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How do you wire 4 8-ohm speakers to get 4 ohm total impedance?


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#1 chap

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Posted 22 May 2010 - 04:37 PM

Hi guys,

How do you wire four 8-ohm speakers to get 4 ohms total impedance for your typical 4x12 Celesion cabinet?

I just did the math ... maybe I'm missing something but I can't figure out how to get a total impedance of 4 ohms from 4 8-ohm speakers without doing something weird like adding in a resistor or something.

If you wire them all in parallel, that gives you a total of 2 ohms. If you wire them all in series that gives you 32 ohms. If you wire 2 in parallel, and the other two independently in parallel, then both sets in series that gives 8 ohms. Or do you have to wire like 3 in parallel and one in series or something?

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#2 Mike Lee

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Posted 22 May 2010 - 04:49 PM

I think you've already figured out that you can't do it. You need 4x 16-ohm speakers if you want a total impedance of 4 ohms - all in parallel. Most traditional Marshall cabs are wire series-parallel for 16 ohms.

If you were two in series to get 16 ohms, and then wire that pair in parallel with the other two in parallel, you can get 3.2 ohms. That's close to 4 ohms, but your amp may or may not like that.

#3 seeker

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Posted 22 May 2010 - 05:03 PM

Unless your amp requires 4 Ohms, go with 8. If it does, as Mike says, need 16 ohmers to do the job.

#4 backinit

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Posted 23 May 2010 - 04:25 AM

I think you've already figured out that you can't do it. You need 4x 16-ohm speakers if you want a total impedance of 4 ohms - all in parallel. Most traditional Marshall cabs are wire series-parallel for 16 ohms.

If you were two in series to get 16 ohms, and then wire that pair in parallel with the other two in parallel, you can get 3.2 ohms. That's close to 4 ohms, but your amp may or may not like that.

I think the nominal rating would still be 4 ohms. I just replaced the speakers in my 412 and even though the speakers were all 8 ohm speakers they all test around 6.2.

Avatar speakers has Celestion Super 65s in 16 ohms in their clearance section. I just installed 3 of them in my cabinet and I'm blown away by how versatile (and loud) they are. I played a practice yesterday and my 33 watt head was on .8 on the clean channel.

#5 chap

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Posted 23 May 2010 - 09:38 AM

thanks for your help guys, 8 ohms it is then... and to get 8 ohms, I worked it out on paper...

2 8-ohm speakers will be wired parallel for a total of 4 ohms.

Then each set of two speakers wired in series for 8 ohms...

At least thats how works on paper. :D

#6 seeker

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Posted 23 May 2010 - 11:20 AM

thanks for your help guys, 8 ohms it is then... and to get 8 ohms, I worked it out on paper...

2 8-ohm speakers will be wired parallel for a total of 4 ohms.

Then each set of two speakers wired in series for 8 ohms...

At least thats how works on paper. :D


Actually, you can do it two ways. Also do the obverse of what you say, series two speaker sets for 16 ohms each, and then parallel the two sets to get 8 ohms.

There is acuallty a discernable difference in sound. One requires 7 wires, and the other 6 wires. My personnal preference is the 6 wire solution, which is how the classic Marshall 4x12 was wired.

A good read at The Amp Garage

Edited by seeker, 23 May 2010 - 11:30 AM.

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#7 Moose

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Posted 23 May 2010 - 07:13 PM

I just replaced the speakers in my 412 and even though the speakers were all 8 ohm speakers they all test around 6.2.


Fair warning -- those aren't "6.2 ohm" speakers... they're 8 ohms. No need to complicate things or worry about minute measurement differences.

The first reason I say this is that you're measuring impedance. Most people measure resistance and assume it's the same...

Second, impedance in a speaker varies significantly based on the frequency. A speaker could be 35ohms impedance at its resonant peak, 6 ohms at 200hz, 8 ohms at 500hz, 16 ohms at 5Khz... and it's still an 8 ohm speaker. The value's really nominal, not precise, due to the nature of the speaker. So, anything near to 8 ohms is 8 ohms.

In other words, that speaker's 6.2ohms with that meter, but still an 8 ohm speaker.

Strangely, I just said something complicated when my whole point is, there's no need to complicate all of this.

#8 GusS

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Posted 25 May 2010 - 09:16 AM

the 8 ohms refers to AC impedance, you measured DC resistance. The relationship between the two is complicated (among others, the impedance varies by frequency), but I've noticed that the DC resistance measured with the multimeter x 1.3 yields the approximate speaker load rating.

Edited by GusS, 25 May 2010 - 09:19 AM.


#9 HSB0531

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Posted 25 May 2010 - 10:30 AM

What everyone else said.

Questions:
1) How many speaker jacks are on your amp.
2) What is the rating for each jack.
3) Does the amp have a variable ohm selector switch ala Marshall.

Jim

#10 chap

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Posted 25 May 2010 - 10:42 AM

This is new to me that the impedance / resistance depends on the frequency but in a way it's similar to my understanding of electrical theory.

I was a auto mechanic at a GM dealership for years and electrical was one of my specialties that I dealt with a lot. I always remember this demonstration from my apprenticeship...

The instructor had two 1AWG battery cables (1 AWG is about 3/4" thick battery cable). One was modified so that only one strand of copper wire connected the battery cable. The other was un modified. He used an DVOM (volt meter) to measure the resistance of each wire ... both measured near zero ohms or almost no resistance.

He then used the unmodified battery cable to crank a starter motor, it worked ... he then did the same with the other copper wire with the single strand connecting the two sides and it took about 2 seconds and the strand got red hot and the starter motor didn't turn. He used a volt meter and inductive ammeter to measure the voltage drop and current across the strand of wire and used those numbers to calculate the actual resistance of the wire under the load that it was designed to operate.

As a young impressionable mechanic, this was a pretty cool demonstration but totally highlighted the fact that resistance of a device should be calculated with the load that its designed to carry.

I remember the instructor said that the ohm meter basically sends about .5 volts through a circuit, measures its current and then uses this to calculate resistance. As you can imagine, sending ,.5V through a 1AWG cable isn't really a good indication of its resistance.

What he also said, and Iremember this now ... is that the ohm meter could possibly damage computer circuits and to NEVER use it without checking the diagram and service manual. .5V sent through a wire designed to carry serial data could actually damage the ECM.

Apologies guys, I got off on a tangent there...

#11 HSB0531

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Posted 26 May 2010 - 01:52 AM

Regarding resistance & impedance:
What is measured by an ohm meter is the electrical resistance of the voice coil.
This is done when the cone is at rest (not moving).
The resistance measurement will be less than the actual impedance of the coil.

Impedance is measured by sending a tone (400 Hz. being one of the standards) into the speaker's voice coil.
This causes the cone to become energized and move, giving you the nominal (average) impedance of the coil.


Jim

Edited by HSB0531, 26 May 2010 - 01:53 AM.