4x12 cab in stereo?

[Farview]

Sorry but... your formula is only good for DC... not AC... the transfer of AC power is very complex, but maximum power is transferred when the load and source impedances match. Impedance and restistance are not the same thing. While a speaker may measure around 4, 8 or 16 ohms resistance, the transformer will see an entirely different load. For an 8 ohm load the amp's output transformer will see anything from < 4 to > 16 ohms based on frequency. The transformers impedance also varies with frequency. Both the speaker and transformer are inductive in nature... but, when the amp's ouput load is mismatched, this plays havoc with the transformer and output circuitry in the amp. The power has to be delivered somewhere and usually the transformer ends up eating it by dissipating it as heat. While you may get away with a mismatch for some time, it's damaging your amp (output transformer and power tubes) in the long run. Also, the frequency response is not optimum.

True, that is the formula for DC, but at low frequencies it works virtually the same for AC, and still illustrates the point without going completely off the technical deep end. You however did a much better job of explaining it than I did. And the end result was the same, when you match the load of the amp and the speaker, there will be no volume difference. The volume diffference comes from the mismatch and the electrical chaos that ensues.

 

[Farview]

My amp has 5 outputs, rated from 4-16 ohms
one for 16 ohms, two for 8 ohms, and two for 4 ohms.

My cabinet has two 1/4 inputs, labeled "left/input" and "right/output". There is a two position switch labeled "mono 16 ohms" and "stereo 8 ohms".

If you were to run 2 speaker cables from your amp to the cabinet in stereo mode, you would hook them to the two 4 ohm connections. There is no reason to do this if you don't have a stereo head. The reason for plugging it into the 4 ohm outs is because two 8 ohm loads (the way the amp is wired) will equal 4 ohms. There is no advantage to this. this would be the same thing you would do if you had two 8 ohm 4x2 cabs.

Until you are running a stereo head or two heads at the same time, just leave the cabinet in mono and plug it into the 16 ohm output.

 

[Farview]

And franky, I'm sorry if I didn't clarify all of my stuff, but there's no need to be a jerk about it,

I'm sorry for sounding like a jerk. It was obvious to me that he had a tube amp and you were giving him advise that was geared toward solid state and could (if interpreted the wrong way) blow up the output transformer on his amp.

I do get testy about these things because in a former life, I did warrantee repairs on tube amps for a living. Most of the catastophic failures that did not involve bad tubes (late 80's - early 90's) were a direct result of impedance mismatches that were done because the owners simply didn't understand the concept, or were trying to turn their 100 watt amp into a 400 watt moster by plugging a 4 ohm cabinet into the 16 ohm output.

 

[boingoman]

By "specified" impedance I'm not sure what you mean.
If there are dedicated speaker outputs, wouldn't you consider this a "specified impedance"?

And just so I understand, and am sure we are on the same page, let me more clearly try to describe this

My amp has 5 outputs, rated from 4-16 ohms
one for 16 ohms, two for 8 ohms, and two for 4 ohms.

My cabinet has two 1/4 inputs, labeled "left/input" and "right/output". There is a two position switch labeled "mono 16 ohms" and "stereo 8 ohms".

Does this indicate to you that in "stereo" that each individual input would be 16 ohms, and I should then run from the two 8 ohm ouputs from my amp???

I promise I will put this to bed once I understand this...

Someone really should have asked what kind of head you are using first of all. Sounds like a Marshall to me. That's what the following is based on.

The cab is 16 ohms in mono. When set in stereo, each input is 8 ohms. The way cabs are wired today, plugging two together runs them in parallel. If they are the same impedance, two together halves the impedance. So if you go amp-cab1in-cab1out-cab2in, and both are 16 ohm cabs, the load is 8 ohms. When you set the cab to 16 ohms, that right/out jack can be used to plug another cab in.

OK. Yes I do consider it a specified impedance. Your outs on the amp are meant to be used as 16 ohm by itself, the 8 ohm outs together or seperately, or the 4ohm outs together or seperately. Do not mix and match.

16 ohm output- run to cab into 16 ohm mono input.

8 ohm outputs- total load should be 8 ohms, ie two 16 ohm cabs paralled, or one 8 ohm cab. If you are gonna plug in a full stack these are the outputs to use. Run one 16 ohm cab to each jack, or parallel two cabs to one jack.

4 ohm outputs- total load should be 4 ohms. So you can hook up:

One 4 ohm cab or

2x8 ohm cabs, one from each output or both paralleled from one output. If you run that cab in stereo, the amp sees it as two 8 ohm cabs. Or
Four 16 ohm cabs, two paralled on each 4ohm out, or all four paralled on one 4 ohm output. two full stacks!

 

[Sonixx]

... I would think the amp is designed to handle one 16 or two 8 or two 4 ohm loads...

better said this way... one 16 ohm or two that equal 8 ohms or two that equal 4 ohms.

 

[boingoman]

True, that is the formula for DC, but at low frequencies it works virtually the same for AC, and still illustrates the point without going completely off the technical deep end. You however did a much better job of explaining it than I did. And the end result was the same, when you match the load of the amp and the speaker, there will be no volume difference. The volume diffference comes from the mismatch and the electrical chaos that ensues.

I was gonna say something as well- yes, that formula is DC, but we work with nominal impedance. It's enough of a fudge to be workable in terms of power output and figuring out acceptable impedance loads, so we use the same formulas.

 

[Sonixx]

yeah... the point is well taken... we all work that way and descibe it in terms of simple logic. but, sometimes a little more understanding of what is really going on is in order.

 

[cellardweller]

Ahh, yes. Sorry for omitting that possibly vital piece of information. My amp is an Ampeg VST-120. It is tube. It has a 12' which is unplugged in favor of the 4x12.

I wish I had a picture to show you, that would make this so much easier, but let me try to explain...

From left to right-

First (single) output labeled "16 ohms",

Next a pair of outputs, immediately under the left it reads "8 ohms", above the two outputs is a bracket pointing to both which says "two 16 ohm".

Next, another pair of outputs, immediately under the left reads "4 ohms", above the two 8 ohm outputs is a bracket pointing to both which says "two 8 ohm".

I will leave the cabinet in 16 ohm mono, and use the single 16 ohm output. I was just curious to experiment for myself, but didn't want to fry my amp. I have been cautioned sufficiently to know not to mismatch impedance on a tube amp, beyond these instilled fears (justified fears) I don't know squat, so didn't want to try it without a bit of advice from you's guy's...

 

[Farview]

I always rewire all my cabs for mono anyway, imagine trying to get stereo with the left and right 1 inch apart in the same cabinet. There was a whole run of Randall cabinets that were wired out of phase when run in mono, if you ran it in stereo mode it was fine. That would be the only good reason to run the cab in stereo mode with one head. manufacturer error.

I'm tired of this thread now

 

[futurestar]

Mesa Boogie has an impedance matching section with diagrams near the end of this manual (pages 28-33):
http://www.mesaboogie.com/manuals/3chRecto.pdf