aberdeen-music

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Series circuit:

RT=R1+R2+R3+R4

Parallel circuit

1/RT=1/R1+1/R2...

RT=total impedance

So for 8 ohm speakers:

series:

RT=8+8+8+8=32 ohms

parallel:

1/RT=1/8+1/8+1/8+1/8

1/RT=4/8

RT=8/4=2 ohms

A lot of amps don't like 2 ohms either

SOLUTION: parallel-series

Wire them in series as pairs, then wire the pairs parallel.

for a pair, RT=8+8=16 ohms

two 16 ohm pairs paralleled

1/RT=1/16+1/16=1/8

RT=8 ohms

This way 8 ohm speakers present an 8 ohm load to the amplifier, 4 would give 4, etc.

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SOLUTION: parallel-series

Wire them in series as pairs, then wire the pairs parallel.

for a pair, RT=8+8=16 ohms

two 16 ohm pairs paralleled

1/RT=1/16+1/16=1/8

RT=8 ohms

This way 8 ohm speakers present an 8 ohm load to the amplifier, 4 would give 4, etc.

There is actually a difference between SeriesParallel (pairs in parallel wired as series) and ParallelSeries (pairs in series wired as parallel). Eg, if one speaker fails in SeriesParallel you have 3 left, whereas if 1 fails in ParallelSeries you have 2 left. Not sure if that is the reason, but i read somewhere that SeriesParallel is more common.

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There is actually a difference between SeriesParallel (pairs in parallel wired as series) and ParallelSeries (pairs in series wired as parallel). Eg, if one speaker fails in SeriesParallel you have 3 left, whereas if 1 fails in ParallelSeries you have 2 left. Not sure if that is the reason, but i read somewhere that SeriesParallel is more common.

I wouldn't think series-parallel is common. Firstly every 4x12 I've looked in has parallel-series wiring.

Secondly, look at what happens when one speaker fails.

P=IV

V=IR

P=V^2/R

parallel-series:

If one of a pair fails the other has it's circuit disconnected. RT increases to 16 ohms so P is halved (well, it isn't actually because the ratio depends on the apparent load in the amp in parallel with the apparent load from the speakers). Both speakers will take, nominally, the same amount of power as they were doing, reducing the risk of blowing another.

series-parallel

if one speaker fails you have 8 ohms from the single and 4 ohms from the parallel giving 12 ohms. So you're getting more power than the parallel-series circuit. Not a problem, you've got 50% more speakers to deal with it.

Except that the single speaker will take 2/3rds of the power, meaning it takes almost double the power it was dealing with in a 4 speaker combination

Plug in some numbers and you'll see, assume V^2=800:

4 speaker working: any configuration

Total power to cab=800/8=100 W

power per speaker=25 W

Total power to cab=800/16=50W

total power per speaker=25W

total power to cab=800/12=66.6 W

8 ohm series speaker will take 2/3rds of power and 4 ohm parallel speakers only 1/3rd

Power per speaker: series 44 W, parallel 11 W

As you can see, one dead speaker could quickly become two dead speakers.

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