# Speaker resistances



## jfk911 (May 23, 2008)

So i picked up and old Marshall jcm800 lead 1960a cab its later 80's as it has the G12T-75 and not the 70s or 65s, today i will take the back off and actually date the speakers. One does have a different sticker backing than the others. They all seem original as they are all made in england rather than china like the newer speakers. all the research i did about those cabs all said 16 ohms, i know its my fault for not checking but when i came home i found out it was an ohm cab. Not not a huge deal cause my head can do 16, 8 0r 4 ohm cabs. Its just id prefer a 16 ohm cab over 8 ohm. Now from my reading i found out speaker resistances can be off from about 1 or two ohms. Now ive figured out how to wire the 8 ohm speakers up to 20 ohm, along with 2, 10.6, 32, and obviously 8 ohm. Now if i wire it to 20 ohms and use my amp on the 16 ohm section is that a huge deal for the amp or am i better off to keep it on 8? Reason i ask is when my band plays gigs we do alot of gear sharing and i would prefer to use my cab rather than other ppls so if i could get it close to 16 that would be ideal so i dont look like the only jack ass who refuses to share cabs.

edit: ok i can also get 13.3 ohms which would be better than the 20 ohms no? or should i still stick with the 8.


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## greco (Jul 15, 2007)

Have you measured the DC resistance of the speakers (understanding that is a basic indication of impedance), or are the speakers marked with the ohms they are rated for (or both)?

Personally, I'd stay with 8 ohms and keep life simple. Why do you prefer a 16 ohm cab?

Cheers


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## jfk911 (May 23, 2008)

i did both yesterday. I just want a 16 ohm cab so when i play shows and the bands share gear i can offer up my cab rather than me having to use somebody elses cause mine is an 8 ohm. i just prefer using my own gear rather than using somebody elses cab cause the resistance of mine dosnt match everybody elses head. cause not all heads have the choice of what resistance you can use.


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## Andy (Sep 23, 2007)

Wire them up for 8 ohms. Any configuration other than 8 (or 2 and 32, but they're more or less useless) will spread power unevenly among the speakers.

Also, 95% of the heads out there will run 8 ohms.


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## jfk911 (May 23, 2008)

i was just going to re post saying i completly did no think of the power distribution. and the 13.3 ohms would be right screwed up 1 speaker having half the power and the other three sharing the other half. so 8 ohms it is. thnx


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## WCGill (Mar 27, 2009)

How can you get 20 ohms out of a 4x12 cab? Or 13.3, 10.6? Never heard of it.


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## greco (Jul 15, 2007)

WCGill said:


> How can you get 20 ohms out of a 4x12 cab? Or 13.3, 10.6? Never heard of it.


I was going to ask the same thing.....but i decided to take the "easier way out". 

Cheers


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## jfk911 (May 23, 2008)

different combination circuits i can maybe draw them on paint tomorrow. its just different combination of series and parallel. for example the the 20 ohm is two in series and the other two in parallel. 10.6 is 3 in parallel and one in series. 13.3 is two in series then those two are in parallel with one other speaker, then that parallel circuit would be in series with the last speaker. But like what stated before the power distribution would not be proper for any combination other than 2, 8, or 32 ohms. then two is all in parallel, 32 is all in series and 8 is series paralleled with the other two speakers that are in series.


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## WCGill (Mar 27, 2009)

Are you using the dc resistances of the speaker? They measure close to the impedance rating, usually a little under, but just use the rating as it's the standard. With 4 16 ohm speakers in a 4x12, you can wire them either 16 ohms or 4 ohms. With 4 8 ohm speakers in a 4x12, it's 8 ohms or 2 ohms. Yes, you can wire them in series for 64 or 32 ohms but it's not really used.


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## Fader (Mar 10, 2009)

I found this site to very informative about cab wiring.
Check it out.

Wiring a 4 X 12 Speaker cabinet


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## Wild Bill (May 3, 2006)

jfk911 said:


> different combination circuits i can maybe draw them on paint tomorrow. its just different combination of series and parallel. for example the the 20 ohm is two in series and the other two in parallel. 10.6 is 3 in parallel and one in series. 13.3 is two in series then those two are in parallel with one other speaker, then that parallel circuit would be in series with the last speaker. But like what stated before the power distribution would not be proper for any combination other than 2, 8, or 32 ohms. then two is all in parallel, 32 is all in series and 8 is series paralleled with the other two speakers that are in series.


What Mr. McGill and others are getting at is that you can't measure a speaker with a DC resistance meter. Speakers are measured in impedance, which is an AC type of measurement that requires rare and expensive equipment.

When you measure with a DC resistance meter you get the resistance of the wire used to make the voice coil, NOT the impedance! However, the DC ohms can provide a good guess. Since speakers are made in standard impedances and the DC resistance is usually a bit less than the impedance figure one can make a good guess. If you measure 2.4 ohms or something, it's a good bet that the speaker is rated at 4 ohms. An 8 ohm speaker might measure 5 or 6 ohms. Obviously, a 12.5 ohm speaker would actually be a 16 ohm unit.

The formulae for impedance work the same as DC ohms, that is you add them up in series or use the DC parallel formula for parallel. Watts and power sharing are the same as well. The only practical difference that a casual tech must worry about is, as I said, you can't use the DC ohms reading as appropriate.

WB


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## stratele52 (Oct 25, 2010)

I'm new to this forum, and I'm french speaking so excuse my english.

If your amp transformer have a 16 ohms output tap, you'll have better sound if you use this tap ( with matching speakers) because you use the maximum winding of the output transformer. If you use 8 ohms tap with 8 ohms speaker the sound will not be as good as the 16 ohms because you use half transformer winding.

Speaker impedance (16, 8, 4 ohms) are close to DC resistance mesuring but NOT the same.


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## greco (Jul 15, 2007)

stratele52 said:


> I'm new to this forum, and I'm french speaking so excuse my english.
> 
> *If your amp transformer have a 16 ohms output tap, you'll have better sound if you use this tap ( with matching speakers) because you use the maximum winding of the output transformer. If you use 8 ohms tap with 8 ohms speaker the sound will not be as good as the 16 ohms because you use half transformer winding.*Speaker impedance (16, 8, 4 ohms) are close to DC resistance mesuring but NOT the same.


Welcome to the forum. Your English is excellent.....nothing to be concerned about at all.

I have never heard this comment before (re: the portion I bolded in your quote)...very interesting. I wouldn't have thought it would make any difference.

Cheers


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## Wild Bill (May 3, 2006)

greco said:


> Welcome to the forum. Your English is excellent.....nothing to be concerned about at all.
> 
> I have never heard this comment before (re: the portion I bolded in your quote)...very interesting. I wouldn't have thought it would make any difference.
> 
> Cheers


If it does make a difference I'll bet it would take a NASA lab to measure it, Dave! The few turns of even the 4 ohm winding is enough for the full frequency range in a well made transformer. Adding more for 8 and 16 ohms would just be over-kill.

It's great that the poster chimed in to share what he believes but I would dearly love to see a technical explanation of why his point is true. It sounds to me like something picked up on one of those pseudo-technical audiophile sites.

Still, I'm not too old to learn something new. It's just that the idea contradicts a lifetime of studying electronics. That's why I would need a technical defense before I could accept the idea.


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## WCGill (Mar 27, 2009)

The theory of using the whole winding for better tone started with Gerald Weber, along with a lot of other unproven and half-baked ideas that he cooked up. Like a true internet myth, it got repeated enough that it eventually became accepted as fact. Unfortunately it ain't so.


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## greco (Jul 15, 2007)

Wild Bill said:


> What Mr. McGill and others are getting at is that you can't measure a speaker with a DC resistance meter. *Speakers are measured in impedance, which is an AC type of measurement that requires rare and expensive equipment........*
> WB


Could someone please link me to a site that describes this piece of equipment. I don't want to buy one, I just want to read about it to educate myself. I tried to find out about it on the internet, but had no real success.

Many Thanks

Cheers


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## Fader (Mar 10, 2009)

Impedance is resistance plus reactance. Speakers produce mostly inductive reactance.

REACTANCE MEASURING EQUIPMENT

Impedance and Reactance


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## greco (Jul 15, 2007)

Fader said:


> Impedance is resistance plus reactance. Speakers produce mostly inductive reactance.
> 
> REACTANCE MEASURING EQUIPMENT
> 
> Impedance and Reactance


Here is a pic for the curious:

Figure 1-14.—Bridge circuits.

Thanks for the link........and I thought my o-scope was complicated !...LOL

Cheers


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## Fader (Mar 10, 2009)

Is it a dual trace silly-scope Dave? Look at the voltage waveform and the current waveform and the difference will give you the impedance.

That bridge looks like something Dr. Emmett Brown used on the Delorean.


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## greco (Jul 15, 2007)

Fader said:


> Is it a dual trace silly-scope Dave? Look at the voltage waveform and the current waveform and the difference will give you the impedance......


This is embarrassing...but I'm not sure. It is quite an old Tektronix model that was well cared for. 

Is the impedance measure on a dual trace scope quite accurate?...in comparison to the DMM estimate method.... or a similar "estimate" ?.

Thanks for telling me about this method....I'll have to do some experimenting!

Wild Bill told me (in a terrific detailed tutorial) how to determine the impedance on an unmarked output jack on the back of an amp....that was a fun task. He even sent me his spread sheet that does all the calculations for this.


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## greco (Jul 15, 2007)

My scope is a model 434...it appears to be dual trace, according to the information on this site

http://www.valuetronics.com/vt/Assets/product_full/TEK_434.JPG

This is the same as my scope....cool 

[video=youtube;jvsH8Awhre8]http://www.youtube.com/watch?v=jvsH8Awhre8[/video]

Cheers


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