The
impedance of a speaker or amplifier is an aspect of pa systems commonly
misunderstood but vitally important and probably the most common reason
for amplifier damage
Definition: ‘Impedance’
is the frequency dependent electrical resistance of the system to AC current.
Speakers have a fixed impedance,
commonly 4 or 8 ohm. Usually noted next to the input, but can be checked
by measuring DC resistance across input terminals. Note that DC resistance
is usually lower than nominal impedance ; this does not imply a fault.
Amplifiers have a minimum load
impedance, usually 4 ohm. The total impedance of the speakers connected
to the amp must be equal to or greater than the minimum impedance.
A stereo amplifier can be considered
as two independent amps ; consider the load on each side as a separate
sum.
Amplifier outputs, and loudspeaker
connections are almost always parallel. To calculate the total load impedance
for cabinets in parallel use the following formula where Z denotes the
impedance of each cabinet ; 1/Z1 + 1/Z2 + 1/Z3…………+1/Zn
= 1/Ztotal
e.g. 2 x 8 ohm cabs = 1/8 +
1/8 = 2/8, therefore Ztotal = 4 ohm
Using
a total impedance too low for the amp will cause it to overheat, and will
cause more power and distortion to be delivered to the speakers, potentially
damaging those also.
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further information on this topic or are looking for personalised information
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