Posted by BRUNO

Wed Nov 28, 2007 1:41 pm

Ok..I hope this will definetely answer most of your questions here about impedance etc.. It may be long and I hope my English won’t be too bad on this

First stop thinking that impedance is difficult to understand …it is not or I would not get it either

– this is one of the things that leads to mistakes what is stereo ? A simple answer would be that mono is one signal path and stereo is two …
you don’t really need more here..just treat your stereo rig the same as TWO mono will have to do twice the cabinet wiring job…as far as amp and cabs are concerned you should have no shared part between the two… exception being those stereo power amps and stereo cabs if you have those just imagine that it is two amps or cabs glued together and it should help ..more on this later .

4 ,8 ,16 ohms CABS and TOTAL LOAD

As we’re talking about a mono amp it is obvious that if I use more than one cab they will work together and become almost like one because the same signal will be directed to those two cabs..this is where what we call TOTAL load apply …
Every speaker do have an impedance..all the speakers of ONE cab wired together are also equal to an impedance value ..this one is the IMPORTANT one for don’t really need to bother with what’s inside if you bought a cab in a shop ..what you need to know is the impedance of the cab will most of the time be 16 or 8 ohms
( rarely 4 ohms)
Now you guess it if you have only one 8 ohms cab you must adjust your amp to 8 ohms on its impedance selector..or use the 8 ohms out (more on this later)
The problem appears when using more than one cab .. it will again be most of the time 2 cabs…
A few things to avoid :
1. using an odd number of 3… not a good idea
2. mixing two cabs of DIFFERENT impedance..not good either

So how do we found this TOTAL load number when using (most of the time again ) two cabs ? Well this is easy… just use this formula :
ex: two 16 ohms cab = 8 ohms total load
two 8 ohms cab = 4 ohms total load

You will probably never be confronted with other “mixing cabs” prob…
This is called using two cabs in parallel ..Series is a different thing and better forget about it cause again it will probably never have to be applied in your system except for inside wiring of a 4 x 12 which is another subject .


Now this is by far THE problem most people have …on one side you have those old Marshall amps with no specific labelling on speaker outputs..and that impedance selector..if you still don’t know where to set it ,at least you know where to plug the cabs ..Now enters the amp with multiple outputs ,some being labelled with similar numbers (4 and 8 most of the time). If you want for once to understand this properly some cleaning in your mind must be done ..And the first thing to do is to imprint in your mind that a (mono) amp DO NOT have several outputs of the same type (4,8,16ohms) ..but ONLY one … repeat it 100 times to be sure you get it

Now you’re going to ask why the hell do I see twice 8 ohms (for ex) on my OD100 …For a simple reason..The manufacturer thought that you may want/need to use more than one cab on this UNIQUE 8 ohms tap of the output tranny of your amp…BECAUSE as seen before you have TWO 16 ohms cabinets you want to use and that EQUALS to 8 ohms TOTAL LOAD
So he was nice enough to install TWO 1/4 jack on the one and ONLY ONE 8 ohms outs ..yes those 1/4 are wired together in parallel to the SAME wire inside the amp .
8 Ohms here being the TOTAL load . So we are going to plug our TWO 16 ohms cabs into the TWO 1/4’s labelled 8 ohms . Those two work together and are NOT USED as DISCRETE 8 ohms outs in this situation cause again this is NOT what they are .
It must be noted here that unless one out is labelled ” USE FIRST ” as on some VHT you can use randomly any of the two 8 ohms labelled 1/4’s when using only ONE 8 ohms cab .

So now back to the Stereo thing..If you have a stereo power amp you will find that it has twice this serie of 1/4’s ..or two impedance selector..if it doesn’t it is probably a mono amp and you’ve been ripped off

A stereo cab will have or NOT a switch to go from mono to stereo.
Some 2 x 12 cabs use CONTACT 1/4 to do the means that both speakers are connected together unless you plug into the second input which have the effect of separating the two speakers
NEVER use two amps in a cab switched to mono … You will fry everything
An ex of stereo cab:
-Marshall 1960 :
– it has a switch set it accordingly
– if using stereo it will virtually become two 8 ohms cabs . NOTHING ELSE
– If used mono you got the choice of two impedance value for the cab
16 ohms or 4 ohms .You have the choice here so use what your amp offers..if it offers both use what you think sounds the best .

That should cover most of it ..of course there are some situations where it will be different but you will unlikely encounter something different than what is explained here .I hope it was not unclear it again and at the end you will realize that is it much more simple to apply than explain .
If something is unclear or if you have more questions just let us know



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