Impedance — Audio and Video Impedance Matching

Impedance (Z)

For balanced audio signal fundamentals, see fundamentals/balanced-vs-unbalanced-audio. For gain structure, see fundamentals/gain-structure.

Impedance (Z) is the opposition to AC current flow in an electrical circuit, measured in ohms (Ω). In AV systems, impedance governs how signals transfer between source and load. The rule for audio and video differ significantly: audio systems use bridging (high input impedance) to avoid loading the source, while video systems require termination (matched impedance) to prevent signal reflections. Getting this wrong causes tonally incorrect audio, reflections in video, or power loss in speaker systems.

Audio Impedance — Bridging

Professional audio equipment uses voltage bridging: the input impedance of the receiving device is much higher than the output impedance of the source. This ensures the source "sees" minimal load and can drive multiple inputs without level loss.

Typical values:

• Line output impedance: 50–200 Ω (professional balanced); some consumer gear: 1 kΩ+
• Line input impedance: 10 kΩ–20 kΩ (professional balanced)
• Microphone output impedance: 50–200 Ω
• Microphone preamp input impedance: 1.5 kΩ–3 kΩ (standard); 5 kΩ–10 kΩ (high-impedance for vintage/transformer mics)
• Rule of thumb: input impedance should be at least 10× the source output impedance for proper bridging

Driving a professional line input (10 kΩ) with a 150 Ω output is correct — the source sees a very light load. Driving the same input with a consumer source at 10 kΩ output into 10 kΩ input creates a 6 dB level drop and a frequency-response error.

Speaker Impedance — Power Transfer

Loudspeaker systems require power transfer, not voltage bridging. The amplifier output impedance should be much lower than the speaker nominal impedance so the amplifier can control the speaker's back-EMF (damping factor).

• 4 Ω — low-impedance speaker; common in portable and some installed systems
• 8 Ω — standard nominal impedance for installed speakers
• 16 Ω — some ceiling speakers; less common
• Damping factor: amplifier output impedance / speaker impedance; higher is better; professional amplifiers typically achieve damping factors of 100–1000

Paralleling speakers reduces impedance (two 8 Ω speakers in parallel = 4 Ω). Amplifiers have minimum impedance ratings — driving below the minimum causes overheating and potential damage.

70V/100V Distributed Systems

Distributed audio systems use constant-voltage (70V or 100V) transmission to allow multiple speakers to be driven from one amplifier without impedance matching calculations. See glossary/70v-distributed-audio.

Video Impedance — Termination

Analog video and SDI systems require matched termination: the cable, source, and load must all be the same impedance (75 Ω for SDI/composite; 110 Ω for AES3 digital audio). Without matched termination, signals reflect at impedance discontinuities and travel back toward the source, appearing as ghosting, ringing, or (for SDI) jitter and bit errors.

• SDI: 75 Ω coaxial cable, 75 Ω source output, 75 Ω load termination — all three must match
• AES3 digital audio: 110 Ω balanced (XLR); connector and cable must be 110 Ω rated — standard microphone cable (typically 40–60 Ω) causes AES3 reflections over long runs
• HDMI/DisplayPort: differential impedance controlled in the cable specification; no external termination required by the user

Never put two video terminations on one SDI output. An SDI distribution amplifier (DA) provides one input termination and drives multiple outputs, each loaded only at the final destination. Bridging two 75 Ω loads on one output produces 37.5 Ω and a 6 dB level drop with severe reflections.

Common Pitfalls

• Driving an audio line input with a microphone-level signal without impedance matching. A mic connected directly to a line input loses 20–60 dB of signal level and may not achieve proper bridging impedance. Fix: always pass microphone signals through a preamp with appropriate mic input impedance before the line-level stage.

• Using standard microphone cable for AES3 digital audio over long runs. Generic mic cable has ~40–60 Ω impedance; AES3 requires 110 Ω. At lengths over 20–30 m, reflections cause bit errors and signal dropouts. Fix: use cable rated 110 Ω for AES3 runs (Belden 1800F or equivalent).

• Double-terminating an SDI signal. Connecting two monitors to an SDI output without a DA places two 75 Ω terminations in parallel (37.5 Ω), causing reflections, level drop, and waveform ringing visible on a waveform monitor. Fix: use a distribution amplifier; never loop SDI through two devices unless the first device has a true loop-through (not a parallel-connected output).

• Ignoring speaker minimum impedance. Wiring three 8 Ω speakers in parallel on one amplifier channel creates a 2.7 Ω load — below most amplifiers' 4 Ω minimum. Fix: calculate total parallel impedance before wiring; use a separate amplifier channel or series wiring to maintain minimum impedance.