What do audio and power systems cover in WJEC A-Level Electronics?

Audio systems cover the audio signal chain: the mixer (a summing amplifier), tone control with filters, voltage and power amplification, gain in decibels, and driving a loudspeaker. Power systems cover producing a steady DC supply from the AC mains: the transformer, half-wave and full-wave bridge rectification, smoothing with a reservoir capacitor, voltage regulation, and electrical safety including fuses and earthing. Both reuse earlier sub-systems such as the summing amplifier, filters and diodes.

Why is a summing amplifier used as an audio mixer?

A summing amplifier feeds each input into the virtual earth of an inverting amplifier through its own resistor, so the inputs add without interacting; adjusting or adding one channel does not affect the others. The output is the inverted weighted sum, and a variable input resistor (a fader) sets each channel's level. This independence and clean summing is exactly what an audio mixer needs, which is why the summing amplifier is the standard mixer circuit.

What is the difference between a voltage amplifier and a power amplifier?

A voltage amplifier raises the signal voltage but supplies only a small current, so it cannot drive a low-impedance load like a loudspeaker directly. A power amplifier supplies both voltage and the large current needed to deliver power into the speaker, moving the cone. In an audio system the voltage (pre-)amplifier brings the small signal up to level, and the power (output) amplifier is the final stage that drives the loudspeaker and so handles the heat.

What are the four stages of a DC power supply?

A DC power supply has four stages. The transformer steps the mains AC down and isolates the circuit. The rectifier (a single diode for half-wave, or a bridge of four for full-wave) converts the AC to pulsating DC. The reservoir (smoothing) capacitor charges on each peak and discharges between peaks to reduce the ripple. The regulator (a Zener or IC regulator) holds the output at a fixed voltage despite load and input changes. Fuses and earthing provide safety.

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WJEC A-Level Electronics Audio and Power Systems: mixers, power amplifiers and DC power supplies explained

A deep-dive WJEC A-Level Electronics guide to Audio and Power Systems. Covers the audio signal chain, the mixer based on a summing amplifier, tone control with filters, voltage and power amplification, gain in decibels, driving a loudspeaker, and DC power supplies (transformer, rectification, smoothing with a reservoir capacitor, regulation, and electrical safety with fuses and earthing).

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What the audio and power section demands
Audio systems
Power supplies and mains
How the audio and power section is examined
The topics, dot point by dot point
For the official specification

What the audio and power section demands

Audio and power systems are two showcase applications in Component 2 that pull together sub-systems from across the course. Audio builds a signal chain from mixing to a loudspeaker; the power supply turns the mains into steady DC. The topic rewards understanding how familiar blocks (summing amplifiers, filters, diodes) combine, plus a few standard calculations.

This guide walks through the two topics in specification order, then sets out the exam patterns WJEC repeats. Each topic has a matching dot-point page with worked exam questions; this overview ties them together.

Audio systems

An audio system is a chain: a mixer (a summing amplifier whose virtual earth keeps channels independent) combines sources, tone-control filters shape the frequency balance, a voltage (pre-)amplifier raises the signal, and a power amplifier supplies the current to drive the loudspeaker. A voltage amplifier raises voltage but little current; a power amplifier supplies the power to move the cone. Gain is often in decibels, $20\log_{10}(A_v)$ for voltage, so a gain of 100 is 40 dB.

Power supplies and mains

A DC supply has four stages: a transformer steps the mains down and isolates; a rectifier converts AC to DC (one diode half-wave, a bridge full-wave using both half-cycles); a reservoir capacitor smooths the ripple by charging on peaks and discharging between them; and a regulator (Zener or IC) holds the output fixed. Full-wave is easier to smooth because its pulses come twice as often with no gaps. Safety uses a rated fuse and earthing of metal parts so a fault blows the fuse.

How the audio and power section is examined

Expect mixer-gain calculations and the virtual-earth reason for using a summing amplifier, the voltage-versus-power amplifier distinction, decibel gain conversions, the four-stage power-supply description with each stage's function, and the half-wave versus full-wave comparison. These are reliable marks built on reused ideas.

The topics, dot point by dot point

Each topic has a dot-point answer page with worked exam questions and cross-links. Browse them from this overview and the subject hub.

For the official specification

WJEC Eduqas publishes the full specification, past papers and mark schemes at eduqas.co.uk. Always revise from the current specification and the board's own past papers, because question style is board-specific.

Sources & how we know this

WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)