Amplifiers are designed to power a specific "load", meaning a speaker or a set of speakers having a specific "AC impedance" measured in ohms.

An impedance resists the flow of electric current and so the higher a speaker's impedance the less current the amplifier must supply.

Conversely, the lower a speaker's impedance the more current the amp must supply.

Hence, a low-impedance speaker is a big workload for your amp.  And a high-impedance speaker is a small load.

Most speakers have an impedance of either 4, 8 or 16 ohms (Ω).  To push more air, several speakers (always of equal impedance) can be wired together in a cabinet.

The cabinet impedance depends on how the speakers are wired and may differ from the impedance of the speakers.  Wiring diagrams and their effect on cabinet impedance are displayed below.

An amp works best when driving a load with the same impedance as the amplifier's "output impedance".  If they don't match, there could be consequences:

• If the speaker impedance is higher than the amp's, the amp might sound weak or dull.
  
  
• If the speaker impedance is lower than the amp's, the amp might sound fantastic but eventually burn out.

For these reasons, amplifier speaker jacks are often marked with the minimum load you should connect (4 Ω for example).  Speakers and cabinets should also be marked with an ohms value.  If not, you can easily measure the value with a multimeter:

NOTE:  A multimeter measures DC resistance, not AC impedance.  However, the two values correlate, resistance being somewhat lower than impedance.  So round up the meter reading to 4, 8, or 16 ohms.

Some amps have an impedance selector switch on the rear panel that you can use to match its output impedance to various cabinet impedances.  Changing cabinet impedance is one of the easiest and most dramatic ways to change the character of a guitar amp.

WARNING:  If your amp has output tubes, be sure to connect speakers whenever the amp is On ‒ otherwise its output transformer can easily be damaged, especially if you're playing through it.

There are two ways you can connect two speakers.  One way is called  "series wiring" and the other is called "parallel wiring".

Wiring two speakers in series doubles the impedance of one speaker.  Wiring two speakers in parallel halves the impedance of one speaker.

For example, the diagram below shows you how you can turn a pair of 8 Ω speakers into either a 16 Ω or a 4 Ω load.

To build an 8 Ω, two-speaker cabinet, use
two 4 Ω speakers in series or two 16 Ω speakers in parallel.

Wiring four speakers in series quadruples the impedance of one speaker.  Wiring four speakers in parallel quarters the impedance of one speaker.

Below, the diagram on the left shows four 4 Ω speakers making a 16 Ω cabinet.  The diagram on the right shows four 16 Ω speakers making a 4 Ω cabinet.

A third way to connect four speakers is called "series-parallel" wiring.  It puts a pair of series-wired speakers in parallel with another pair of series-wired speakers.  The combined impedance matches that of one speaker.

For example, the diagram below shows a 4 Ω cabinet containing four 4 Ω speakers.

Use this wiring with 16 Ω speakers to emulate a classic, 16 Ω Marshall^® cab.

To build an 8 Ω cabinet, use four 8 Ω speakers in series-parallel.

The diagram below shows the wiring of a Marshall^® 2 × 12" speaker cab having a Mono/Stereo selection switch and two specialized input jacks.

When the Mono/Stereo switch is set to "Mono", the two 8 Ω speakers are wired either in series (making a 16 Ω cab) or in parallel (making a 4 Ω cab), depending on which jack you use.

When the switch is set to "Stereo", each jack connects to a separate 8 Ω speaker and you use both jacks, one for the amp's left output and one for its right output.

To build a four-speaker Mono/Stereo cabinet, replace each 8 Ω speaker above with either two 4 Ω speakers wired in series or two 16 Ω speakers wired in parallel.  The cabinet impedances will be the same as those listed on the jacks above.

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