Small Signal Audio Design

Douglas Self

• Publisher: Routledge
• Year: 2014
• Print ISBN: 9780415709743
• eBook ISBN: 9781134635207
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Acoustical Engineering
• Index: Technology & Engineering

Learn to use inexpensive and readily available parts to obtain state-of-the-art performance in all the vital parameters of noise, distortion, crosstalk and so on. With ample coverage of preamplifiers and mixers and a new chapter on headphone amplifiers, this practical handbook provides an extensive repertoire of circuits that can be put together to make almost any type of audio system.

A resource packed full of valuable information, with virtually every page revealing nuggets of specialized knowledge not found elsewhere. Essential points of theory that bear on practical performance are lucidly and thoroughly explained, with the mathematics kept to a relative minimum. Douglas' background in design for manufacture ensures he keeps a wary eye on the cost of things. Includes a chapter on power-supplies, full of practical ways to keep both the ripple and the cost down, showing how to power everything.

Douglas wears his learning lightly, and this book features the engaging prose style familiar to readers of his other books. You will learn why mercury cables are not a good idea, the pitfalls of plating gold on copper, and what quotes from Star Trek have to do with PCB design.

Learn how to:

• make amplifiers with apparently impossibly low noise
• design discrete circuitry that can handle enormous signals with vanishingly low distortion
• use humble low-gain transistors to make an amplifier with an input impedance of more than 50 Megohms
• transform the performance of low-cost-opamps, how to make filters with very low noise and distortion
• make incredibly accurate volume controls
• make a huge variety of audio equalisers
• make magnetic cartridge preamplifiers that have noise so low it is limited by basic physics
• sum, switch, clip, compress, and route audio signals

The second edition is expanded throughout (with added information on new ADCs and DACs, microcontrollers, more coverage of discrete op amp design, and many other topics), and includes a completely new chapter on headphone amplifiers.

CHAPTER 1

Basics

Signals

An audio signal can be transmitted either as a voltage or a current. The construction of the universe is such that almost always the voltage mode is more convenient; consider for a moment an output driving more than one input. Connecting a series of high-impedance inputs to a low-impedance output is simply a matter of connecting them in parallel, and if the ratio of the output and input impedances is high there will be negligible variations in level. To drive multiple inputs with a current output it is necessary to have a series of floating current-sensor circuits that can be connected in series. This can be done [1], as pretty much anything in electronics can be done, but it requires a lot of hardware, and probably introduces performance compromises. The voltage-mode connection is just a matter of wiring.

Obviously, if there’s a current, there’s a voltage, and vice versa. You can’t have one without the other. The distinction is in the output impedance of the transmitting end (low for voltage-mode, high for current-mode) and in what the receiving end responds to. Typically, but not necessarily, a voltage input has a high impedance; if its input impedance was only 600 Ω, as used to be the case in very old audio distribution systems, it is still responding to voltage, with the current it draws doing so a side issue, so it is still a voltage amplifier. In the same way, a current input typically, but not necessarily, has a very low input impedance. Current outputs can also present problems when they are not connected to anything. With no terminating impedance, the voltage at the output will be very high, and probably clipping heavily; the distortion is likely to crosstalk into adjacent circuitry. An open-circuit voltage output has no analogous problem.

Current-mode connections are not common. One example is the Krell Current Audio Signal Transmission (CAST) technology which uses current-mode to interconnect units in the Krell product range. While it is not exactly audio, the 4–20 mA current loop format is widely used in instrumentation. The current-mode operation means that voltage drops over long cable runs are ignored, and the zero offset of the current (i.e. 4 mA zero) makes cable failure easy to detect – if the current is zero, you have a broken cable.

The old DIN interconnection standard was a form of current-mode connection in that it had voltage output via a high output impedance, of 100 kΩ or more. The idea was presumably that you could scale the output to a convenient voltage by selecting a suitable input impedance.

The drawback was that the high output impedance made the amount of power transferred very small, leading to a poor signal-to-noise ratio. The concept is now wholly obsolete.

Amplifiers

At the most basic level, there are four kinds of amplifier, because there are two kinds of signal (voltage and current) and two types of port (input and output). The handy word ‘port’ glosses over whether the input or output is differential or single ended. Amplifiers with differential input are very common – such as all opamps and most power amps – but differential outputs are rare and normally confined to specialised telecoms chips.

To summarise the four kinds of amplifier:

TABLE 1.1 The four types of amplifier

Amplifier type	Input	Output	Application
Voltage amplifier	Voltage	Voltage	General amplification
Transconductance amplifier	Voltage	Current	Voltage control of gain
Current amplifier	Current	Current	?
Transimpedance amplifier	Current	Voltage	Summing amplifiers, DAC interfacing

Voltage amplifiers

These are the vast majority of amplifiers. They take a voltage input at a high impedance and yield a voltage output at a low impedance. All conventional opamps are voltage amplifiers in themselves, but they can be made to perform as any of four kinds of amplifier by suitable feedback connections. Figure 1.1 a shows a high-gain voltage amplifier with series voltage feedback. The closed-loop gain is (R1 R2)/R2.

Transconductance amplifiers

The name simply means that a voltage input (usually differential) is converted to a current output. It has a transfer ratio A = I_out/V_in, which has dimensions of I/V or conductance, so it is referred to as a transconductance amplifier. It is possible to make a very simple, though not very linear, voltage-controlled amplifier with transconductance technology – differential-input operational transconductance amplifier (OTA) ICs have an extra pin that give...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Dedication
• Preface
• Acknowledgments
• Acronyms
• Chapter 1 Basics
• Chapter 2 Components
• Chapter 3 Discrete transistor circuitry
• Chapter 4 Opamps and their properties
• Chapter 5 Opamps for low voltages
• Chapter 6 Filters
• Chapter 7 Preamplifier architectures
• Chapter 8 Moving-magnet inputs: levels and RIAA equalisation
• Chapter 9 Moving-magnet inputs: archival and non-standard equalisation
• Chapter 10 Moving-magnet inputs: discrete circuitry
• Chapter 11 Moving-magnet inputs: noise and distortion
• Chapter 12 Moving-coil head amplifiers
• Chapter 13 Volume controls
• Chapter 14 Balance controls
• Chapter 15 Tone controls and equalisers
• Chapter 16 Mixer architectures
• Chapter 17 Microphone preamplifiers
• Chapter 18 Line inputs
• Chapter 19 Line outputs
• Chapter 20 Headphone amplifiers
• Chapter 21 Signal switching
• Chapter 22 Mixer sub-systems
• Chapter 23 Level indication and metering
• Chapter 24 Level control and special circuits
• Chapter 25 Power supplies
• Chapter 26 Interfacing with the digital domain
• Index