One source of confusion we frequently run across is the concept of speaker impedance. This confusion is also a likely cause of many blown power amplifiers. This article is intended to explain the meaning of speaker impedance and guide the reader in connecting multiple speakers to an amplifier.
The ohm is the unit of measure for impedance, which is the property of a speaker that restricts the flow of electrical current through it. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. The impedance of a speaker is a physical property that (ideally) does not change value, although from an engineering standpoint, there are many complex characteristics that make up speaker impedance For this reason, the rating of a speaker is called its ‘nominal’ value, which pretty much means “in name only”. For the average audio user, the nominal impedance is the dominant characteristic and for the purposes of this discussion, we will use the nominal value of the speaker’s impedance.
Why are ohms important?
Two reasons:
(1) If you connect your amplifier to the wrong speaker impedance, you risk damaging the amp. In tube amps, too high a load impedance (or a disconnected load) can result in damage to the output tubes or output transformer, while in solid state amps, if the speaker impedance is lower than the amplifier impedance, the amplifier will tend to overheat and more power is used up in the amplifier than is delivered to the speaker. Too many speakers on a solid state amp can burn up the power output section.
(2) The amplifier will deliver maximum power (volume) to the speaker when the speaker impedance matches (is equal to) the internal impedance (called the OUTPUT IMPEDANCE) of the amplifier. Too low of an impedance will result in weak output and poor tone. If the speaker impedance is higher than that of the amplifier, its power output will again be less than it is capable of.
There is a relationship between voltage, current and impedance. Voltage is what moves the electrical current. Thus, an increase in voltage can increase the current as well, as long as the impedance does not change. On the other hand, lowering the voltage also lowers the current. George Simon Ohm formulated this relationship by writing down Ohm’s Law. This formula states that in a circuit, the current is directly proportional to voltage and inversely proportional to impedance.
Knowing about speaker ohms is important because it is a safety issue. Connecting too many speakers to a device such as an amplifier can result in overloading. When this happens, there is a risk that it may blow up. Amplifiers give their maximum power to the speaker when the impedance of the speaker is similar to the output impedance of the amplifier. If the speaker impedance is lower than the amplifier’s, then the amplifier has the tendency to overheat. In the opposite situation when the speaker impedance is lower, the result is a poorer quality of sound.
Speakers can be connected to each other in two methods: parallel and series. If the speakers are connected in a series, the impedances are added to each other. For example, two 8-ohm speakers in a series have a total impedance of 16 ohms. In cases where speakers of the same impedance are connected in a parallel, the total is the impedance of one speaker divided by how many speakers there are. For parallel connections of speakers with differing impedance levels, the total impedance is computed by multiplying the impedance of the speakers. The product is then divided by the sum of the speakers’ impedance.
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