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Problems with impedance measurement
Problems with impedance measurement
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Old 6th March 2018, 08:03 PM   #1
sese317j is offline sese317j  Germany
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Default Problems with impedance measurement

Hey,
i recently got myself a pair of Dayton Nd105-4 drivers, and now im trying to measure the impedance curve with a crude self-build circuit and REW/Arta.

Click the image to open in full size.
Click the image to open in full size.
The cable with the yellow connector is connected to the output of my behringer u-controll UAC222, the red rca is connected to the right input and used as the refference, the silver one is used as the main probe.
Click the image to open in full size.

I tested the Nd105-4 with Arta and Limp, and from both programms i got pretty similar results, but they got some differences compared to the datasheet provided by Dayton.

Click the image to open in full size.

This above is the impedance i measured.

Click the image to open in full size.

And this is how the impedance curve should look like.

Click the image to open in full size.

This above are both curves overlayed, the yellow one is from Dayton, and the red one is from my own measurements.

My main problem is that the phase curve is nowhere near the datasheet. Besides that, the impedance rises as the frequency goes down. Above 3khz there is exact the opposide problem, the impedance does not rise as mentioned in the datasheet.

I dont know what is going on with my measurement, and already tried some things, for example to switch the rca cables (the reference and the main probe).
I tested both speakers and both times got the same problems

Thank you in advance for any help.
Attached Images
File Type: png FImp.png (48.2 KB, 21 views)
File Type: jpg GetImage(1).jpg (800.2 KB, 14 views)
File Type: jpeg GetImage123.jpeg (810.1 KB, 19 views)
File Type: jpg GetImage.jpg (728.4 KB, 20 views)
File Type: png Gudde.png (57.5 KB, 14 views)
File Type: png Nd105-4.png (45.7 KB, 9 views)
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Old 6th March 2018, 08:17 PM   #2
mbrennwa is online now mbrennwa  Switzerland
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Make sure your circuit is exactly according to the software requirement. Different software may need different circuits. It's a bit tricky to understand your circuit from the photos.

For testing purposes, it may be useful to replace your speaker / driver with a plain resistor. You should get an impedance "curve" that is a flat line at the resistor value. If not, something is not right with your setup.
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Old 6th March 2018, 08:24 PM   #3
EarlK is offline EarlK  Canada
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Your red trace looks like it was made while the woofer was mounted in a bass reflex box (?).

Manufacturers impedance measurements are just about always made with the transducer
free hanging ( ie; not mounted in an enclosure or near any reflecting surface ).

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Old 6th March 2018, 08:50 PM   #4
Lojzek is online now Lojzek  Croatia
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Default Correct impedance jig for ARTA

This one should work accurately.
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File Type: png IMP Jig.png (25.6 KB, 131 views)
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Old 6th March 2018, 09:43 PM   #5
giralfino is offline giralfino  Italy
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I have the UCA202, and the jig posted by Lojzek with a 20R resistor. If I measure a resistor instead of a flat line I obtain a flat line only above 200-300Hz, and a curve under, like the effect the OP experiences. No problems at higher frequencies though. I minimized the problem at the lower frequencies connecting the yellow probe to the phones output with the volume at maximum.
I very recently bought an used M-Audio Mobilepre MKII and solved completely the problem...

Ralf
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Old 6th March 2018, 10:05 PM   #6
sese317j is offline sese317j  Germany
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Default A bit of success

Im using the exact circuit Lojzek mentioned. The Idea of meassuring a resistor with a known value realy helped me to understand whats worng. I realized that my calibration was completey destroyed by whatever, recalibrated, and immediately got more promissing results.

Click the image to open in full size.
This is the new measurement, the problem with the phase is gone, and the rolloff at higher frequencies as well, only the problem with the high impedance at low frequencies remains.

Click the image to open in full size.

Click the image to open in full size.

This are the measurements of a resistor with an impedance of 3.9 Ohm.

Thank you all for your helpfull suggestions.
Attached Images
File Type: png Nd105Bessa.png (47.1 KB, 11 views)
File Type: png resi2.png (39.5 KB, 8 views)
File Type: png resistor.png (47.5 KB, 9 views)
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Old 6th March 2018, 11:09 PM   #7
Lojzek is online now Lojzek  Croatia
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Interesting, the last IMP plot of the 3.9R resistor appears as if the output of the soundcard had a capacitor in series with it, yet there is this strange thing, phase being positive and not negative as it should have been with a capacitor. I wonder if this could be by-passed with a wire and remedied to a flat line.
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Old 7th March 2018, 04:18 AM   #8
studiophone is offline studiophone  United States
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REW recommends using 100 ohm non-inductive resistor and the headphone out of the interface. I use an 82 ohm and it works fine. I would double your 47 ohm resistors. It also helps to measure the exact resistance and plug in that number instead. (82.5 in my case) Is that line out on that interface? If it is I'm not sure that's hot enough. You're close though. just a little more...
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Old 7th March 2018, 06:27 AM   #9
mbrennwa is online now mbrennwa  Switzerland
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Ok, you're getting there. Your soundcard input probably has a DC blocking capacitor, forming a high-pass filter with the dummy resistor. This would explain why the low-frequency rise shifts to lower frequency with the lower value dummy resistor. You could test this by adding another capacitor to the input and see if the effect gets worse.

Do you know the value of the DC blocking capacitor? Can you modify / change it? Can you use a different soundcard?
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Old 7th March 2018, 08:28 AM   #10
giralfino is offline giralfino  Italy
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Quote:
REW recommends using 100 ohm non-inductive resistor and the headphone out of the interface.
Quote:
I minimized the problem at the lower frequencies connecting the yellow probe to the phones output with the volume at maximum.
I used the phones output but the problem is still there. From some files I have, with that configuration a 3R3 resistor measures 5.1 Ohm at 20Hz, and a 4R7 resistor measures 6.4 Ohm at 20Hz, way lower than the roughly 16 Ohm shown in the OP graph for a 3R9 resistor. I don't remember if I tried also the combination phones out+higher value resistor though. For the OP this should be worth a try, for me changing the sound card solved the problem...

Quote:
Do you know the value of the DC blocking capacitor? Can you modify / change it?
Unless I missed something obvious, there is nothing you can do/change/remove on that sound card.

Ralf
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