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Post subject: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 4:13 am
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Hello

A Hot Rod Deluxe with Eminence Canabis Rex . Amp look to be made in 2012

Speaker is reading 6.6 ohms. Is coil is begining to be short ?


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 6:06 am
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Most people asking that type of question, I'd just reply that DC resistance always measures less than AC impedance, and that an 8 ohm speaker should read in the 6 to 7 DC ohm range. (Some types will even read in the high 5 range.)
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But I presume you know that.

Have other Cannabis Rex speakers measured higher for you? If so, what's their typical reading? +/-0.1 I'd chalk up to normal tolerances.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 6:10 am
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Thank you for your answer .
I have few used / old 8 ohms speakers of many brands, reading are from 6 to 8 ohms.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 6:52 am
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Thread title should reference the C.Rex specifically if you're wondering if that one is reading lower than it should.

I'm curious about your 8 ohm speakers that actually measure 8 ohms. That would worry me a lot more than a 6.6 reading. Make me wonder if the copper coil has oxidized, a solder joint has developed some resistance, or if it's actually a 16 ohm with a partially shorted coil.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 7:55 am
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strayedstrater wrote:
Thread title should reference the C.Rex specifically if you're wondering if that one is reading lower than it should.

I'm curious about your 8 ohm speakers that actually measure 8 ohms. That would worry me a lot more than a 6.6 reading. Make me wonder if the copper coil has oxidized, a solder joint has developed some resistance, or if it's actually a 16 ohm with a partially shorted coil.



Not developed some resistance, loose some resistance , drop from 8 to 6.6 ohms

My question is ; With time and heat at high volume ,could a speaker 's coil may develop short between some winding ?


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 9:11 am
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With my limited experience rebuilding speakers I've found that the initial reading on a voice coil depends on the finished product being vertically centered on the pole according to the depth of the winding's. Also, If you press up or down on a speaker, the reading changes. That being said, an 8 ohm voice coil is an 8 ohm voice coil. Upper 6's is fine.

UN


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Tue Jul 31, 2018 9:24 pm
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stratele52 wrote:
strayedstrater wrote:
Thread title should reference the C.Rex specifically if you're wondering if that one is reading lower than it should.

I'm curious about your 8 ohm speakers that actually measure 8 ohms. That would worry me a lot more than a 6.6 reading. Make me wonder if the copper coil has oxidized, a solder joint has developed some resistance, or if it's actually a 16 ohm with a partially shorted coil.



Not developed some resistance, loose some resistance , drop from 8 to 6.6 ohms

My question is ; With time and heat at high volume ,could a speaker 's coil may develop short between some winding ?


No, I was saying your 8 ohm speakers that read 8 ohms may have developed some extra resistance. I've never heard of or encountered a speaker where the DC resistance matches the nominal AC impedance.
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Yes, it's possible for any coil to partially short (to have a few loops of the wire shorted).

Very common with traditional Fender single-coils, somewhat common with transformers. Never read about it with voice coils, but it's possible.

The only way to know if that C. Rex is reading lower than it should is to find out what the average DC resistance of other C. Rex's is.

If there's no rubbing and the customer has complained that the speaker sounds different than it used to, I wouldn't worry about it.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Wed Aug 01, 2018 2:06 am
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You write ;
" . I've never heard of or encountered a speaker where the DC resistance matches the nominal AC impedance"

________________________

I read many speaker ( different brand ) since yesterday and see few 8 ohms reading 7,5 or 9 ohms .
I have a 15 ohms speaker read 13.5 ohms.

Now you can say you have heard :lol:


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Wed Aug 01, 2018 8:00 am
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The general rule of thumb is:

(DC ohms) X 1.3 = (AC ohms)

It's a rough approximation. AC impedance is a nominal figure -- the resistance varies with frequency. Different manufacturers use different methods to arrive at their impedance ratings. A few use the minimum impedance, others measure at an arbitrary Hz, still others average the readings over some part of the driver's frequency response, and others simply multiply the DC resistance by 1.3 (or 1.5).

7.5 for an "8 ohm" speaker is a little high but probably ok. 13.5 for a "16 ohm" speaker is normal.

9 ohms DC for an "8 ohm" speaker would worry me if it's any major brand of speaker. ("Worry me" as in making me wonder if some part of that speaker's circuit has developed extra resistance.) If it's an off-brand driver I'd just chalk it up to crappy tolerances. I might hesitate to use it in a MusicMan or some Marshalls that are finicky about impedance mismatches.
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When I measure "8 ohm" speakers I expect to see something close to 5.4 to 6.8 DC resistance.

With "16 ohm" speakers I expect to measure 12 to 14 ohms DC.
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A 6.6 DC resistance for an 8 ohm C.Rex is what I'd expect. If it was much less than 5 ohms or if it was 8 ohms or higher I'd suspect something was wrong with it.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Wed Aug 01, 2018 8:07 am
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Thank's for your answer,

Where come from this formula ? ; The general rule of thumb is: (DC ohms) X 1.3 = (AC ohms)


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Wed Aug 01, 2018 9:13 am
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stratele52 wrote:
Thank's for your answer,

Where come from this formula ? ; The general rule of thumb is: (DC ohms) X 1.3 = (AC ohms)


Who invented the wheel?

That formula has been around since the late 1940's when permanent magnets began to replace field coils in dynamic drivers.

I don't know the ins & outs of the early audio industry and how/why they decided to rate speakers by AC impedance instead of DC resistance.


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Post subject: Re: 8 ohms speaker read 6.6 ohms
Posted: Wed Aug 01, 2018 10:25 am
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Thank's ,


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