# Impedance measurement with power amp



## JAmBer (Dec 22, 2013)

Following the advice in the help file, and this thread: http://www.hometheatershack.com/forums/rew-forum/62352-impedance-measurements-w-power-amplifier.html I constructed a little PA for doing impedance measurements, with a ~10.9ohm sense resistor. 

This works wonderfully for reducing noise (see the high frequency difference in attached file). Unfortunately, the calibration system seems to assume that you're always using a 100 ohm resistor and so the impedance numbers are all wrong. 

See attached plots showing the measured impedances for a Wharfedale Diamond 10.0 speaker when using the recommended 100 ohm value (correctly measures ~6ohm impedance) and when using the 11 ohm impedance (measures with an offset and scale error). I suspect this is the problem, because during calibration, it always claims a "perfect" (loopback) calibration with a 100ohm value.

A secondary problem is that my amp is inverting. Now the phase is calculated at 180degrees, rather than 0. The "invert" checkbox in the preferences, which I understand is supposed to correct this, makes no difference.

The shape and everything is correct, there's just an offset for some reason. Has anyone else experienced this? Have I done something wrong, is there a fix or have I uncovered a bug?


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## JohnM (Apr 11, 2006)

The '100' when you calibrate is a proxy percentage reading, not an impedance. Per the help file: "_The completed measurement shows the level of the measurement channel (usually right) compared to the reference channel, where a reading of 100 Ohms corresponds to 100%, 99 Ohms would be 99% etc._"

Impedances that are offset by the value of the sense resistor usually indicates left and right channels are swapped.


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## JAmBer (Dec 22, 2013)

Haha... you're absolutely right, I had the channels swapped. The 180deg out-of-phase should also have been a clue. Doh! Why didn't I think of that?! Now I feel like an idiot! 

Thanks for your help!


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