# Answer to a PM over speaker impedance



## Wild Bill (May 3, 2006)

Here's a question that came to me in a PM that I thought might be useful to others as a general post:

*I have a quick question for you that is still unclear to me... I just got myself a 1989 JCM 800 2210 Canadian Edition. As I am sure you already know that these amps only have 4 ohm and 8 ohm output options. I am running this head through a 1960A cab. So on the back of this cabs there are 2 inputs and a switch. Here are my options: 16 ohm mono, 4 ohm mono, 8 ohm right stereo and 8 ohm left stereo. Now, I am not sure which combination I should be using to maximize tone...? Also (this may be a really stupid question), what is the difference between the stereo and mono switch on the back of the cab? The mono says 300 watts rms and the stereo says 150 watts rms...?
Any help would be greatly appreciated!*

The stereo switch actually splits the speakers! In effect, you will have two 2-12" cabs in one box. That way you can run a left channel and a right channel, or two different amps into the same box.

When you only have one amp, there's no point in running in stereo mode, except maybe to cut the volume a bit by running only 2 speakers. The reason you can run more power in mono is 'cuz you have double the speakers to handle the power.

When you wire up multiple speakers you have a couple of options about the combined speaker load, or impedance. This depends on how you wire them. Four 16 ohm speakers can give either 16 ohms or 4 ohms. That's your 'mono' option running all speakers together. When you run stereo you then have 2 separate 2-12" combos. You can have 8 ohms or you can have 32 ohms. Since nobody runs 32 ohm outputs we can forget about that one. So in stereo mode you have 8 ohms each side, only.

Hope that makes sense!

:food-smiley-004:


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## nicmat42 (Dec 31, 2008)

Thanks so much Wild Bill (and sorry for the PM, I completely understand). I understand now what's going on and one more quick question to touch on that subject; in the back of my head there are 2 output jacks, would it be possible to run 2 speaker wires (one from each output jack) to the 2 input jacks on my cab set to stereo? So basically all 4 speakers would be on...?

Also, what is the differences in sound when it comes to 4ohm matches or 16 ohm matches? I get the feeling that people mostly use 16 ohms, is this correct and why?


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## Wild Bill (May 3, 2006)

nicmat42 said:


> Thanks so much Wild Bill (and sorry for the PM, I completely understand). I understand now what's going on and one more quick question to touch on that subject; in the back of my head there are 2 output jacks, would it be possible to run 2 speaker wires (one from each output jack) to the 2 input jacks on my cab set to stereo? So basically all 4 speakers would be on...?
> 
> Also, what is the differences in sound when it comes to 4ohm matches or 16 ohm matches? I get the feeling that people mostly use 16 ohms, is this correct and why?


If you run two speaker wires, you would have the same thing as running two 2-12" in parallel. Two 8 ohm loads combine in parallel to give 4 ohms. So you would have exactly the same thing as running one cable with the switch set at "Mono-4 ohms'. Why spend the money for an extra speaker cable?

As for differences in sound with different loads, I doubt if there's any human ear that could detect it, Nick! The output transformer in the amp has different taps brought out for 4, 8 and 16 ohm loads. As long as the speaker load and the switch setting is the same the amplifier couldn't care less if it's running into 4 ohm or something different. It's just a convenience having the different output taps, since different cabinet speaker arrangements will give different loads.

That being said, if you are running a LONG speaker cable like 50 feet or more you should try to run a 16 ohm speaker load (and the amp set to 16 ohms as well, of course!) This is an unusual situation I know but it can happen. Running a higher speaker load means the total power losses from the wire used in the speaker cable will be less. With the typical shorter speaker cable the difference would be "mice nuts".:smile:

Deliberate mismatching amps and cabs, like setting the amp to 8 ohms and running a 16 ohm cab, will change the tone slightly. It will also stress the output tubes in your amp. Tubes are tough and in a pinch if I didn't have the right cab for an amp I wouldn't hesitate to do it but still, it will take a few hours off the life of those tubes.

:food-smiley-004:


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## greco (Jul 15, 2007)

Wild Bill said:


> "mice nuts".:smile:


Wild Bill...thanks for reviewing this topic again. 

I always enjoy your rodent anatomy phrase (above) ....LOL

All the best

Dave


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## mkaye (Jan 25, 2009)

just remember that the speaker load is reflected back on to the output tubes so it is important to use speakers that match the desired output impedance of the amp

mark


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