THE PHYSICS OF QUIET / 09

RF in Analogue Audio Circuits

How radio-frequency energy can enter an analogue circuit, meet a nonlinear junction and become a lower-frequency error.

A circuit does not need to reproduce a radio signal directly for radio-frequency energy to matter. If RF reaches a susceptible analogue junction, it can be rectified or mixed into an offset, modulation product or other measurable error.

This page explains established RF coupling and conversion mechanisms and how they inform Quiescent’s research into working audio systems.

Illustration of a radio-frequency wave entering an analogue circuit and producing a lower-frequency residual.

RF can arrive by more than one route

Signal cables, loudspeaker leads, mains wiring, chassis structures and circuit traces can couple radio-frequency energy by conduction, capacitance, induction or radiation. Which route matters most depends on frequency, geometry, screening, reference paths and the circuit itself.

The presence of RF near a system is therefore only the first part of the question. The important next question is whether a path carries enough of it to a susceptible part of an analogue circuit.

Illustrative exploration

RF coupling paths in an analogue amplifier

Choose a path to examine the entry route, circuit boundary and resulting interaction. Read from the incoming RF route, through the susceptible boundary, to the lower-frequency residual that conversion can produce. The diagrams illustrate these relationships rather than measured product responses.

Illustrative path: blue indicates the incoming RF carrier, warm marks a nonlinear boundary, and green indicates a possible lower-frequency residual.

State 01 of 03

Input and signal ground

RF can be coupled onto a signal conductor, its shield, or the reference shared by a sensitive input stage. The coupling strength depends on source, geometry, impedance and frequency.

Potential entry: An input cable, connector or signal-reference path can carry a small high-frequency voltage or current into the amplifier.
Circuit boundary: Input protection, semiconductor junctions and other non-ideal boundaries can respond nonlinearly at high frequency.
Possible consequence: Rectification or mixing can create a baseband component, offset or intermodulation product within the circuit’s operating band.

The resulting error depends on the RF reaching the input and the stage’s susceptibility. Measurement establishes its level; listening evaluation assesses its significance.

Illustrative path: blue indicates the incoming RF carrier, warm marks a nonlinear boundary, and green indicates a possible lower-frequency residual.

State 02 of 03

Supply and reference

High-frequency energy can also arrive through a supply, a reference network or a return path shared between stages. A circuit only knows the voltage and current that appear across its terminals.

Potential entry: Mains-borne energy, capacitive coupling and internal current loops can all influence a local supply or reference impedance.
Circuit boundary: Regulators, decoupling networks, active devices and the ground/reference structure have finite impedance and frequency-dependent behaviour.
Possible consequence: If a reference moves or a nonlinear junction demodulates the carrier, small unintended terms may enter the signal or feedback environment.

The effect depends on the disturbance spectrum and level, shared impedance, circuit topology and the receiving stage.

Illustrative path: blue indicates the incoming RF carrier, warm marks a nonlinear boundary, and green indicates a possible lower-frequency residual.

State 03 of 03

Speaker cable and output

The output terminals connect a high-current amplifier stage to a cable and a reactive loudspeaker load. Those same terminals can form an RF coupling and return path.

Potential entry: A speaker cable can behave as part of an antenna system at some frequencies, bringing RF to the amplifier–load boundary.
Circuit boundary: The output network, feedback boundary, protection circuitry and output devices all meet at a low-impedance, high-current node.
Possible consequence: RF can alter conditions around that boundary or generate unwanted components through nonlinearity. This is distinct from audio-band loudspeaker back EMF, although both use the output terminals.

Coupling depends on the cable geometry, surrounding RF field and amplifier output network. These relationships determine the disturbance reaching the circuit boundary.

A nonlinear junction can detect RF

A simplified nonlinear current relationship can be written as i(v) ≈ a₁v + a₂v² + a₃v³ + …. The squared and cubic terms matter because they can create new frequency components when more than one signal is present.

For an amplitude-modulated RF carrier, rectification can create DC and lower-frequency content related to the carrier envelope. In an audio circuit, that can appear as an offset, a low-frequency artefact or an intermodulation product. Circuit symmetry, filtering, and layout reduce susceptibility; RF-immunity measurements establish how effectively they work across the relevant frequency range.

Input and signal ground

RF that becomes differential at the input can disturb a sensitive reference point and create an amplified error.

Supply and reference

RF current sharing an impedance or reference path can appear as rail or bias disturbance where supply rejection is finite.

Speaker cable and output

At the output boundary, RF current can interact with compensation, protection and stability while the amplifier is delivering audio current.

What this can mean in a power amplifier

The relevant concern is how RF-induced disturbance affects an amplifier’s linear operating margin. Rectified offsets, reference disturbance, additional high-frequency current and changes in feedback behaviour can interfere with accurate signal delivery.

When an amplifier is driving a demanding reactive load, its voltage, current and stability margins are already important. Testing RF exposure alongside load behaviour reveals whether unwanted RF produces additional output error or changes clipping, limiting or recovery.

From circuit behaviour to working audio systems

RF coupling and rectification are established electronic mechanisms. Quiescent’s own research has found that effects described in the literature also apply within working audio systems. This experience informs our investigation of the routes through which unwanted high-frequency energy reaches sensitive circuitry.

Our engineering approach is to control that energy before it interacts with the musical signal. The relevant route and strength of the interaction depend on the equipment, connections and surrounding conditions.

During our development and listening evaluations, reducing unwanted disturbance has repeatedly produced a clearer, more stable musical presentation.

Measurements help identify the physical changes; listening evaluations assess their significance in music. Together, they guide the development of treatments for the connected system.

Continue the Physics of Quiet

Follow the signal from a circuit mechanism to a reactive loudspeaker load, or return to the wider system view.

Loudspeaker load

Power Amplifiers and Loudspeaker Loads

Why voltage, current, back EMF and headroom need to be considered together.

Explore

Changing conditions

Tracking Error in Audio Systems

Why structured error differs from a steady background noise.

Explore

Connected paths

The Physics of Quiet

Return to the wider system view of energy, boundaries and connected routes.

Explore

Technical notes and sources

The diagrams illustrate RF entry routes and nonlinear conversion rather than measured product responses. Practical assessment defines the RF frequency, level and modulation, coupling route, circuit operating conditions and resulting output error.

The references explain established susceptibility and conversion mechanisms. Quiescent’s research and development informs their application to working audio systems.