# Impedance measurement on Beta



## DanTheMan (Oct 12, 2009)

What a great feature for the speaker designer! Now REW can get you everything you really need to know especially considering the ease of getting many overlaid responses. I'm going to build a rig to do these measurements. Has anyone else done this yet? For some reason I'm a bit hesitant even though it seems so simple. I should have it built by the end of the day, but I'd sure like to look at what some others have done before I do mine.

Dan


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## laser188139 (Sep 19, 2009)

Dan, I've only been thinking about which parts I need. I'm sure I have a ~100 ohm resistor somewhere from years gone by. I was thinking of using alligator clips to make it easy to connect to the back of the speaker. If I had a flexible wire between the resistor and the alligator clip, I could just clip it upstream of the resistor for the loopback calibration connection, shorting across the resistor. 

Bill


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## DanTheMan (Oct 12, 2009)

Thanks Bill.

Anyone know exactly which pin of the XLR connectors go where? should the "neg" and "g" pin be tied together, or should the ground just float?

Thanks,

Dan


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## Chester (Feb 19, 2007)

Has anyone figured out what to do about the ground yet? I have been trying to make impedance measurements on a pair of headphones (curious what the curves are) however the graph I get appears to be a straight line within 0.1 ohms of the resistance of the Sense resistor (checked with 47.1 and 101.0 ohm resistors and this was the case); after ~1-2 khz, the line drops off slowly (resistance goes down on the graph).... I think it has to do with not grounding things properly; I have also tried 'switching' the sense resistor and the headphone contacts however this yields a graph of ~0 ohms resistance which peaks at 0.3 ohms @ 20khz. If you have a working impedance measurement apparatus and could take a closeup picture of two, I would appreciate it


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## laser188139 (Sep 19, 2009)

Matt, here's a photo of my quick and dirty impedance testing rig. 









I started with a RCA cable, cut it in half, then stripped the wires. My old Klipsch LaScalas have screw connector strips so I used a spade connector for one of the sides. As the stripped wires are very thin, this gave me a good point to solder the three grounds together. The hot side of the source (red), I soldered to one side of a 100 ohm resistor, along with one of the return cables (white). The other return cable (red) I soldered to the other end of the resistor. For the hot side to the speaker, I used a short scrap of stranded wire to an alligator clip, as I can clip the alligator clip to the speaker or to the other side of the resistor, to short it out for the calibration step. In hindsight, I could have used a smaller wattage resistor than the 2W I chose. 

In case you're curious, here are the measurements I took on my two front speakers. After replacing the crossovers, the nominal impedance should be 6 ohms which I see on the left; for some reason the average impedance of the right is 7 ohms. 









The reactive component stays within ±20° down to 70Hz. You can see the phase change at ~500Hz and ~4kHz where the crossovers are between the three drivers. 









Bill


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## DanTheMan (Oct 12, 2009)

Thanks Bill! Much appreciated.

Dan


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## Chester (Feb 19, 2007)

Thank you for the pictures; so basically, the ground is common between the soundcard output, the speaker, and the line in.. Correct? I have a 5 watt resistor I am going to be using  wanted to use a non-inductive resistor, figured that wouldn't be good when trying to measure inductance...


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## laser188139 (Sep 19, 2009)

Chester said:


> ... so basically, the ground is common between the soundcard output, the speaker, and the line in.. Correct? ...


And both lines-in, yes. 



Chester said:


> ... I have a 5 watt resistor I am going to be using  wanted to use a non-inductive resistor, figured that wouldn't be good when trying to measure inductance...


I was a little casual, just wanting to see how the measurement technique worked. So I did not worry too much about any residual inductance in the resistor; I assumed it should be very small relative to the resistance. 

The soundcard is just not going to put out very much power, especially when configured for headphone output. I thought about using a larger resistor, but decided 2W would be more than enough, and it was overkill. The 5% resistors I had on hand seemed too low a wattage, and I figured it did not need to be perfectly accurate provided I used a meter to measure the actual resistance and did not just assume it was 100 ohms. 

Have fun,
Bill


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## Ricci (May 23, 2007)

Wait...What? When did this feature get added?


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## laser188139 (Sep 19, 2009)

John introduced this in the version of June 19th. I did not get around to building a jig myself until August, just to see how it worked. Neat, eh? Details on how to set it up, what the wiring needs to be, what steps to take, are all in the helpfile. 

Bill


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## Kenneth R. (Oct 20, 2010)

ooooooooooooh this looks fun. I want to try it.


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