Impedances in audio: typical values

Impedance is the opposition a circuit offers to alternating current, measured in ohms. In audio it decides how devices interact when they are connected, and the numbers are easier to remember when listed together.

Typical values

Compare these with the rule from Voltage bridging versus power matching: a source drives a load at least ten times its own impedance.

Why high-impedance sources need care

A source with high impedance forms a low-pass filter with the capacitance of the cable. A guitar pickup of 10 kΩ and a 6 m cable of 100 pF/m, which is 600 pF, give a cutoff of about 27 kHz. With a larger source impedance or a longer cable, the cutoff falls into the audio band and treble is lost. Test combinations with the cable capacitance tool.

Hence the DI box: it converts a high-impedance, unbalanced signal into a low-impedance, balanced one for long cables, see Balanced and unbalanced connections.

Impedance and frequency

Impedance usually depends on frequency. A loudspeaker may be called 8 Ω but reads 4 Ω at some frequencies and 30 Ω at others, see Amplifiers, loudspeakers and ohms. A guitar pickup behaves like an inductor with a resonance. A single number is only a summary, usually measured at 1 kHz or the minimum above resonance.

Measuring

A simple way to find an unknown output impedance is to measure the voltage unloaded and with a known load, then calculate the ratio. The interconnection tool does the arithmetic for both output and input impedances.

Pitfalls

1. Plugging a low-impedance headphone into an output meant for loudspeaker-level or high-impedance load can distort or overload the source.
2. Connecting several inputs to one output lowers the load impedance (the inputs are in parallel). Ten 10 kΩ inputs make 1 kΩ.
3. Impedance mismatches in the other direction, a microphone into a low-impedance input, cost level and change the frequency response of some microphones.

Draft: written by an AI assistant, awaiting technical review.