Resistance of test leads

Discussion in 'Official REW (Room EQ Wizard) Support Forum' started by djcarpentier, May 13, 2019 at 1:31 PM.

  1. djcarpentier

    djcarpentier New Member
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    Hi all,

    First post here. I have a set up with a 100 ohm 0.1% resistor and after calibration i go to make a measurement of my test lead resistance. I consistently get a flat line value of 3.2 ohms. This doesn't make much sense. I really want to use REW for calculating T/S parameters. Does anyone have any insight on this? 3.2 ohms.png
     
  2. John Mulcahy

    John Mulcahy REW Author

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    Which REW version are you running? Did you run impedance calibration?
     
  3. djcarpentier

    djcarpentier New Member
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    REW 5.19. Yes i run the impedance calibration and it seems to come out ok.
     
  4. John Mulcahy

    John Mulcahy REW Author

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    What is the full setup - interface being used, connections etc.
     
  5. djcarpentier

    djcarpentier New Member
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    I followed the impedance design page for a standard measurement system. Just running it off my PC's onboard audio ins and outs. I do have an audio interface, but haven't tried to use it yet. I have very good electrical knowledge, so i'm thinking this has something to do with my onboard sound card. My test leads are also on the longer side as i need space to do any real measurements. I've attached some photos of my test rig. I guess my nest step would be to try my audio interface. It has fully balanced ins and outs though, so i was trying to avoid it. I also don't have the greatest work are (living room).

    Thanks very much for having a look John, much appreciated.

    20190514_124937.jpg 20190514_124950.jpg
     
  6. John Mulcahy

    John Mulcahy REW Author

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    The long leads are the ones to and from the onboard audio then. Even in that case 3 ohms seems high, that's around 10 m of 30 AWG. Could try a version with short leads to see how much difference that makes. If you measure a 100 ohm resistor do you get a 103 ohm result?
     
  7. djcarpentier

    djcarpentier New Member
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    The longer one is to the sound card output. Short one is input. Then there are the two speaker leads that are fairly long ~ 1m. R sense measures at 99.7 ohms. This is about right as it is supposed to be a 100 ohm 0.1% resistor. Going to hook up my Focusrite Forte with this system and see what happens. I'll let you know.
     
  8. John Mulcahy

    John Mulcahy REW Author

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    I wasn't referring to the sense resistor, I was meaning to take a 100 ohm resistor and measure it with the rig. Should have picked a different value :)
     
  9. djcarpentier

    djcarpentier New Member
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    Ahh. Will try that and let you know.
     
  10. djcarpentier

    djcarpentier New Member
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    Just made a measurement with my Focusrite Get a flat line value of 2.87 ohms. Put in my second precision 100 ohm 0.1% resistor and now i get a value of 102.8 ohms. Somehow my measurement rig has an impedance of just under 3 ohms. Frustrating. I guess i can resolder everything, but i fail to see that their will be a significant improvement.
     
  11. djcarpentier

    djcarpentier New Member
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    Re-soldered everything with short leads. Exactly the same output. Don't think its worth investing more time and money in parts. Time for plan B.
     
  12. John Mulcahy

    John Mulcahy REW Author

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    Strange one. Difficult to think of a mechanism that would produce an offset like that, perhaps something a little odd with the ground connections? Best of luck with plan B in any case.
     

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