Measured Impedance of Speakers

[xtian]

I bought an expensive-ish, true RMS meter (Meterman 38XR). It's great at measuring everything EXCEPT estimating the resistance of coils. When trying to measure speaker resistance, the value never settles. Same with the resistance of OT primary coils. I have to use one of my cheap meters for these jobs (as I don't have an inductance meter).

Why?

I have a Meterman 37XR.

Great meter!

I had better luck with I tried again. I think I have to clip the leads on, and then keep my hands out of the mix.

[fperron_kt88]

Please ignore if this has been suggested before.

Pick a low value resistor that you know well (that's the though part to get started, perhaps you have a 0.1% resistor? A 1% is not bad to start with...). You certainly have some left from a tube bias circuit mod you added on an amp...

Put the known R in series with the speaker and apply some low voltage source (Vs) across both the R and speaker. Measure Vr the *voltage* across R with your DMM. Measure Vs across the source.

Vr = Vs * R / (R + Rspeaker)

You isolate Rspeaker, the only unkown left:

Rspeaker = (Vs / Vr - 1 )*R

(look at the wikipedia page: http://en.wikipedia.org/wiki/Voltage_divider. search for "Solve for R1" on the page. The drawings are well made and the math is clear.)

Try to pick R close to the expected value of Rspeaker. This will help minimize errors.

The 0-300mV range of any DMM or regular multimeter will almost certainly have a better accuracy and precision than any resistance scale for the value of DC resistance of a speaker. It takes 2 measurements and a known resistor + some math, but you will have a value that makes sense.

I never had any luck (or patience) trying to avoid contact resistance for values in this range...

That or use a 4-wire resistance meter...

As stated above, the impedance of a speaker is not a fixed value. So measuring DC resistance is only one part of the story. Zspeaker is a curve that varies with frequency... but that is another subject.

Let me know if this helps in some way... Again, feel free to discard accordingly...

[Turbojunkie]

Never knew that measuring speaker impedance could be a hassle, guess I just got lucky with the ancient Fluke 77 I got at a yard sale for $8.00...all my 16ohm speakers are right around 13.5 and all 8ohm versions around 6.5...My 8ohm alnico SRO's are the only ones that measure lower at 5.7 & 5.9...

[fperron_kt88]

Yup, it can be a little more involved if you want precision and accuracy.

But, as you mentioned, for a ball park, the DMM on "Ohms" range is fine.

Contact resistance is a real bummer in this context. I've seen up to 4-5 Ohms easy...

[Turbojunkie]

Gotcha....I get 2 or 3 bits of great, useful info every time I jump on TAG, thanks to everyone for sharing the knowledge. GREAT people on this forum...