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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