Speaker impedance is the nominal electrical load a loudspeaker or cabinet presents to an amplifier. Guitar and PA equipment commonly uses ratings such as 4 Ω, 8 Ω and 16 Ω.

Use our Speaker Impedance Calculator to work out cabinet loads instantly.

Speakers in series

Series impedances add together. Two 8 Ω speakers in series produce a nominal 16 Ω load. Two 16 Ω speakers in series produce 32 Ω.

Speakers in parallel

For parallel connections, the reciprocal impedances add. With two identical speakers this is simple: two 16 Ω speakers in parallel produce 8 Ω, while two 8 Ω speakers in parallel produce 4 Ω.

Four-speaker cabinets

A 4×12 guitar cabinet often uses a series-parallel arrangement. With four identical speakers, the common two-series-pairs-in-parallel arrangement produces the same nominal impedance as one individual speaker. Four 16 Ω speakers can therefore form a 16 Ω cabinet.

Why amplifier matching matters

Solid-state and valve amplifiers do not always respond to impedance mismatch in the same way. A solid-state amplifier normally specifies a minimum load. Going below that minimum can demand excessive current. Valve amplifiers commonly use an output transformer with labelled taps, so the intended cabinet load should normally be connected to the corresponding output.

Always follow the amplifier manufacturer’s specified load limits. Do not assume that a mismatch which happens to work on one amplifier is safe for another.

Speaker impedance is nominal

A speaker marked 8 Ω does not measure exactly 8 Ω at every frequency. Loudspeaker impedance changes with frequency. The labelled value is a nominal figure used for system matching and cabinet design.

Output transformers and reflected load

In a valve amplifier, the output transformer reflects the loudspeaker load back to the output valves. The transformer’s impedance relationship depends on the square of its turns ratio. Our Output Transformer Impedance Ratio Calculator demonstrates that relationship.

If you have a faulty guitar amp, powered speaker or PA system, request a repair quote from Electronic Repairs UK.