Impedance Matching

Impedance is the opposition to signal flow in a circuit or cable, measured in ohms (Ω). It combines resistance (frequency-independent) and reactance (frequency-dependent), and it determines how signals transfer between devices. Proper impedance relationships prevent signal reflections, level loss, and distortion — and the rules differ significantly between audio, video, and RF applications.

What Impedance Is (Technically)

Impedance (Z) is expressed as a complex number: Z = R + jX, where R is resistance and X is reactance. Reactance comes from capacitive (C) and inductive (L) elements in cables and circuits. At audio frequencies (20 Hz–20 kHz), wavelengths range from 17 km to 17 m — far longer than typical cable runs — so reactive effects are minimal. At RF frequencies and in digital video, wavelengths are short enough that cable impedance governs signal behavior critically.

The fundamental principle: when a signal encounters an impedance boundary (Z₁ → Z₂), a reflection coefficient determines what fraction of the signal reflects back toward the source:

ρ = (Z₂ - Z₁) / (Z₂ + Z₁)

A perfect impedance match (Z₁ = Z₂) yields ρ = 0 — no reflection. A short circuit (Z₂ = 0) yields ρ = -1 — full reflection inverted. An open circuit (Z₂ = ∞) yields ρ = +1 — full reflection in phase.

Bridging: The Audio Standard

Professional audio systems use bridging rather than impedance matching. The output impedance is kept low (typically 50–600 Ω) while the input impedance is kept high (10 kΩ–100 kΩ). This intentional mismatch has two advantages:

• One low-impedance output can drive multiple high-impedance inputs simultaneously with minimal level loss
• Signal levels are governed by the output device's voltage, not the load impedance

The "600 Ω" figure in legacy audio specifications is a historical relic from telephone engineering, where true matched 600 Ω systems maximized power transfer over long lines. Modern professional audio equipment uses 600 Ω outputs only for backward compatibility with older gear; the actual driving impedance is usually well below 100 Ω.

Typical Audio Impedances

Amplifier damping factor = speaker impedance / amplifier output impedance. A damping factor of 100 with an 8 Ω speaker means the amplifier output impedance is 0.08 Ω. Higher damping factors give the amplifier tighter control over woofer motion, reducing bass overhang in poorly damped enclosures.

Impedance Matching for Speakers

Speakers are the one place in audio where impedance ratings are operationally critical:

• Amplifiers are rated for minimum load impedance (e.g., "stable at 4 Ω minimum")
• Connecting speakers with total impedance below the amplifier's minimum causes excessive current draw, thermal stress, and potential failure
• Connecting speakers with total impedance above the minimum is safe but reduces output power

Series vs. parallel speaker wiring:

• Two 8 Ω speakers in series = 16 Ω total
• Two 8 Ω speakers in parallel = 4 Ω total
• Two 8 Ω speakers in series-parallel (2×2) = 8 Ω total

For 70V/100V distributed speaker systems, individual speaker impedance is irrelevant — all speakers connect to the constant-voltage bus through matching transformers. See speaker-placement for 70V system design.

Video Impedance: 75 Ω and Critical Matching

Analog video and uncompressed digital video (SDI, component, composite) use 75 Ω characteristic impedance throughout. Unlike audio bridging, video systems require impedance matching because video signal wavelengths are short enough that reflections create visible artifacts:

• Ghosting — a delayed duplicate of the image caused by a reflected signal arriving microseconds late
• Ringing — oscillations after edges causing smearing or streaking
• Level errors — power loss from mismatched termination reducing white level and color accuracy

Proper 75 Ω video signal chain:

• Source output impedance: 75 Ω
• Cable characteristic impedance: 75 Ω (use cable specified for video, not general data cable)
• Load termination: 75 Ω at the final destination

In analog video distribution with multiple display points (loop-through configurations), only the last device in the chain terminates at 75 Ω. All intermediate devices use high-impedance inputs (typically > 10 kΩ) to avoid loading the line. Terminating every device causes a voltage divider effect that drops the video level severely.

SDI and Digital Video

SDI (Serial Digital Interface) is especially sensitive to impedance. The standard specifies 75 Ω with return loss of 15 dB minimum up to 270 MHz for SD-SDI, and tighter requirements for 3G-SDI (2.97 Gbps) and 12G-SDI (11.88 Gbps). A single impedance discontinuity — a poor connector, wrong cable, or splice — creates reflections that corrupt data at the bit level. Errors manifest as sparkle noise, blue-screen dropouts, or complete signal loss.

For SDI, use only 75 Ω coaxial cable specified for the signal grade. Belden 1694A is a common choice for HD-SDI; Belden 4855R for 4K 12G-SDI. Do not substitute general-purpose RG-59 or RG-6 cable — these have 75 Ω nominal impedance but do not maintain impedance tolerance at high frequencies.

Ethernet: 100 Ω Differential

Cat5e, Cat6, and Cat6a are specified at 100 Ω characteristic impedance (differential pair). This is why Dante, AES67, and AV-over-IP systems require properly installed structured cabling: sharp bends, excessive untwisting at terminations, or cross-pinned patch cables all create impedance discontinuities that increase jitter and bit error rate. At Gigabit speeds, even small return loss violations affect performance.

Maximum untwist at terminations: Cat6 allows 13 mm; Cat6a allows 6 mm. More untwisting than this raises pair impedance and can cause link instability in high-density Dante networks.

Transformers for Impedance Conversion

Transformers are the traditional tool for converting between impedance levels — particularly for interfacing 600 Ω legacy equipment to modern low-impedance inputs, or for providing galvanic isolation (eliminating ground loops). The turns ratio determines the impedance transformation:

Impedance ratio = (turns ratio)²

A 1:10 turns ratio transformer steps voltage up 10× but steps current down 10×, transforming impedance by 100×. A 600 Ω source through a 1:3.87 transformer presents 600 × 15 = 9,000 Ω to the load (approximately).

Jensen Transformers and Lundahl are recognized high-quality audio transformer manufacturers. For balanced-to-unbalanced conversion and ground isolation, passive DI boxes use transformers. Active DI boxes buffer instead, presenting the same high input impedance without a transformer.

RF and AES3 Digital Audio

RF coaxial cables (antennas, satellite, cable TV) use 75 Ω for video and 50 Ω for data/transmitter applications. At RF, proper matching is critical for maximum power transfer — a mismatch creates standing waves that reduce transmitted power and can damage transmitters.

AES3 digital audio cable uses 110 Ω balanced twisted-pair (XLR connectors). The specific 110 Ω impedance is required to minimize reflections at the data rates involved. Using standard microphone cable (typically 35–50 Ω) on AES3 runs causes data errors at high sample rates (96/192 kHz) even though the cable physically fits the XLR connectors. Cable marked "AES/EBU" is specified to 110 Ω. See aes for AES3 specifications.

Common Pitfalls

• Using audio cable for video — Audio cables have different impedance characteristics (often not 75 Ω) and inadequate shielding for video. The result is ghosting, color errors, and intermittent signal loss. Always use purpose-built video cables.
• Unterminated or double-terminated analog video loops — In loop-through video distribution, only the last device in the chain terminates. Terminating an intermediate device halves the signal level for everything downstream. Forgetting to terminate the last device causes reflections.
• AES3 cable impedance — Substituting standard mic cable for AES3 runs causes high-frequency data errors. At SD sample rates (48 kHz) the errors may be intermittent; at 192 kHz they can be constant. Use 110 Ω cable labeled AES/EBU.
• Speaker impedance below amp minimum — Connecting 4 Ω speakers to an amp rated "8 Ω minimum" risks thermal shutdown, distortion, and eventual failure. Always verify total speaker impedance (accounting for parallel runs) against the amp's minimum.
• Ignoring SDI cable spec for high-data-rate signals — Cable rated for SD-SDI may pass 3G-SDI over short runs but fail over longer runs or through connectors. Specify cable rated for the signal grade (1694A for HD, 4855R for 12G).
• Damping factor neglect in installed sound — Very long speaker cable runs (over 100 feet) at 8 Ω effectively reduce the amplifier's damping factor, causing bass bloat. Use appropriately gauged cable (12 AWG minimum for long runs) or install amplifiers close to speakers.