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Old 30th December 2006, 11:11 PM   #1
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Default simple impedance question

If I put a multimeter to the + and - of a driver, what is that impedance rating I am getting? What does it mean in terms of the impedance that driver will have towards my amplifier?

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Old 31st December 2006, 12:09 AM   #2
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You cannot use a multimeter/vom to check the impedance accurately of any driver. The only correct way is to use an impedance meter.
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Old 31st December 2006, 12:12 AM   #3
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Assuming you have disconnected the driver from the amp, the ohms you read should be about 25 or 30% lower than the impedance the driver will present when it is playing music.

So an 8 ohm driver will read about 5.8 ohms or so on the multimeter, a 4 ohm driver will read about 3 ohms or so.
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Old 31st December 2006, 12:14 AM   #4
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These are for nominal ratings. The actual impedance will vary according to frequency.
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Old 31st December 2006, 12:19 AM   #5
Vikash is offline Vikash  United Kingdom
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Hi patchwork,

The first thing to know is a driver's resistance changes with frequency. So a single figure doesn't tell you everything. You want to view the driver's impedence plot to know what you're amp will really be driving.

Re is a driver's DC resistance which is measured with a MM with the driver in free air.

The nominal impedence is a kind of average taken from the impedance plot of the driver. Usually means very little.
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Old 31st December 2006, 12:23 AM   #6
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Quote:
The actual impedance will vary according to frequency.
Thus the reason to school someone properly to use the correct equipment.

There are too many variables in trying to use a VOM/multimeter. First and formost how accurate is the meter?

I always say the proper tool for the job. This is why we mention the usage of books by Morgan Jones and other people noted for their profound knowledge to school people and provide a proper knowledge base.
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Old 31st December 2006, 02:47 AM   #7
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Maybe he just wanted to know if he had a 4 ohm or 8 ohm woofer.
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Old 31st December 2006, 12:37 PM   #8
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kelticwizard

My logic is very simple but some may feel it is flawed. In an ideal world you would teach someone how to use the proper tools.
It would be very simple for patchwork to drive down to his local stereo shop and borrow an accurate impedance meter. This meter would provide him with all the necessary information he would need to evaluate his current driver. Example:
It would provide the impedance at different frequencies. It would also tell him at a glance if there was any voice coil damage to the driver. This is why I push the idea of using this tool which has been designed specifically for this purpose instead of trying to use a VOM meter. I don't care what you factor in for your percentage of impedance it simply will not provide accurate information.

Story time now....

I work for a sound consultant in the midwest. My job consists of fixing what people "in the know" screw up. In other words when a sound system (usually a large one like a stadium system) gets screwed up I fix it. I have had my hands full of people trying to use a VOM meter to analyze a speaker load and making changes based on what their VOM meter tells them. It is simply incapable of providing accurate results and therefore shouldn't be used period.
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Old 31st December 2006, 12:45 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi Burned,
is there a diy version (cheap and easy to source/build) that can do the job properly?

Can you tell us how to use it after we have built and calibrated it?

I have used the Constant current method and it is certainly not simple.
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Old 31st December 2006, 12:46 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi Patch,
you have gone very quiet.
Have you got your answer?
Or do you want more?
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