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    Speaker Loads and Wiring




LOADS

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 must supply.

So, a low impedance speaker is a big load for your amp and a high impedance is a small load.




OHMS

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.  Specific wiring diagrams and their effect on load impedance are provided below.

If a load impedance doesn't match what the amplifier expects, there may be consequences.  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 wiring".

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.



Series & Parallel Wiring - 2 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 one speaker.

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.



Series & Parallel Wiring - 4 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 speakers.  The combined impedance equals that of one speaker.

The 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.



Series-Parallel Wiring - 4 Speakers


To make a four speaker, 8-ohm cabinet, wire four 8-ohm speakers in series-parallel.











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.



Mono/Stereo Speaker Cabinet Wiring

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.




Additional Resources:


Marshall® is a registered trademark of  Marshall® Amplification plc.



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