| Parallel and series speaker wiring |
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| Friday, 21 January 2005 18:26 | |||
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I have often seen people ask how to wire multiple speakers to their amp to get a higher output power. This is achieved by decreasing the impedance of the load to get the highest output power the amp will provide, as shown by the formula P = I²R where P is power in Watts, I is current in Amps and R is resistance in Ohms. It is important to remember that Impedance and Resistance are not the same thing, as Impedance takes into account the effects of resistance, inductance and frequency. However, in this case it is safe to assume that resistance and nominal Impedance are the same thing.
Wiring speakers in series increases the overall resistance of the system. In a series circuit, the speakers are wired from output to input in a chain, where the same current is shared between all speakers in the circuit. 
Figure 1 shows two speakers wired in series for both the left and right channels. Let's concentrate on just the right channel, and assume speakers A and B both have a nominal Impedance of 8 Ohms. By simply adding the Impedance of both speakers in the circuit, we can see that 8 + 8 = 16, so the amp would be presented with a 16 Ohm load. Bear in mind this is a fairly high resistance and in many situations would not be practical, it does not make use of the maximum output power available from the amp.
Figure 2 shows two speakers wired in parallel for both the left and right output channels. This decreases the overall resistance of the system, and with two identical loads, say 8 Ohms each, we can simply half this value giving a total load resistance of 4 Ohms. If your amp puts out 500w per channel at 4 Ohms, wiring two 250w 8 Ohm speakers in parallel would be suitable for use with the amp, although it is often accepted practice to use an amp with a higher power rating than that of the speakers, helping to avoid clipping and undue stress on the amp. If you want to calculate the total resistance for two different loads, you can use the formula 1/Rtot = 1/R1 + 1/R2 ...etc. However, it is not normally a good idea to mix speakers of different impedance in the same system.
As shown above, combining series and parallel circuits allows even more speakers to be driven from the same output, but remember that you wont get more output power than the amp can provide, it will simply be shared between the speakers in the system. If all speakers on the left channel in figure 3 had an impedance of 4 Ohms, the amp would also see a total impedance of 4 Ohms. This is because A an B are wired in series to give 8 Ohms, as are C and D, and these two series circuits are combined in parallel to give 4 Ohms. In many cases changing from an 8 Ohm to 4 Ohm system can help increase output without the need for buying a new amp. However, don't expect that doubling the wattage will double the volume, it won't. Also bear in mind the specifications of your amp, don't try and drive a load that has a resistance too low for the output stage of your amplifier.
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