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Old speaker, impedance unsure, question on DVM reading

little-al

little-al

Active Member
I have two old Fisher branded 12" woofers, one early/mid 1960s the other early 1970s, but appear essentially identical. Both alnico. One was sold to me as an 8 Ohm and the other as 4 Ohm.

With a DVM, one reads about 4.2 Ohm (the advertised 4 Ohm), the other about 5.2 Ohm (the advertised 8 Ohm).

I haven't seen too many speakers advertised as 6 Ohm, although I suppose they do exist.

Based on this measurement, what do you think the actual average impedance is for these?

I'm really looking for a 4 Ohm driver in this case. Usually a DVM reading of a speaker reads a bit low not high, that is for an 8 Ohm speaker is it often a reading of around 6-7 Ohm. But perhaps that isn't always true?

Would either of these be OK for an application where I want a 4 Ohm speaker?

Thanks!
Al
 
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The most accurate test would be with a Woofer Tester 1~3 unit to look at the overall impedance curve. Only being one ohm apart, but still above four ohms I would still rate these as 8 ohm types. Static and reactive loads are differant, the reactive loads tend to rise above a measured static measurement, not go lower.
DC
 
One of them is from the early '70's, and they're BOTH alnico? Boy, you don't see that too-often (since most speaker-makers had switched to ceramic magnets by then). Could one be a factory-repair maybe, to match up with the other one? Who knows...
 
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Never trust a vintage speakers specs and measurements. Always do your own.

If an 8 Ohm speaker measures 5.2 Ohms that't a bad sign. I would take out the crossover and measure individual drivers. If they measure not bellow 7.2 Ohms then the crossover is due for recap and bad solder checkup. Pots also need Deoxit treatment.

I wouldn't recommend connecting an 8 Ohm speaker that measures 5.2 Ohm to 4 Ohm amp.
 
Thanks for the replies.

Both are bare drivers, pulled from Fisher consoles. Both speakers are Fisher drivers, and although 5-10 years apart are clearly nearly identical in manufacture with only minor cosmetic differences such as one having a larger fabric dust cap. Both are alnico, one from 1971 one from mid 60s.

I measured both speakers directly on their lugs with a DVM. They were not wired into any crossovers or other components. That's why I thought the 4-5 Ohm impedance was interesting/odd.

They are not malfunctioning, play well, and have no damage. And even though they are 5-10 years apart in age, measure nearly identically. So I don't think either is damaged or otherwise electrically unsound.

So is the consensus that these are likely nominally 8 Ohm?

Thanks!
Al



P.S.

Check out this other thread:

http://www.audiokarma.org/forums/showthread.php?t=473769


It looks like this fella also has a vintage 12" speaker that tests 5.9 Ohm. So maybe some of these older large 8 Ohm speakers just test static impedance lower than modern units?
 
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When you measure across the speaker terminals with an ohmmeter you are measuring the DC resistance not the impedance. To measure the impedance you will need to run an AC signal and vary its frequency over the audible range. This is not too hard to do if you have a signal generator, amplifier and a voltmeter that can accurately measure AC voltage across the audible frequency range.
 
tcdriver said:
When you measure across the speaker terminals with an ohmmeter you are measuring the DC resistance not the impedance. To measure the impedance you will need to run an AC signal and vary its frequency over the audible range. This is not too hard to do if you have a signal generator, amplifier and a voltmeter that can accurately measure AC voltage across the audible frequency range.
Click to expand...

Interesting! I was wondering too why out of about 7 pairs of speakers i checked with a dmm that they all read less ohms than what there stated at
 
tcdriver said:
When you measure across the speaker terminals with an ohmmeter you are measuring the DC resistance not the impedance. To measure the impedance you will need to run an AC signal and vary its frequency over the audible range. This is not too hard to do if you have a signal generator, amplifier and a voltmeter that can accurately measure AC voltage across the audible frequency range.
Click to expand...

Thanks, yes I realize the difference. But I was more interested in quantifying the quick DC resistance check most do to make an educated guess at a speaker's nominal impedance without a complex test in light of the results I observed.

Most of my experience, little though it is, usually has 8 Ohm speakers show around 6 Ohm, not down into 4-5 Ohm. But doing some additional reading and seeing some other responses, it appears this also is not that unusual.

So I'm going to treat both of these as 8 Ohm units for my purposes. It will be close enough.

Thanks again for all the quick responses.

Al
 
tcdriver is right on.
There are some tools available that make the testing easier and give you fancy graphs, but the old-fashioned method he gives is the cheap way. Here's a great article that explains the concepts and shows how it's done.
 
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