Open just about any equipment manual and you'll come across terms like input impedance, output impedance, impedance matching and so on — but what do they all mean, and does it matter anyway?
Most people feel comfortable with the concept of electrical resistance — the higher the resistance of a circuit or component, the more voltage you need to push a given current through it. Resistance is measured in ohms (Ω), and Ohm's law states that R (resistance) = V/I — where V is the voltage across the circuit, and I is the current (in Amps) flowing through the circuit.
Useful Terms
Once you know Ohm's law, impedance isn't too difficult to define — it's essentially the resistance a circuit puts up, not to DC (Direct Current), but to an alternating current, such as an audio signal. And impedance is still measured in ohms. In a purely resistive circuit, resistance and impedance are the same thing, but in the case of a reactive circuit (one containing capacitors or inductors), the impedance will vary depending on the frequency of the signal being applied. In the case of audio equipment, the circuitry is designed (where possible) so as to keep the impedance reasonably constant over the entire audio range. However, this isn't always possible, and loudspeakers in particular show a significant change in impedance at or near the resonant frequency of the drivers.
It isn't difficult to understand input impedance — any collection of components strung onto the inside of an input jack must load (take power from) the signal being fed into that input. But how does output impedance work? If input impedance is related to how much electrical current is soaked up by a circuit, output impedance is related to how much current an output can supply. If you like analogies, think of a person pedalling a bike. A cyclist has an output impedance related to his or her ability to turn the pedals, whereas the pedals themselves have an input impedance — they put up resistance to being turned. And that leads us nicely onto the concept of impedance matching.
A Perfect Match
Strictly speaking, a properly matched circuit is one where a circuit with a given output impedance feeds an input which has the same value input impedance. In other words, a 600Ω output feeding a 600Ω input is perfectly matched. This is important in any circuit where we're concerned with transferring the maximum amount of power from one circuit to another, for example, an amplifier driving a loudspeaker. Referring back to the cyclist, this situation is roughly analogous to the bike being geared so that the cyclist can achieve the maximum speed without having to work so hard as to become exhausted. If the wrong gears are chosen, the cyclist either pedals like hell and gets nowhere, or the pedals are so hard to turn that it becomes impossible to continue. Either situation is a mismatch between the energy provider (the cyclist) and the energy receiver (the pedals).
In Practice
When it comes to low level audio signals, we're not concerned about transferring lots of energy — we just want to get the signal voltage from equipment A to equipment B with as little loss as possible. In other words, we want our bike geared so as to be easy to pedal — we don't care if it doesn't get anywhere so long as the pedals go round. To achieve this situation when matching audio signals, it's usual for the source impedance (the output impedance of the equipment providing the signal) to be between five and ten times lower than the load impedance (the device accepting the signal). Not only does this prevent the signal from being unduly loaded, it also enables one source to drive two or three loads simultaneously if required, for example, by means of a splitter lead.
A typical mixing console has an input impedance of around 1kΩ, while a low‑impedance dynamic microphone might have an impedance of 200Ω or so. This satisfies the criteria for a good match. Similarly, most line inputs have an impedance of around 50kΩ, whereas most equipment designed to provide a line level output will have an output impedance of 10kΩ, or, as is often the case, considerably less. However, if you plug a high‑impedance mic, with a 47kΩ output impedance, into the mic input of a desk (1kΩ), the signal from the mic will be severely loaded by the mixer, resulting in a drastic loss in level and possible distortion. In other words, the guy on the bike just fell off!
Summary Of The Rules
When matching amplifiers to loudspeakers, the output impedance of the amplifier should match that of the loudspeakers as accurately as possible, to ensure that the amplifier is able to deliver its maximum rated power. If the speaker impedance is lower than that of the amplifier's output impedance, the amplifier will be forced to work too hard, which will cause overheating and possibly failure. If the amplifier has an overload protection circuit built‑in, this may operate and shut down the amplifier, either partially or completely.
If, on the other hand, the speaker impedance is significantly higher than the amplifier's output impedance, the amplifier will be unable to deliver its full power rating, but should in all other respects work normally. Taking an example, if an amplifier with an output impedance of 4Ω is connected to an 8Ω loudspeaker, the maximum power available will be half the rated power of the amplifier. Never run valve amplifiers without a speaker connected, as damage may result. This does not apply when using solid state amps, however.
When matching audio signals (eg, mic or line level signals), unlike with the amplifier and speaker example above, it is not power transfer we require, but simply the most efficient transmission of the signal voltage. As a result, the ideal setup here is when the source impedance is significantly lower than the load impedance, ideally by a factor of 10 or greater. If the source impedance is the same as the load impedance, the circuit will work as it should, but the maximum available signal level will be halved. Finally, if the load impedance is lower than the source impedance, the available level will drop, and there may also be audible distortion.
A Few Tips
- Providing the load impedance is considerably less than the source impedance, it should be possible to split an audio signal to drive two or more loads. However, never join two outputs together in order to feed one input. This must only be done using a mixer circuit; directly connecting two outputs may cause circuit damage, as well as signal degradation.
- When using very long cables (in excess of 10 metres), use low impedance sources to avoid signal degradation caused by cable impedance and external interference. Balanced systems offer greater immunity to interference than unbalanced systems.
- With very high impedance sources such as electric guitar, keep the cable length as short as possible, and choose a cable designed for guitar use (or make your own, using low capacitance cable).
- When handling digital signals such as the S/PDIF outputs from DAT machines and CD players, don't use any old cable, as at these frquencies the cable impedance must also be matched. If you make your own cables, select an RF cable with an impedance of between 70Ω and 75Ω, or buy a proprietory digital cable. Keep the lead lengths as short as is practical. Failure to use the correct cable may result in errors, which manifest themselves as random clicks or bursts of distorted sound.