Voltage bridging versus power matching

Modern audio equipment connects a low-impedance output to a high-impedance input. Older textbooks teach matching the two impedances. The difference between these two approaches is the reason why connecting audio devices is easier today than it used to be.

Two ways to connect a source and a load

A source has an output impedance Z_out, and the load an input impedance Z_in. They form a voltage divider:

voltage at the load = source voltage · Z_in / (Z_in + Z_out)

• Power matching: set Z_in equal to Z_out. The load then receives the maximum power from the source. The price is that only half the source voltage arrives, a loss of 6 dB.
• Voltage bridging: make Z_in much larger than Z_out, typically ten times or more. Almost the full voltage arrives, and the source delivers very little power, which is all an audio input needs.

The numbers

The first row is the power matched case. The others are typical of modern equipment. Calculate your own combinations with the interconnection tool.

Why 600 Ω existed

Early audio technology borrowed from telephony. Long telephone lines needed matched terminations to avoid reflections, and the power available from the amplifiers was limited. The standard impedance of 600 Ω and the unit dBm come from this world. dBm means decibels relative to 1 mW, whatever the impedance. Only in a 600 Ω circuit does 0 dBm correspond to 0.775 V, which is where the voltage reference of dBu comes from. Studios kept the habit for decades.

With transistor and integrated circuit amplifiers it became easy to build low-impedance outputs and high-impedance inputs, and the reasons for matching disappeared.

Why matching is unnecessary in audio

• Wavelength is long. A 20 kHz signal in a cable has a wavelength of about 10 km, and typical cables are a tiny fraction of this. The cable behaves like a simple wire, without reflections that would need terminating.
• Power is not needed. An input only has to sense the voltage.
• Matching costs level. Loading a microphone with its own impedance would cost 6 dB of signal.

Where matching still matters

• Radio frequency and video, where cables are long relative to the wavelength, use 50 Ω or 75 Ω matching.
• Some vintage equipment, especially ribbon microphones and certain transformers, are designed for specific loads.
• Long, balanced lines at high frequency, and digital audio cables (AES3 110 Ω) are designed as transmission lines.

Loudspeakers

A loudspeaker amplifier is not power matched either. It has an output impedance far below 1 Ω, so that the voltage across the loudspeaker follows the signal regardless of the speaker's impedance variations. Power matching would waste half the power and give the amplifier no control over the cone's motion, see Amplifiers, loudspeakers and ohms.

Rule of thumb

Make the input impedance at least ten times the source's output impedance. For microphones, preamp inputs of 1.5 to 10 kΩ with sources of 50 to 200 Ω follow this rule.

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