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Old 24th June 2002, 02:48 PM   #1
alvaius is offline alvaius  Canada
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Default Help Needer on Dual Woofer\Mid-Base Cross-Over

Help!!!

I am already an experienced DIY (and some professional) electronics audio buff. I had no expectations of being a DIY Speaker Buff, but I seem to have been forced into it.

I recently bought a set of 2 main channel speakers and a center channel speaker from a small speaker company that may be on its last legs. I bought based on my listening experience from about a year previous. Here is the problem. While the cabinet, drivers, etc. are fine, they seem to have just thrown the cross-overs together. The units are large towers consisting of two 8" mid-bass drivers and a 1" open-weave silk dome tweeter. The lower 8" driver cross-overs at a lower frequency with only the upper driver extending much into the mid-range. Based on some measurements I have taken, and some guess work, I expect the cross-over frequency should be about 2KHz. The mid-bass is pretty good up to that range based on some dirty tweaks I have tried.

It is a basic ported design, however, the unit is stuffed pretty solid with dampening material to behave more like a non-ported system (confirmed a year ago when talked to the designer).

I have most of the specs for the woofer and tweeter (except a response\impedance curve). I can run a crude response curve and an impedance curve will be relatively easy to come up with.

So, my question(s):

1) Is there any book, article, etc. that will allow me to easily figure out how to configure the cross-overs for such a design? I.e. where to cross-over lower base unit, mid-base, etc. Phase compensation, etc.

2) A couple suggestions on free\shareware modelling software for cross-overs.

3) A suggestion for a reasonable cost microphone with a flat response or at least one that is well characterized so that I can flatten the response.

I am pretty up on the components so sourcing them is not a problem.

FYI, the speakers only cost me $650 U.S. for the set of mains and $200 for the center channel which was about 1/4 of the original list. I guess I will now have to consider this a reasonable cost "DIY KIT". The cabinet construction is fantastic and the drivers are actually really good. Below 2KHz, they sound far better than my Energy 22.2s and the bass is very clean with no air pumping. I purchased them from the factory at www.zetag.com

Thank you in advance!
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Old 23rd July 2002, 07:17 PM   #2
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You should look voor a book called "cookbook on loudspeakerdesign" by Vance Dickenson.
This book describes all you need to know about speakerdesign and filter development.

rudy
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Old 23rd July 2002, 10:40 PM   #3
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Default Good Woofers?

Would you consider the 8" woofers they have for sale separately to be a good bargain? You seem to say that they produce good bass. I'm looking for <500Hz only from an 8.

I second that book reccomendation. They also list good microphones for testing. I use a Radio shack SPL meter, which comes with a chart of on and off-axis mic response.

Cheers, Dan
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Old 24th July 2002, 12:09 AM   #4
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I couldn't get the Zetag URL to work...

the XO (1st order most likely) for the bottom woof is determined by the baffle-step frequency.

The other XO could be almost anything -- have you traced the existing XO.

dave
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Old 29th July 2002, 02:45 PM   #5
alvaius is offline alvaius  Canada
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To Planet 10 - Wow! Excellent response and exactly what I am looking for. So essentially the second woofer is giving me the extra oomph at the bottom edge to compensate for the bass response of the baffle step. That is something I can work with and my amp has tons of current to handle the lower impedance at low frequencies.

I had the link wrong www.zetagcorp.com

To another poster, yes, the woofers actually seem pretty good.

I have a pair of Energy 22.2s that are in theory down 2db at 28Hz. They are front ported and at that frequency, the speaker is essentially a hand dryer with tons of air coming out of the port. The response may only be down 2db, but I would not call what is put out at that frequency sound (These were reasonably good speakers 10 years ago). The Kinimas on the other hand actually still put out real sound at this frequency. The lower bass is far superior.

I have traced out the cross-over. Good for a laugh really. They use multiple inductors to get the intended inductance. They are all jammed close together and the mutual inductance is sufficient enough that when I took off one of the speakers and replaced it with a resistor to measure the cross-over, I had sound coming out of the other speaker. Not loud, but sound non-the-less. I had to do a double take as I was 100% sure it was disconnected. It was! The positive and negative were touching on the input closing the circuit on the other woofer. The mutual inductance between the coils was driving the other speaker.

For the woofer, the impedance\frequency curve I measured is as follows (mounted in the cabinet):

F ZS
200 6.768306011
300 6.752777778
500 7.379827089
1000 9.305031447
2000 12.93925926
2300 13.61509434
2600 14.70434783
2900 15.63539095
3500 19.37941176
4000 17.47174888
4500 19.17756098
5000 20.25360825

For the upper woofer, the capacitor is about 3.4uF and the inductor is around 0.8mH. For the bottom woofer, the capacitor is 6.8uF and the inductor is 1.4mH.

For the tweeter, I measured as follows:

F ZS
200 6.372056515
400 6.981481481
600 7.970036101
700 8.450922509
800 8.548339483
900 8.849070632
950 8.832352941
1000 8.820109689
1050 8.896703297
1100 8.8039783
1200 8.772597865
1300 8.616666667
1400 8.380775717
1500 8.506913997
1600 8.293421053
1800 7.913841369
2000 7.43
2300 7.093103448
2600 7.058723404
2900 6.706451613
3500 7.325773196
4000 8.010869565
4500 7.26
5000 7.887234043

I kept thinking that I made a mistake, but I could not find a real impedance peak. I tried smooth stepping through the frequencies with no luck finding any significant peak.

The capacitor was measured at 4.59uH and the inductor was 0.6mH.

All measurements were done with a computer generated sine wave that I confirmed on the oscilloscope. I used a true RMS meter that is accurate to within 1% over a 5KHz bandwidth and 3% over a 20KHz bandwidth.

Alvaius
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Old 29th July 2002, 03:59 PM   #6
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Quote:
Originally posted by alvaius
I had the link wrong www.zetagcorp.com
Right, i have actually been there. Nice girls. Lousy website.

dave
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Old 2nd August 2002, 06:40 PM   #7
alvaius is offline alvaius  Canada
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The width of the enclosure is 11 inches suggesting a baffle step frequency of just over 400Hz from your emails.

Built a first pass cross-over with a first order low pass on the bottom woofer at 400Hz, a second order LW on the upper woofer at 2.3KHz, and a second order on the tweeter at the same frequency with the phase inverted.

WOW!! I knew the drivers had potential! I went from being ripped off (still working on that), to having some excellent sounding speakers. The woofers really are very good. Not sure the tweeter is up to the same standards, I may try a better one if I can find a good match. Using Solen caps and some cheap air core inductors for now. I will upgrade and respond back on the results.

Thank's for the help.

Alvaius
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