?

Other thought on this speaker. The tweeter has an open pole piece and the enclosure the tweeter is in is open to the woofers. Is this bad?.... i.e. would the tweeter be flapping in the wind so to speak in tune to the woofer movement?


For the DIYers who want to have a good laugh, this is what is in the cross-over now:

Lower Base: 1.4mH and 6.8uF in a Link-Witz Riley.
Upper Base: 0.8mH and 3.4uF in a Link-Witz Riley.
Tweeter: 0.6mH and 4.7uF in a Link-Witz Rileya and it was not reversed in polarity!!!

For reference: The woofer resistance is 6.2ohm. Impedance rises fairly smoothly from about 6.8ohms at 200Hz to 13.6ohms at 2.3Khz and 20.4ohms at 5KHz. The tweeter has an impedance of around 7.1ohms at 2.3KHz and rises to about 8ohms at 4KHz and it not much higher at 5KHz. Before you ask, no, I forgot to measure the resonance frequency.. had to sleep. That will happen tonight.
 
OK, let's see, if i got you right, you've got a system with two identical 8" woofers in a vented cabinet and a 1" tweeter with a hole through the pole.
One of the woofers works from the subbass to approx. 5KHz, the other from sub to approx. 3KHz.
First, the crossover freqencies are much to high. As experience shows, 8 inchers often have resonances at around 2Khz and above.Also that they have different filters is a weird thing that ends up in messy phase response over a great bandwidth.
Second, the drilled up pole is indeed a problem when coupled to the bass cabinet, the dome will be driven by air pressure caused by the woofer, that ends up in the Doppler effect wich causes frequency modulation in the upper high frequencies.

Let's see what we can change.
I'd use both woofers with a common filter and a crossover of about 1.8 KHz, the tweeter exactly in center of the woofers.
If your box isn' buildt that way you should go for a new baffle to exchange with the old one.Just place the tweeter in the middle of the baffle and put the woofers as close as possible to it.Of cause you should provide the tweeter with a hood of approx. 1/2 to 1 litre and stuff it with some damping material.
You can reuse your reflex pipe because we didn't change the cabinet dimensions, place it near the lower woofer.
This combination performs similar to a coaxial system.
The Volume behind the tweeter allows it to have more extension than a closed one, in combination with it's 1" diameter it should be able to perform down to 1,8 KHz.

Filters: the recommended filter structure for this alignment is a 3rd order Riley-Linkwitz.
If the woofers are too present at the xover point, try a bigger inductor in the lowpass filter.

Hope this helps, please keep me informed.

Regards

Arne
 
It does help. Thank you. Changing the front baffle is not an option. At that point, I will start from scratch.

I did some more tests on the woofers. They appear to have a mass limited breakup around 4KHz. [Couple that with a peak in the cross-over response at the same frequency and you can imagine what the female vocals sound like.]

I found a bit of information on the tweeter that suggests the Fo = 1400 Hz, but when I did a manual impedance curve with a true RMS voltmeter that was in theory good to 20KHz, I could only detect a slight increase in the impedance around 900Hz. It was unusual in that it was only about 2 ohms different from the rest of the response. I am a bit suspect of my measurements. I blew my line-in on my sound card a few nights ago. When I fix that, I will do a proper impedance curve.

I also found that the tweeter is not completely open at the back, but is a dual chamber design. The tweeter is built to the speaker companies specs [zetag], by VIFA, but it appears to be a modified version of what I attached.

http://www.d-s-t.com/vifa/data/d27tg-35-06a.htm

I know that internally it is an open pole piece, but I am not sure how well that plastic back on the tweeter seals it, how it effects the frequency response, or whether it is even a good thing.

The following link is the woofers in the system.

http://www.shoporium.com/shops/womblemillhollow/view_item.php3?id=67583

So you don't think that running one of the woofers with a first order cross-over at say 400Hz will give a better point source for the mid-range?

And of course, here is the actual speaker I bought....

http://www.zetagcorp.com/hig4.htm

They were about $650U.S. for a pair.

Looking forward to responses:

Alvaius