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Author Topic:  Testing Speaker Ohms
John Goux

 

From:
California, USA
Post  Posted 2 Dec 2018 9:35 am    
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What does the Ohm reading of a speaker indicate?

Lately sellers are posting photos of vintage speakers with the multimeter showing the digital ohm reading.

Some of these old speakers, like JBLs, are down from 8 ohms to around 6.5 or 5.5.
I borrowed an original D120 from a friend, which sounds great, and reads 6.5.

The question is, what is the effect on the sound, or reliability of the speaker, with an Ohm reading?

John
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Jerry Overstreet


From:
Louisville Ky
Post  Posted 2 Dec 2018 9:47 am    
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You probably know it's just the DC Resistance of the driver.

The important thing is to match a speaker with the amp. Amplifiers are designed to work with a certain amount of DC resistance. Too much is generally OK, but in some cases too little will fry an OT.

Amplifiers warning labels caution users to never go with less than the recommended ohms reading.

Far as sound quality and reliability, I don't know that this factor has any effect within intended use.

Generally, a vom reading will be a tad less than the stated measurement, but somewhere close as in the JBL unit you reference.
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Jon Light


From:
Saugerties, NY
Post  Posted 2 Dec 2018 9:50 am    
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Publishing of Ω reading is for the purpose of proving that the speaker indeed has the impedance being claimed. An 8Ω speaker will read around 6 on a meter. That's just a quirk of the impedance realities. Nobody would be trying to imply any significance of minor deviations of some decimal points. You just know that a 6Ω reading means it is rated for use in an amp calling for an 8Ω speaker and a ±3Ω reading would designate a 4Ω speaker, 15Ω, a 16Ω speaker.
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Greg Cutshaw


From:
Corry, PA, USA
Post  Posted 2 Dec 2018 11:36 am    
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Most rated speaker impedances are measured at 400 hertz and not at zero frequency or DC (as with an ohmmeter). The chart below shows a typical relationship between DC resistance and impedance. Note that for a classic tube amp, maximum power is delivered to the load or speaker when the speaker's impedance matched the internal impedance of the amplifier. Too low an impedance and the plate and transformer current will be very high and can reach damaging levels (think approaching a dead short). Too high and the transformer secondary voltage levels can increase and cause arcing (think approaching an open circuit).

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