Unpredictable impedance in open circuit

setiawan

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I'm trying to rig up a little bit of wiring, so that I can measure the T/S of a driver. I've used a 100ohm resistor as the sense resistor

As per the guide, I have hooked up the wires and resistor like so:
Measurement.jpg


In guides I've seen, it shows that they get a flat response from the open circuit calibration. However, I get some fluctuation under 150hz, and it changes between measurements. The following five were done one after another, and as you can see they are all different.

1.jpg


2.jpg


3.jpg


4.jpg


5.jpg


The short circuit calibration and reference calibration are also similarly unreliable/different from the guide.

If anyone has any potential explanations, or suggestions on how to improve the results, please let me know. Thanks!
 

John Mulcahy

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Those combo connectors have a stereo headphone output and a mono mic input, they are no good for impedance measurements.
 

setiawan

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Those combo connectors have a stereo headphone output and a mono mic input, they are no good for impedance measurements.
Sorry I should have clarified, I'm using one of those cheap USB soundcards. This one supposedly has stereo mic input.

Looks like I don't have enough posts on this forum to link to it, but it's locatable on Amazon under the brand Koopao.
 

John Mulcahy

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Check the Windows audio properties for the soundcard input to make sure the default format is stereo and the same sample rate as the output and REW.
 

setiawan

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The default format for the microphone in Window's sound control panel settings is already set to "2 channel, 16 bit, 48000 Hz (DVD Quality)" (though it is greyed out):

mic properties.jpg


It's also set to 48 kHz in REW's preferences, as well as the "Make a measurement" box.

Any other ideas what might cause something like this?
 

John Mulcahy

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Have you used a mono connector for a stereo signal anywhere? Your diagram shows one, but it seems you aren't using that so a diagram of what you actually are using might have some clues.
 
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