Impedance - Speaker Cabinet Wiring
There are a number of important technical issues relating to the matching of speakers
to amplifiers, and to the wiring of multiple speaker systems (eg 2 x 12", 4 x 12", etc.).
Impedance - Cabinet Wiring
The impedance of a speaker is basically a measurement of how hard the amplifier will have to
work to drive that speaker. The impedance is measured in Ohms and is marked on the back of
most speakers. The impedance of the speaker(s) installed in an amplifier or cabinet will be
quoted in the operating instructions (If you have them).
It is essential, particularly in the case of tube (valve) amplifiers that the impedance or ‘load’
presented to the amplifier is correct; otherwise this can damage the amplifier. Many tube
amplifiers are set up with multiple ‘taps’, eg. 8 and 16 Ohms. In such cases, it is important that
the impedance of the speaker system matches the impedance value set on the amplifier.
Most transistor amplifiers are designed to drive 8 Ohm loads. Although the impedance matching
is not as critical as with tube amplifiers, the impedance of the speaker(s) should be at least equal
to the minimum impedance of the amplifier. The majority of guitar speakers are available in a
choice of 8 and 16 Ohm impedances. However, the way in which multiple speakers are wired
(in a 4 x 12" cabinet for instance) determines the overall impedance of the speaker system.
Hence wiring is critical.
There are basically two ways of wiring multiple speakers: Series and Parallel.
Here are diagrams of the most typical wiring configurations.
If multiples of identical speakers are used in a combo or cabinet, the overall power rating is
calculated by multiplying the individual speaker rating by the number of speakers in the cab,
eg: 4 x 25 watts = 100 watts.
However, when speakers are mixed, the lower rating is used to
make the calculation, as both speakers draw the same power from the amp.
For example, in the case of one 60 watt speaker and one 25 watt speaker in the same cab, the
effective power rating is calculated as follows: 1 x 25 watts + 1 x 25 watts = 50 watts total.
It is not critical to match the power handling of a speaker system precisely with the power output
of the amplifier. But as a general rule, the power rating of the speaker(s) should be at least equal
to that of the amplifier.
Phase / Polarity
All guitar speakers have just two connectors, one marked positive (+) and the other negative (-).
If one speaker is wired the wrong way around in a multiple speaker configuration, it will not damage
the amplifier. However, it will be ‘pushing’ when the other speakers are ‘pulling’, resulting in the
cancellation of some frequencies in the overall sound. It is therefore important that the correct
wire goes to the correct terminal.
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