Jump to content

Unpredictable impedance in open circuit


Recommended Posts

I'm trying to rig up a little bit of wiring, so that I can measure the T/S of a driver. I'm following this guide: https://audiojudgement.com/measure-thiele-small-parameters-using-free-software/

 

Basically it involves putting together a cable that connects to a driver's terminals on one end, and to the input/output of a computer's soundcard on the other end, with a 'sense' resistor:

Measurement.jpg.e9504d4414eff5833f0bd09770650a73.jpg

 

The guide makes use of REW, and its impedance measurement. The first part is to calibrate the system, and the first calibration is for the open circuit.

 

TBH I'm way out of my element here. My electronics understanding is very poor, and my understanding of how this calibration fits into the bigger picture is poorer yet. What I do know however is that the open circuit impedance graph I get isn't nearly as flat as the one in the guide. Worse yet, when I produce these open circuit impedance graphs one after another, the results vary. Typically in the lower frequencies, under about 150hz, the impedance response fluctuates, and unpredictably between measurements:

 

1.jpg.593e43fc15ac32041bb66b190f3050e2.jpg

 

2.jpg.d22c80785b97d2605fcf30721f61acaa.jpg

 

3.jpg.c431c12d1ff4b354a4b680f5a85cde3e.jpg

 

4.jpg.1a2b5fb1b7a81753ba1dc79986892a38.jpg

 

5.jpg.b3899072852898a1dff68e13f2fff318.jpg

 

 

From this, I have two questions. Firstly, would this fluctuation in the lower regions affect my ability to accurately measure T/S parameters? Secondly, what may be some reasons that these fluctuations exist? Poor wiring? Low quality soundcard?

 

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

Link to post
Share on other sites

Without digging too deep, did you actually use a 100ohm (brown black brown) resistor as required, or a 4.7Kohm (yellow purple red) resistor, as shown in the diagram? What I mean is, did you just copy the colour coding, or actually use a 100ohm resistor?

Link to post
Share on other sites
7 minutes ago, bob_m_54 said:

Without digging too deep, did you actually use a 100ohm (brown black brown) resistor as required, or a 4.7Kohm (yellow purple red) resistor, as shown in the diagram? What I mean is, did you just copy the colour coding, or actually use a 100ohm resistor?

 

I bought a 100ohm 1w resistor from Jaycar. I didn't take much notice of stripes, but looking now it appears to be brown black brown gold/yellow. I've also measured it with a multimeter and it turns up pretty much 100 ohms, +- 0.2ohms.

 

Should I try a different resistor value anyway? And if so, should I be going up or down in resistance?

Link to post
Share on other sites

No, it was just that when I saw that pic, the error in colour coding struck me straight away. I can pretty much read them on sight without working it out in my noggin.

I haven't tried doing what you are doing, so can't really comment further on that.

Link to post
Share on other sites


  • Recently Browsing   0 members

    No registered users viewing this page.




×
×
  • Create New...