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Old 4th January 2003, 10:40 PM   #11
Wizard of Kelts
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Simulation with an Re of about 4. The 3.8 Ohms reading will change this only marginally.

Forget about the low impedance near 200 Hz. Your crossover components will riaise that. Your inductor is probably about 4.7 mH or so.

The impedance vaalley at the tuning frequency is probably just a bit higher than shown. It is a rule of thumb that the impedance at the tuning frequency of a bass reflex is between 1 and 1.5 ohms higher than Re. However, most of the time it measures a bit higher.

I would say that you could probably hook these up as they are. To be safe, I would consider an 1 ohm resistor in series. It will cut your midpoint sensitivity by 2 dB, but will put you safely over the 6 ohm line. You could even consider parallelling two 1 ohm resistors to give a 1/2 Ohm value. That would cut midrange sensitivity by less than 1 dB.

Here is the chart.

The light blue line is your response on 2.83 Volts. I watt for an 8 ohm speaker, 2 watts for a 4 ohms speaker.

The purple line is the impedance.

The yellow line is the impedance phase angle.
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File Type: gif ilianh woofer impedance.gif (20.5 KB, 372 views)
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Old 4th January 2003, 11:13 PM   #12
Ilianh is offline Ilianh  Canada
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Thanks for the help guys, i'll go get some mega 1 ohm resistor now.. i hope i can find one.... or i'll par some others...
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Old 5th January 2003, 12:01 AM   #13
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Quote:
Originally posted by kelticwizard
I would say that you could probably hook these up as they are. To be safe, I would consider an 1 ohm resistor in series.
Keep in mind that the 6 ohm limit is for using the amp with all 5 channels driven. If you are only using it to drive a pair of speakers the current that would be available for the other channels is now available for the two channels used so you have a little more leeway.

Also, they will have given themselves some margin for error. One of the reasons being that a speaker will often be speced at a higher impedance than it really is so that the sensitivity spec can be exaggerated.

I'd just try them. Listen for strain as you turn the volume up. Then stop and back them off. That is the limit of your amp. Then you can try it with the resistor. If it doesn't play louder with it in then you don't have a problem. If it does then the amp is very sensitive to low impedance loads.

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Old 5th January 2003, 05:18 AM   #14
Ilianh is offline Ilianh  Canada
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I'll try that, thanks


And when calculating the crossover, I need to take in account the 1 ohm I added (if i do), right?
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Old 5th January 2003, 05:36 AM   #15
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Quote:
Originally posted by Ilianh



And when calculating the crossover, I need to take in account the 1 ohm I added (if i do), right?
Whoa. From your first post, I thought that these speakers were already built, crossed over, housed, finished, and up-and-running!
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Old 5th January 2003, 05:43 AM   #16
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By the way, your parameters did not contain a spec for Le. I modelled the speaker with an Le of 1mH, but that was a guess.

I noticed your Thiele-Small parameters had such numbers as Fc, Qmc, etc., that indicates that this speaker was used in a closed box. Do you have a response graph of the speaker either unenclosed or in a closed box?
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Old 5th January 2003, 09:09 AM   #17
Ilianh is offline Ilianh  Canada
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The multimeter I have has a resolution when measuring inductance of 0.01 H

I could test its Le, but at this resolution..

Quote:
Whoa. From your first post, I thought that these speakers were already built, crossed over, housed, finished, and up-and-running!
I said this in the 4th reply


Quote:
The speakers I made have an external crossover so I can change stuff on it and make tests easily.


Quote:
Do you have a response graph of the speaker either unenclosed or in a closed box?
Not yet, I look forward to using speakershop for this but my sound card is half duplex (well, theoriticaly, it should be full duplex...but.. speakershop sais its not)
well I'm getting a new sound card soon because I burned the preamp stage of this one...
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Old 5th January 2003, 12:07 PM   #18
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Ilianh:

A) Oops, sorry, missed your 4th post where you said that. That explains a lot.

I mean, I was just wondering, if you had not built the speakers yet, why you would be trying to use woofers that you were thought might not be suitable. I thought, "If this guy is worried, why doesn't he just use 6 or 8 ohm rated woofers to use in his speakers?"

As for the response graph, I noticed that your parameters contained such numbers as Fs and Fc, Qms and Qmc, Qes and Qec, etc. Those numbers that end in c mean that this speaker was enclosed in a sealed enclosure, and those measurements were taken and listed along with the parameters in free air. I was just wondering if your speaker came with a response graph in a sealed enclosure, since those "c" numbers indicate that measurements were taken in a sealed enclosure.

By the way, what is the brand and model of your woofer?
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Old 5th January 2003, 12:10 PM   #19
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Full duplex sound card? I don't know what your finances are, but you can get a decent full duplex sound card for $10 delivered at www.pricewatch.com. Just click on Multimedia-Sound Cards and take a look. that is how I got my sound card.

I ordered a generic sound card with the Crystal 4280 chipset and I used a freeware program to arrive at a frequency response given in this thread:
cabinet design is giving me issues!

The link to the test freeware is given on that page as well.

At first glance, the response doesn't look good, but then you look closely and see that the sound card is -3 dB at 35 Hz and only -6 dB at 20 Hz. Not bad performance, but more importantly, predictable and therefore can be used to test equipment under 20 Hz.

The sound card came without softwaare, but I downloaded the driver at www.drivers.com without a hitch.

PS: If you do download the freeware for the 4280, for some reason I found it easier to put it into a folder in Program Files and then install the sound card through the Add Hardware part of the Control Panel. When the time came to choose, I chose Have Disk and specified the C drive and the specific folder the 4280 was in. For some reason, it was only interested in the "inf" file.
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Old 5th January 2003, 06:01 PM   #20
Ilianh is offline Ilianh  Canada
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Kelticwizard:

Well theres the CMI 8738 4.1 channel and its 14$ cdn and the Diamond DT-588 4 channels too for same price, at a store 1 street away.
The CMI8738 is full duplex, I think i'll get this one.

Quote:
I was just wondering if your speaker came with a response graph in a sealed enclosure, since those "c" numbers indicate that measurements were taken in a sealed enclosure.
Well its a AR 8"speaker and its writen d01-0007 on the back, I had to find the thiel-small parameters myself because AR didnt respond to my emails. I used the sealed enclosure method to fnd the specs.
This driver is used in the ar-318ps..
http://www.supremevideo.com/audio/ac...ch/ar318ps.htm
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