makin' simple measurements

[steve_mak]

Hi all,

I cannot believe how hard it is proving for me to measure Re !!

I feel as though the gods are against me from the start !!

This is the scenario. I have bought a flash u-beaut (meaning fancy) digital LCR meter and have also got a cheaper Asian DMM.

The digital LCR meter measures R using a sine wave ... (you can hear it when it is connected to the speaker) so that renders the LCR meter useless for this.

My DMM gives me an unstable figure above 9 ohms because sound and pressure on the cone creates voltages ! (Poke the cone and the meter takes off !)

I take the driver to work and use a Fluke DMM and get 5.7 ohms. I use a no-name cheapie DMM also and get 5.8 ohms. I use a lab grade benchtop DMM made by HP and ... the figure ranges from 5.2 to 6.1 because the DMM (resolving to 3 decimal places) is measuring instantaneous voltages ! (the speaker is a microphone !!)

I have tried placing a known 8.055 ohm resistance in series with the speaker and powering the lot with a 1.5V battery and using the dc voltmeter (with its integrator functionality) to effectively measure Re but I get 11.4 ohms !!! How is this possible ? Why doesn't this work ?

Aaargh !! All I want is a dc resistance meter of some accuracy !

Helpful suggestions please ! How accurate is 'good enough' ?

Cheers.

[SY]

Use a simple analog meter. That's really the best choice for inductors.

Sadly, accuracy IS important here. If you can beg, borrow, or steal an old Simpson 260, all will be well.

[jackinnj]

Originally posted by steve_mak

I feel as though the gods are against me from the start !!

Ask Iphigenia if she want's another go at it.

[steve_mak]

Thanks SY,

Yes, since everything else seems to have almost 24bit resolution I thought it might be best to get to the second decimal point !

I am surprised others haven't shown where my techniques are wrong. I'm particularly baffled by the battery method. That sure is weird.

I'll do the testing in front of a few engineers and see what they think !

[hooha]

Lay the driver face down on a table and take the measurement. That will ensure no cone movement when you're measuring. 2 decimal places is a treat but 1 will be fine. I agree with the analog meter tho...

Mark

[SY]

By "accurate," I meant to 0.1 ohm or better. You don't need 0.01 ohm precision.

[steve_mak]

Back again after some work ...

No wonder I thought I was going mad, I have discovered that I have one driver at 6.05 ohms and the other at 11.49 ohms. Is this a worry ? Surely QA is better than this !

I have checked with various instruments now and am getting repeated results.

Expreriences please !

[simon5]

This is definitely a big problem.

Sorry but I need to laugh a bit

Not at you, but that's ridiculous. It seems one woofer is 8 ohms nominal and the other is a 16 ohms nominal unit. Maybe those woofers were used with a "special" crossover or something.

[AndrewT]

Hi,
I wish I had so much exotic/mundane equipment that I was capable of getting conflicting results.

Seriously, a DMM set to 200ohms is usually good enough to measure to 0.1r, but remember to allow for test leads and other series resistance errors and subtract them from your reading.

I am very surprised that the battery (current source) and series combination failed. Must try this when I get home.

[steve_mak]

Guys,

It looks as though the drivers have two wires in parallel for the voice coil and one of them is clearly disconnected (going by the measurements - not by sight) !!

Since the drivers are brand spanking new I am going to return one of them hopefully.

If for some reason they wont take them back, can a voice coil be replaced easily ? Are voice coils available for a large number of drivers as a rule or only a limited selection of drivers ?

Oh, and before you ask ... no the coil wasn't damaged during testing. One would not blow one of two fuses in parallel !!

What a start to speaker building !!