This month's article is an introduction to one of the important properties of your amp, cabinet or speaker - the impedance.
The impedance is measured in Ohm while the Greek symbol Omega "Ω" is used for representing ohm values. I won’t be dealing with the technical and theoretical description of impedance here because of the scope of this article. For simplicity, let's say the impedance of the speaker is the load of the speaker for the amp.
Guitar cabinets are usually built with the following impedance values: 4 Ohm, 8 Ohm or 16 Ohm. Your amp also has its own impedance related to its output transformers.
There are fixed impedance amps and variable impedance amps. The latest ones have the possibility, using the selector, to choose one of the three most common values (like Marshall heads do). It is important to align the impedance values between the amp and the cabinet. For example: if your cabinet is 16 Ohm then select 16 Ohm on your amp too.
If you must use a non matched hookup, it is possible to use the cabinet with higher impedance value with the amp using a lower setting.
For example: 8 Ohm cabinet with 4 Ohm amp setting. This type of hookup will also influence the overall instrument timbre due to the fact that the output transformers will have to deal with different load (this is true for tube power amp section, not the solid state one).
A hookup to avoid is where you have a lower impedance value cabinet connected to the higher impedance value amp. For example: 4 Ohm cabinet connected to the 8 Ohm amp.
This type of hookup will stress output transformers and may cause damage.
Don't use a tube amp without being connected to the cabinet (or some dummy load). Also, never use an instrument cable for connecting the amp and the cabinet.
Speaker cable - picture by www.armorgoldcables.com
The instrument cable is not suited for this type of application because of high power signal travelling between the amp and the cabinet. Even though at first glance the instrument and the speaker cables may look identical, usually the speaker cables are more robust and most of the times have the caption "speaker cable".
At some point you may want to connect more than one cabinet to your amp. Remember the good old days when guitarists played in front of the wall of Marshall cabinets?
Probably you won't build a wall of Marshalls but 2 cabinets may be your case. Cabinets can be connected either in series, parallel and series/parallel.
Series hookup means that the amplifier's "+" is connected to the "+" of the first cabinet, then "-" of the same cabinet is connected to the "+" of the second cabinet and so on.
The last cabinets "-" pole is connected to the "-" of the amp. The overall impedance is the sum of the single cabinet’s impedances.
Ex: two 8 Ohm cabinets connected in series will produce 16 Ohm, two 4 Ohm cabinets will result in 8 Ohm.
In the case that one speaker gets broken, the hookup gets interrupted.
Parallel hookup means that all the "+" poles of the amp and cabinet are connected together and all the "-" poles of the amp and cabinet are connected together.
In the parallel configuration, the total impedance (in the case we're using 2 cabinets) will be: Rt=(R1xR2)/(R1+R2)
So, two 8 Ohm speakers connected in parallel will result in a total of 4 Ohm: (8x8)/(8+8)=4
In the case that a speaker gets broken, the hookup is still working.
This is the type of hookup used within amps that can handle more than one cabinet/speaker.
Two 16 Ohm cabinets will result in 8 Ohm impedance, so we will set our amp for 8 Ohm when connecting these cabinets.
This type of hookup is a mix of the two. This type of wiring is used inside the 4x12 Marshall cabinets.
To measure the impedance of your cabinet/speaker, all we need is an affordable multimeter. Connect the "-" and "+" respectively and read the values on the display.
Values may vary for 25%-30% related to the effective value. If you read 12 Ohm on the tester, your speaker is 16 Ohm.
Once you understand the principle, it becomes easy to understand what kind of wirings we can make based on the speakers available.
With 2 speakers, you only have 2 choices: series or parallel. You cannot connect 2 speakers in the series/parallel mode.
In case we have two 16 Ohm speakers, we actually have only one possible option because parallel wiring will result in a total of 8 Ohm and the series wiring will result in 32 Ohm. Impedance of 32 Ohm is not common with guitar amps.
With 4 speakers/cabinets we also have the option of a series/parallel wiring.
As you may noticed, in the guitar world, it is important to have the total impedance of 4 Ohm, 8 Ohm or 16 Ohm.