Audio Applications and Amplifiers

Summary

This chapter focuses on audio-specific circuit applications, bringing together concepts from throughout the course. Students will learn about audio amplifier types including preamplifiers for low-level signals and power amplifiers for driving speakers. The chapter addresses critical audio quality metrics including signal-to-noise ratio, noise sources, and various forms of distortion. Understanding these concepts enables students to design and evaluate audio circuits that meet professional quality standards.

Concepts Covered

This chapter covers the following 11 concepts from the learning graph:

Audio Amplifier
Preamplifier
Power Amplifier
Signal-to-Noise Ratio
Noise Floor
Thermal Noise
Audio Distortion
Harmonic Distortion
THD
Intermodulation Distortion
Clipping

Prerequisites

This chapter builds on concepts from:

title: Audio Applications and Amplifiers description: Design audio circuits including preamplifiers, power amplifiers, and learn to measure audio quality generated_by: chapter-content-generator v0.03 date: 2026-01-30

Introduction: Making Music with Electrons

Audio electronics bridges the gap between the abstract world of circuits and the visceral experience of sound. When you crank up your favorite song and feel the bass hit your chest, you're experiencing the final output of a carefully designed chain of amplifiers, each contributing to the signal's journey from a tiny electrical impulse to a room-shaking sound wave.

This chapter brings together everything you've learned—op-amps, filters, frequency response, Fourier analysis—and applies it to the specific challenges of audio. We'll explore how preamplifiers handle delicate signals from microphones and guitars, how power amplifiers drive speakers without distortion, and how to measure whether your audio circuit is any good.

The goal isn't just to amplify—it's to amplify faithfully. A perfect audio amplifier would be an invisible wire that happens to be louder. Real amplifiers fall short of this ideal in measurable ways, and understanding those shortcomings is the key to designing better ones.

Audio Amplifiers: The Signal Chain

An audio amplifier increases the power of an audio signal—making quiet signals loud enough to drive headphones, speakers, or recording equipment.

The typical audio signal chain:

Source (microphone, instrument, playback device)
Preamplifier (boosts low-level signals to line level)
Signal processing (EQ, effects, mixing)
Power amplifier (drives speakers)
Transducer (speaker converts electrical to acoustic)

Each stage must preserve signal quality while providing the necessary gain.

Preamplifier

A preamplifier (preamp) amplifies weak signals to a standard "line level" for further processing.

Input signal levels:

Source	Typical Level	Required Gain
Dynamic microphone	-60 dBV (1 mV)	40-60 dB
Condenser microphone	-40 dBV (10 mV)	20-40 dB
Electric guitar	-20 dBV (100 mV)	10-20 dB
CD player	0 dBV (1 V)	0 dB (line level)

Preamplifier requirements:

High input impedance (avoid loading the source)
Low noise (don't add hiss to quiet signals)
Adequate gain range
Low distortion
Appropriate frequency response (20 Hz - 20 kHz minimum)

Typical topology: Inverting or non-inverting op-amp circuit, sometimes with selectable gain.

Power Amplifier

A power amplifier drives loudspeakers, converting a line-level signal into substantial current and voltage swings.

Power amplifier characteristics:

Specification	Typical Values
Input sensitivity	1-2 V RMS for full output
Output power	10W - 1000W+
Speaker impedance	4Ω, 8Ω, 16Ω
Damping factor	50-500 (higher = better bass control)

Classes of power amplifiers:

Class A: Always conducting, lowest distortion, lowest efficiency (~25%)
Class B: Each transistor handles half the waveform, higher efficiency (~70%), crossover distortion
Class AB: Combination, reduced crossover distortion, ~60% efficiency
Class D: Switching amplifier, very high efficiency (>90%), requires output filtering

Class	Efficiency	Distortion	Heat	Typical Use
A	~25%	Lowest	High	High-end audio
AB	~60%	Low	Moderate	Most amplifiers
D	>90%	Varies	Low	Portable, subwoofers

Diagram: Audio Amplifier Chain

Audio Amplifier Chain

Type: microsim Bloom Level: Understand (L2) Bloom Verb: explain Learning Objective: Students will explain the function of each stage in an audio amplifier chain by observing signal levels through the system. Visual elements: - Block diagram: Mic → Preamp → Mixer → Power Amp → Speaker - Level meters at each stage (dBV or dBu) - Signal waveform display at selected stage - Gain annotations showing dB boost at each stage - Clipping indicators Interactive controls: - Slider: Input signal level (simulating mic sensitivity) - Slider: Preamp gain - Slider: Master volume - Dropdown: Select stage to view waveform - Toggle: Show/hide noise floor Annotations: - Mic level: "~-50 dBV" - After preamp: "Line level ~0 dBV" - Power amp output: "X watts into 8Ω" Clipping demonstration: - Overdrive preamp to show soft clipping - Overdrive power amp to show hard clipping Default parameters: - Typical vocal recording chain - Signal levels showing healthy headroom Canvas layout: - Signal chain diagram: 600 × 350 pixels - Control area: 100 pixels below Implementation: p5.js

Signal-to-Noise Ratio

Signal-to-noise ratio (SNR) measures how much louder the desired signal is compared to the unwanted noise:

\[SNR = 20\log_{10}\left(\frac{V_{signal}}{V_{noise}}\right) \text{ dB}\]

Higher SNR means cleaner sound. For quality audio, you need at least 60 dB; professional equipment aims for 100 dB or more.

Noise Floor

The noise floor is the level of background noise present when no signal is applied—the quietest sound the system can reproduce.

\[\text{Noise Floor (dBV)} = 20\log_{10}(V_{noise,rms})\]

Typical noise floors:

Equipment	Noise Floor
Quiet preamp	-90 to -100 dBV
Consumer gear	-80 to -90 dBV
Poor equipment	> -70 dBV

Thermal Noise

Thermal noise (Johnson-Nyquist noise) is fundamental noise caused by random thermal motion of electrons in resistors.

\[V_{noise,rms} = \sqrt{4k_B T R \Delta f}\]

Where: - \(k_B\) = Boltzmann's constant (1.38 × 10⁻²³ J/K) - T = Temperature in Kelvin - R = Resistance in ohms - Δf = Bandwidth in Hz

Example: A 10 kΩ resistor at room temperature (300K) over 20 kHz bandwidth: [V_{noise} = \sqrt{4 \times 1.38 \times 10^{-23} \times 300 \times 10000 \times 20000} = 1.8 \text{ μV}]

This is the absolute minimum noise you'll get from any 10 kΩ resistor—physics says so!

Reducing thermal noise:

Lower resistance values (where practical)
Narrower bandwidth (filter out frequencies you don't need)
Lower temperature (cryogenic cooling for extreme cases)

Noise Adds as RMS

Multiple noise sources add in quadrature (root of sum of squares), not linearly. If you have two equal noise sources, the total noise is only √2 times one source, not 2×.

Audio Distortion: When Amplifiers Lie

Distortion is any change to the signal shape that wasn't intentional. Perfect amplification would multiply the signal by a constant; distortion adds extra components.

Harmonic Distortion

Harmonic distortion adds new frequencies that are multiples of the input signal. If you input a pure 1 kHz tone, harmonic distortion adds 2 kHz, 3 kHz, 4 kHz...

Sources:

Non-linear transistor or op-amp characteristics
Saturation effects
Asymmetric clipping

Even vs. odd harmonics:

Even harmonics (2nd, 4th, 6th): Often described as "warm" or "musical"
Odd harmonics (3rd, 5th, 7th): Harsher, "edgy" sound

THD (Total Harmonic Distortion)

THD combines all harmonic distortion into one percentage:

\[THD = \frac{\sqrt{V_2^2 + V_3^2 + V_4^2 + ...}}{V_1} \times 100\%\]

Where \(V_1\) is the fundamental and \(V_n\) are harmonic amplitudes.

THD quality levels:

THD	Quality
< 0.01%	Inaudible, excellent
0.01% - 0.1%	High-quality audio
0.1% - 1%	Acceptable consumer
1% - 5%	Noticeable distortion
> 5%	Significant coloration

Intermodulation Distortion (IMD)

Intermodulation distortion creates new frequencies that are sums and differences of the input frequencies—not harmonically related.

If you input 1 kHz and 1.5 kHz, IMD creates:

Sum: 2.5 kHz
Difference: 500 Hz
And more complex combinations

IMD is particularly objectionable because the new frequencies are not harmonically related to the input—they sound dissonant and unnatural.

Clipping

Clipping occurs when the output signal tries to exceed the amplifier's maximum capability (typically the power supply rails).

Types of clipping:

Hard clipping: Abrupt flattening at the rail (transistor saturation)
Soft clipping: Gradual rounding as the limit approaches (tube-like)

Effects:

Adds strong harmonic content (odd harmonics for symmetric clipping)
Creates harsh, buzzing sound at high levels
Can damage tweeters with excessive high-frequency energy

Diagram: Distortion Types Visualizer

Distortion Types Visualizer

Type: microsim Bloom Level: Analyze (L4) Bloom Verb: compare Learning Objective: Students will compare different types of audio distortion by observing waveform changes and spectral effects. Visual elements: - Input waveform (pure sine) - Output waveform (with distortion) - Frequency spectrum showing added harmonics - THD percentage display - IMD product identification (for two-tone test) Interactive controls: - Dropdown: Distortion type (none, soft clip, hard clip, asymmetric, IMD) - Slider: Input level (drive into distortion) - Slider: Distortion amount (for adjustable types) - Toggle: Single tone vs two-tone (for IMD) - Audio playback option (if available) For each distortion type: - Waveform showing the specific effect - Spectrum showing added frequencies - THD calculation Annotations: - "Harmonic content added" for THD - "Non-harmonic products" for IMD - "Clipping at rails" for hard clipping Default parameters: - Soft clipping - Moderate drive level - 1 kHz test tone Canvas layout: - Waveforms: 300 × 200 pixels (stacked) - Spectrum: 300 × 300 pixels (side) - Control area: 100 pixels below Implementation: p5.js

Measuring Audio Quality

Key audio measurements:

Specification	What It Measures	Typical Target
THD	Harmonic distortion	< 0.1%
THD+N	Distortion plus noise	< 0.1%
SNR	Signal vs. noise	> 90 dB
Dynamic range	Loudest to quietest	> 90 dB
Frequency response	Gain flatness	±0.5 dB, 20 Hz-20 kHz
Crosstalk	Channel separation	> 60 dB

Test signals:

1 kHz sine: Standard for THD, SNR measurements
Two-tone (e.g., 60 Hz + 7 kHz): Reveals IMD
Swept sine: Maps frequency response
Pink noise: Tests full spectrum, sounds balanced to ear

Practical Design Considerations

Grounding and Shielding

Star grounding: All grounds connect at one point to avoid ground loops
Shielded cables: For low-level signals susceptible to interference
Balanced connections: Cancel induced noise (professional audio)

Power Supply Rejection

Op-amps and power supplies add noise/ripple. Choose components with good PSRR (Power Supply Rejection Ratio).

Component Selection

Low-noise op-amps: For preamp stages (e.g., NE5532, OPA2134)
Low-tolerance resistors: 1% metal film for consistent gain
Quality capacitors: Film or ceramic for signal path, not electrolytic

Self-Check Questions

1. A microphone outputs 2 mV RMS. What preamp gain (in dB) is needed to reach line level of 1V RMS?

Gain required: [A = \frac{V_{out}}{V_{in}} = \frac{1\text{V}}{2\text{mV}} = 500]

In decibels: [A_{dB} = 20\log_{10}(500) = 20 \times 2.7 = 54 \text{ dB}]

The preamp needs about 54 dB of gain.

2. An amplifier has THD of 0.5%. If the fundamental is 2V RMS, what is the total RMS voltage of all harmonics?

THD is defined as: [THD = \frac{V_{harmonics}}{V_{fundamental}}]

Therefore: [V_{harmonics} = THD \times V_{fundamental} = 0.005 \times 2\text{V} = 10 \text{ mV}]

The combined harmonic content is 10 mV RMS.

3. Why is intermodulation distortion often considered more objectionable than harmonic distortion?

Harmonic distortion produces frequencies that are integer multiples of the input—these are musically related and can sound like a richer version of the original.

Intermodulation distortion produces sum and difference frequencies that are NOT harmonically related to either input tone. For example, 1 kHz + 1.5 kHz creates 500 Hz and 2.5 kHz—frequencies that have no musical relationship to the originals.

These inharmonic products sound dissonant and "dirty," making IMD more subjectively unpleasant even at the same percentage level as THD.

4. What's the theoretical thermal noise from a 1 kΩ resistor at 25°C over a 20 kHz audio bandwidth?

Using the thermal noise formula: [V_{noise} = \sqrt{4k_B T R \Delta f}]

With T = 298K (25°C), R = 1000Ω, Δf = 20,000 Hz: [V_{noise} = \sqrt{4 \times 1.38 \times 10^{-23} \times 298 \times 1000 \times 20000}] [V_{noise} = \sqrt{3.3 \times 10^{-13}} = 0.57 \text{ μV}]

This is the minimum noise floor imposed by physics—no amplifier can be quieter than this with a 1 kΩ source impedance.

Summary

Audio circuit design combines all your circuit skills with the specific requirements of human hearing:

Audio amplifiers form a chain - From microphone level through preamp to power amp to speaker
Preamplifiers handle weak signals - High gain, low noise, high input impedance
Power amplifiers drive speakers - High current, various efficiency classes
SNR measures signal quality - Higher is better; 90+ dB is professional grade
Thermal noise sets the theoretical limit - It's fundamental physics, not poor design
Harmonic distortion adds multiples - Even harmonics sound warmer than odd
THD quantifies total harmonic distortion - Under 0.1% is high quality
IMD creates non-harmonic frequencies - Often more objectionable than THD
Clipping is the ultimate distortion - Occurs when the amplifier runs out of headroom
Good design minimizes noise and distortion - Component selection, grounding, and shielding all matter

Audio electronics is where engineering meets art. The specifications tell part of the story, but ultimately, audio circuits are judged by how they sound. Understanding the science behind sound quality gives you the tools to design circuits that not only measure well but sound great too.