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Problems with impedance measurement
Problems with impedance measurement
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Old 7th March 2018, 08:40 AM   #11
Lojzek is offline Lojzek  Croatia
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By the calculations of Xc according to the impedance of 3.9R, around 20 Hz, something like 470 uF. Can this plastic covering the souncard be removed without damaging it?
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Old 7th March 2018, 04:09 PM   #12
sese317j is offline sese317j  Germany
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Default Behringer UAC 222

This is the datasheet of the behringer uac 222

Click the image to open in full size.

Click the image to open in full size.

And yes, the plastic cover can be removed relatively easy.


Click the image to open in full size.
Click the image to open in full size.

But the first thing im going to try now is to use a resistor with a higher value, and if this doesnt work maybe i will try to measure with a power amplifier.

Click the image to open in full size.
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Old 7th March 2018, 04:29 PM   #13
EarlK is offline EarlK  Canada
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Hi,

It looks to be an impedance mismatch ( to my reckoning ) which is upsetting the sense ratios .

If you can jury-rig up another adapter then try the headphone output on the UCA222 ( which has a much lower output impedance )

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Old 7th March 2018, 04:46 PM   #14
sese317j is offline sese317j  Germany
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Default Resistor with higher value doesnt work

I now tried 47R, 85R and 123R resistors and there was no improvement at all, the impedance at 5hz rose even further.
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Old 7th March 2018, 07:12 PM   #15
Lojzek is offline Lojzek  Croatia
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Thanks for the photos, sese. There appear to be some capacitors (can't read values) near the in/out terminals but I'd leave it alone as is. Who knows what kind of damage one might introduce by messing with these.
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Old 8th March 2018, 08:08 AM   #16
Mario Pankov is offline Mario Pankov  Europe
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Problems with impedance measurement
Here`s your solution:

Do not use the RCA output of the behringer but the headphones output - the RCA`s cannot drive the load, thus the decline in impedance above 10Khz. When you measure with the headphone jack, set the volume to maximum. It has a 4560 inside which can drive it, if you want more - there are mods with ICs from Analog Devices that can deliver more current (and sound better). I bet if you measure a ported enclosure, the second (lower) peak will be absent (so it will look pretty much like a closed box).

Then, you need ot measure with the driver screwed onto something and away of reflective surface. Otherwise you will have bumps in the impedance plot.

If you want to be completely nerdy, you can order a non-inductive (Ayrton-Perry wound) resistor with 0.1% precision. Audio resistors have 10-5% tolerance and are inductive (MOX as in your pic inclusive).

Last edited by Mario Pankov; 8th March 2018 at 08:11 AM.
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Old 8th March 2018, 09:48 AM   #17
Lojzek is offline Lojzek  Croatia
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Mario, can this small Behringer sound card measure a resistor impedance to be a flat line or not, from start to beginning? Inductive audio resistors are not a problem, they measure ruler flat to 20 kHz.
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Old 8th March 2018, 10:52 AM   #18
AllenB is offline AllenB  Australia
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Post 6 shows a resistor. Looks like it has a capacitor in series. Could this be due to too small an internal coupling cap? At least it looks accurate higher up where the action is.
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Old 8th March 2018, 02:00 PM   #19
studiophone is offline studiophone  United States
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Have you done the calibration? under the impedance measurement tab set resistor to 0 and wire it up according to the instructions. (Basically no Resistor out and in L R) this might compensate for the off curve. Even if you canít fix the problem like Alan said you can see the important stuff. You can still design for the driver.
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Old 8th March 2018, 02:58 PM   #20
giralfino is offline giralfino  Italy
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I'll show you 3 images.

First, a scenario like the one posted, done with a Behringer UCA202, connecting to the Line-out. The measure refers to a 3R9 resistor:

Problems with impedance measurement-resistor-1a-jpg

Second, the same resistor with the same sound card, but connecting to the phones-out, and with the volume at maximum:

Problems with impedance measurement-resistor-2a-jpg

Third, the same resistor on a different sound card (M-Audio MobilePre), but using the same jig:

Problems with impedance measurement-resistor-3a-jpg

You can draw the conclusions, and decide if the UCA can be used or not for impedance measures.

Ralf
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