Speaker Loads and Wiring
Instrument and P.A. amps are built to power a "load"
(a speaker or a set of
speakers) having a certain AC "impedance",
measured in ohms.
"Impedance" is a hindrance to the flow of
current. The less the impedance, the more juice your amp
So, a low impedance speaker is a big load for your amp and a high
impedance is a small load.
A single, raw speaker usually has an impedance of 4, 8, or 16 ohms.
To push more air, several matching speakers can be wired together inside a cabinet.
The combined impedance of the whole cabinet may differ from that of the raw speakers.
diagrams and their effect on load impedance are provided
If a load impedance doesn't match what the amplifier expects, there may be
If the load impedance is too high, the amp might sound weak or dull.
If the load impedance is too low, the amp might sound
fantastic but eventually blow up. For this reason, amps are often labeled with
the minimum ohms you can safely connect.
An ohms (Ω) rating is often marked somewhere on a speaker or cabinet.
If not, you can measure the value with an ohmmeter or multimeter.
These meters measure DC resistance, not impedance, and so will read low. Round up the reading to
an even 4, 8, or 16 to arrive at the load's AC impedance.
Some amps have an impedance selection switch or various speaker jacks you
can use to connect alternate impedances. Changing load impedance is one of the easiest
and most dramatic ways to change the sound of a guitar amp.
If your amp uses power tubes (6L6, EL34, etc.), always have your speakers connected when the amp
is on. Without a load, the amp's output transformer can
easily be damaged, especially if you're playing through it.
CABINET WIRING - TWO SPEAKERS
There are two ways you can wire together two speakers.
One way is called "series wiring", the other is called "parallel
Series wiring results in twice the impedance of one speaker.
Parallel wiring results in half the impedance of one speaker.
The schematic diagram below shows how to wire a 16-ohm
cabinet or a 4-ohm cabinet using two 8-ohm speakers.
To make a two speaker, 8-ohm cabinet, wire two 4-ohm speakers in series or two
16-ohm speakers in parallel.
CABINET WIRING - FOUR SPEAKERS
Four speakers wired in series quadruples the
impedance of one speaker. Four speakers wired in parallel quarters the impedance of
The schematic diagram below shows how to make a 16-ohm
cabinet using four 4-ohm speakers or a 4-ohm cabinet using four 16-ohm speakers.
CABINET WIRING - SERIES-PARALLEL
A third way to wire four speakers is called "series-parallel". In this method,
a pair of series-wired speakers is wired in parallel to a second
pair of series-wired
The combined impedance equals that of one speaker.
schematic below shows how to make a 4-ohm cabinet using four, 4-ohm speakers.
Use 16-ohm speakers to get a classic, 16-ohm Marshall cab.
To make a four speaker, 8-ohm cabinet, wire four 8-ohm speakers in
CABINET WIRING - MONO/STEREO
This schematic below shows the wiring of a Marshall® 2x12"
speaker cab with a Mono/Stereo switch and two specialized input jacks.
When the Mono/Stereo switch is set to Mono, the two 8-ohm
speakers are either wired in series (making a 16 ohm cab) or in parallel (making
a 4 ohm cab) depending on
which jack you plug into.
When the switch set to
Stereo, each jack connects to its own, independent, 8-ohm speaker.
In this mode you use both jacks, one for the power amp's left output and
one for its right output.
To make a Mono/Stereo cabinet with four speakers, replace each 8-ohm speaker with two 4-ohm speakers wired in series or
with two 16-ohm speakers wired
in parallel. All the resulting impedances will be the same as the
above, two speaker version.
Marshall® is a registered trademark of
Marshall® Amplification plc.
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