Chapter 1: This chapter explains passive components such as resistors and capacitors, going far beyond the basics. Different types of resistors and potentiometers are covered, including how the materials they're made from affects their performance. A brief tutorial explains how various types of capacitors are constructed, which affects their suitability for different purposes. This chapter also describes parasitic elements of capacitors, such as small amounts of inductance that are effectively in series, and how the applied voltage severely reduces the capacitance of some ceramic types. It also shows how resistors and capacitors can create simple passive filters, including the basic formulas. Further, it explains that inductors are best avoided because they're large, expensive, prone to distortion, and can pick up hum in the air from nearby power transformers.

Chapter 2: This chapter presents detailed information about diode and transistor semiconductors, including several lesser known facts. It explains the various diode types: silicon, germanium, Schottky, and Zener, and shows the logarithmic relation between applied current and forward voltage drop in a diode junction. Then it shows how a single diode can be used to create a fairly competent voltage-controlled attenuator, including a table that relates the current through a diode to its resistance. This chapter also explains gain stages that amplify audio by a modest amount, and transistor followers that can buffer a passive filter. Further, it shows how both junction and FET transistors are used as switches, with all of the relevant circuits provided as files in the popular LTspice format.

Chapter 3: This chapter explains op-amps in depth, including a brief overview of their history. It shows that while op-amps have a very large amount of open-loop gain, most circuits use negative feedback to reduce that gain to a more useable amount, which flattens their frequency response and reduces distortion. It also shows how op-amps are used open-loop as comparators. Both inverting and non-inverting circuits are presented along with their gain formulas, as well as a differential circuit that amplifies the difference between two voltages regardless of their relation to ground. Active filter basics are also shown using a capacitor to change the gain of an op-amp at select frequencies. Finally, the op-amp specs most important for audio are explained in detail, including input noise and slew rate.

Chapter 4: This chapter presents more than 40 circuit elements ranging from simple passive attenuators to a complete guitar pick detector. Formulas are included to calculate the attenuators, though the companion spreadsheet available online can do those calculations for you. It also includes schematics with detailed explanations for peak followers, constant current sources, sample and hold circuits, active half- and full-wave rectifiers. as well as simulated inductors and capacitance multipliers. The section about comparators explains hysteresis in detail, and also explains why dedicated comparator ICs are often a better choice than op-amps. Three different VCA circuits are also shown, including the famous dbx VCA. Further, log converters used in many analog synthesizers are presented, along with timers and logic components. Seven different types of oscillators are shown and explained, along with simple but effective ways to generate musical octaves. Finally, a clever method is presented to incorporate an audio transformer in the feedback path of an op-amp to reduce its distortion and flatten its frequency response.

Chapter 5: This chapter presents every active filter type commonly used for audio, including crossovers and even a formant filter to emulate the sound of a human voice electronically. Filter formulas are also shown, though the online Active Filters spreadsheet will save the time and effort of manual calculations. This chapter also shows that active filters based on a single op-amp section can have one, two, or three poles, then it describes modern active 2-way and 3-way crossovers based on 4-pole Linkwitz-Riley low-pass and high-pass filters. Several filter types are shown in each category, including the Twin-T band-pass and band-stop designs that can achieve incredibly narrow bandwidths. Also shown are all-pass filters that provide phase shift needed for various audio tasks, such as creating a comb filter for a phaser effect or stereo synthesizer. It also explains setting the relevant parameters in LTspice for viewing a frequency response Bode plot, and using a "PWL" list of discrete level and time steps to output nearly any series of voltages. Finally, methods are shown to vary filter parameters via voltage control using a diode as a variable resistor.

Chapter 6: This chapter presents more than two dozen complete audio devices that can be customized in various ways, based on the building blocks and filters in Chapters 4 and 5. It begins by discussing power supplies, then shows a typical mixer section that can combine both mono and stereo sources without interaction between their volume and pan controls. Several preamplifier circuits are shown with both high and low input impedance, as well as a clever way to generate high quality pink noise. Six musical instrument "pedal" effect circuits are also presented, including a fuzz-tone, tremolo, and auto-wah. Three different compressor circuits are shown, along with five equalizer types, plus a unique filter that removes popping Ps from a live audio stream. Beyond showing useful circuits, many important educational topics are covered including switching power supplies and phantom power for professional microphones. The preamp section explains equivalent input noise (EIN) and describes how it's measured. Finally, simple formulas show how to calculate intermediate frequencies that are logarithmically related.

Chapter 7: This chapter presents a collection of audio circuits that can be combined to create a complete analog synthesizer. Two Voltage Controlled Amplifier (VCA) circuits are shown, including the original VCA Robert Moog designed in the 1960s for his MiniMoog. Also included is the classic MiniMoog Voltage Controlled Filter (VCF). The Voltage Controlled Oscillator (VCO) circuit is the author's own design - it's simpler than the MiniMoog VCO because it uses op-amps instead of discrete transistors, but it works well and can output all the usual wave types. The LTspice software used for all of the circuits in this book doesn't filter frequencies higher than half the sample rate from its Wave file output, so an 8-pole anti-aliasing filter is also presented. Finally, every synthesizer needs an LFO for modulation effects, as well as an ADSR, and the circuits shown in this chapter are fully capable and highly flexible.

Chapter 8: LTspice is an amazing program that simulates electronic circuits and examines their performance, but there's a bit of a learning curve. Therefore, this chapter includes a starter file with all of the important setup parameters defined, so readers new to this program can get started right away. Basics are shown for entering circuits, defining voltage sources including Wave files, along with numerous tips and tricks for monitoring points in a circuit for voltage, current, and power. LTspice offers a number of Dot commands, so the most useful ones are listed and explained, including the incredibly powerful .meas command. Digital logic in LTspice is handled in a unique and clever way, so this chapter also explains the various logic component types, and shows how to override their default parameters. Finally, some common errors and their solutions are described, along with steps to import components not included in the standard library.

Chapter 9: This chapter contains an in-depth tutorial about PC boards, beginning with their history. It then explains the construction and design of modern multi-layer boards, concluding that 4-layer boards are often the best choice for audio circuits. Two free programs that handle both schematic entry and PC board layout are described and compared, and both can verify that the board layout matches the circuit's schematic. This chapter also explains that many modern audio devices are built using surface mounted devices (SMDs) that are soldered to pads on top of the board rather than to traces on the bottom. Numerous tips are presented showing how best to make use of this technology, including a method for routing bi-polar power around a board on a single layer.

Chapter 10: This chapter presents a number of tables showing electronic units and symbols, resistor codes and values, musical note frequencies, and the standard Octave and Third-Octave Frequency Bands.

Chapter 11: This chapter describes the four spreadsheets included with this book on its companion web site. The Active Filters spreadsheet calculates resistor and capacitor component values for every filter type included in the book. The Decibels spreadsheet converts between every combination of voltage and decibels, including relating distortion amounts to their decibel equivalents. It also calculates resistor values for both "L" and "H-pad" attenuators, as well as converting between dBu, dBV, and RMS volts. The Ohm's Law spreadsheet calculates every combination of volts, amps, ohms, and watts, and also converts between RMS, peak-to-peak, and average voltage amounts. Finally, the Graphic EQ spreadsheet solves all of the calculations needed to determine component values for the simulated inductors used in that type of equalizer.

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