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Inconsistent Ribbon Measurements
Inconsistent Ribbon Measurements
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Old 11th June 2019, 09:58 PM   #1
ericimage2 is offline ericimage2  United States
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Default Inconsistent Ribbon Measurements

Hi, I have a technical question if anyone can answer, I was measuring the DCR of my ribbon tweeters of my GT3R GT Audio works speakers and I am confused, If I use my $30 SouthWire multi-meter i get a resistance of 3.7 ohms but if I use my fancy $400 BK LCR meter using DCR setting I get 19.2 ohms but if I measure the resistance (R) or Impedance (Z) at 1 Khz I get 3.675 ohms and 3.854 ohms respectively.

Also w hen I measure the DCR with the BK LCR Meter the ribbon tweeter moves a lot but with no sound, and when measuring R or Z I can hear the expected 1 Khz tone. In the other hand with the SouthWire MM It does not move or make any noise when I measure the Ohms.

Why does it moves so much with the BK measuring DCR and why is the DCR value so high on the BK?

BTW I tested the BK and is calibrated!

Thanks a lot for the answers!
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Old 14th June 2019, 10:07 PM   #2
bolserst is offline bolserst  United States
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What Model of B&K LCR Meter do you have?

Most of them do not measure true DC resistance. Instead, they take an AC impedance measurement(usually at ~100Hz) and calculate the resistive and reactive parts of the impedance at that frequency and display the resistive portion. You mentioning that you can see the ribbon moving makes me think this is the case and the AC excitation frequency is close to the resonance of the ribbon.

Some of the newer B&K meters also give the option to choose between modeling the impedance as two elements(resistive, reactive) in series or two elements in parallel. I know the B&K Models 878 and 879 default to the parallel model when resistance is selected which may be what is happening in your case. Switching to the series setting should get you a resistance reading much closer to the true DC .

For example, 4ohm in series with 200uF if measured with the parallel model would display 19.8ohm instead of 4 ohm.
Attached figure created with spreadsheet posted here:
ESL electronics 101 for the electronics challenged
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