Ohm’s Law and Impedance
Electrical resistance becomes important once you start to connect different pieces of equipment together, and describes a circuit’s opposition to DC (direct current) flowing through it, much as a narrow hosepipe might restrict the flow of water. The higher the resistance of a circuit (the narrower the hosepipe), the more voltage (water pressure) you need to push a given electrical current through it. Conductive materials such as copper and aluminium have a low electrical resistance, so current flows easily in them, whilst materials, such as rubber, bakelite, glass and some types of plastic have very high resistances and are therefore less conductive. These are known as insulators, as a negligible amount of current can pass through them.
Technically, resistance really only applies to DC voltages and currents, and is measured in ohms. The simple formula known as ohms Law that we all learned at school takes the form: R = V/I, where R is resistance (in ohms), V is the voltage across the circuit, and I is the current (in amps) flowing through the circuit. If you know any two of the three values, you can use Ohm’s law to work out the unknown one.
When dealing with alternating (AC) voltages such as audio signals, the circuitry ceases to behave as a pure resistor, and we talk about its ‘impedance’ (often abbreviated to ‘Z’) to current flow which may vary depending on the signal frequency. In a purely resistive circuit, resistance and impedance are the same thing, but most electronic circuits also include what we call reactive components—capacitors and inductors —which are frequency dependent. So, impedance can be thought of as ‘AC resistance’, and although still measured in ohms, it may have very different values at different frequencies.
‘Input impedance’ relates to how much current the input terminals of a device draw from a signal source at a given frequency. The lower the input impedance, therefore, the more current the sending device will be required to supply. Similarly, ‘output impedance’ is a measure of how much current an output stage can supply at a given frequency, with a lower output impedance indicating that the unit can deliver more current.
In audio we tend to be concerned mainly with signal voltages rather than impedances, so the usual strategy is for device inputs to exhibit a much higher impedance than device outputs. This ensures that minimal electrical loading takes place when connecting a signal source to a receiving device, and applies whether we are talking about microphones connected to preamps, or mixer outputs into power amplifiers. It is a technique called ‘voltage matching’ and Line-level equipment usually has an input impedance of several tens of thousands of ohms—47 kilohms being a typical figure. The output impedance, on the other hand, is typically just a few tens of ohms.
The same is true of sources such as microphones—the mic preamp of a mixing console usually has an input impedance of roughly 1.5k kilohms, whereas a dynamic microphone designed to be plugged into it will have an output impedance of 200 Ohms or less.
One reason audio equipment is generally designed with such a low output impedance is that it allows long cables to be driven without significant signal loss. When using cables in excess of 10 metres, low-impedance sources minimise signal degradation caused by the cable’s own impedance (a cable is equivalent to a small series resistor and parallel capacitor, with a bit of inductance thrown in!).
One significant exception to the low-impedance source rule, though, is the electric guitar with passive pickups. These have a relatively high output impedance. Consequently, it is important to keep the cable length reasonably short, and to choose a cable designed for guitar use, as excess cable capacitance forms a low-pass filter with the source impedance and can result in treble loss.
Excerpt from The SOS Guide to Live Sound: Optimising Your Band’s Live-Performance Audio by Paul White © 2014 Taylor & Francis Group. All Rights Reserved.