Valve Amplifiers

Front Cover

Valve Amplifiers

Copyright Page

Contents

Preface

Dedication

Acknowledgements

1. Circuit Analysis

Mathematical Symbols

Electrons and Definitions

Batteries and Lamps

Ohm’s Law

Power

Kirchhoff’s Laws

Resistors in Series and Parallel

Potential Dividers

Equivalent Circuits

The Thévenin Equivalent Circuit

The Norton Equivalent Circuit

Units and Multipliers

The Decibel

Alternating Current (AC)

The Sine Wave

The Transformer

Capacitors, Inductors and Reactance

Filters

Time Constants

Resonance

RMS and Power

The Square Wave

Square Waves and Transients

Random Noise

Active Devices

Conventional Current Flow and Electron Flow

Silicon Diodes

Voltage References

Bipolar Junction Transistors (BJTs)

The Common Emitter Amplifier

Considering DC Conditions

Input and Output Resistances

The Emitter Follower

The Darlington Pair

General Observations on BJTs

Feedback

The Feedback Equation

Practical Limitations of the Feedback Equation

Feedback Terminology and Input and Output Impedances

The Operational Amplifier

The Inverter and Virtual Earth Adder

The Non-Inverting Amplifier and Voltage Follower

The Integrator

The Charge Amplifier

DC Offsets

References

Recommended Further Reading

2. Basic Building Blocks

The Common Cathode Triode Amplifier

Limitations on Choice of the Operating Point

Conditions at the Operating Point

Dynamic, or AC, Parameters

Cathode Bias

The Effect on AC Conditions of an Unbypassed Cathode Bias Resistor

The Cathode Decoupling Capacitor

Choice of Value of Grid-Leak Resistor

Choice of Value of Output Coupling Capacitor

Miller Capacitance

Reducing Output Resistance of the Previous Stage

Guided-Grid, or Beam, Triodes

The Tetrode

The Beam Tetrode and the Pentode

The Significance of the Pentode Curves

Using the EF86 Small-Signal Pentode

The Cascode

The Charge Amplifier

The Cathode Follower

Sources and Sinks: Definitions

The Common Cathode Amplifier as a Constant Current Sink (CCS)

Pentode Constant Current Sinks

The Cathode Follower with Active Load

The White Cathode Follower

Analysis of the Self-Contained White Cathode Follower

The White Cathode Follower as an Output Stage

The μ-Follower

The Importance of the AC Loadline

Upper Valve Choice in the μ-Follower

Limitations of the μ-Follower

The Shunt-Regulated Push–Pull Amplifier (SRPP)

The β-Follower

The Cathode-Coupled Amplifier

The Differential Pair

Gain of the Differential Pair

Output Resistance of the Differential Pair

AC Balance of the Differential Pair and Signal at the Cathode Junction

Common-Mode Rejection Ratio (CMRR)

Power Supply Rejection Ratio (PSRR)

Semiconductor Constant Current Sinks

Using Transistors as Active Loads for Valves

Optimising rout by Choice of Transistor Type

Field-Effect Transistors (FETs) as Constant Current Sinks

Designing Constant Current Sinks Using the DN2540N5

References

Recommended Further Reading

3. Dynamic Range: Distortion and Noise

Distortion

Defining Distortion

Measuring Non-Linear Distortion

Distortion Measurement and Interpretation

Choosing the Measurement

Refining Harmonic Distortion Measurement

Weighting of Harmonics

Summation and Rectifiers

Alternative Rectifiers

Noise and THD+N

Spectrum Analysers

Digital Concepts

Sampling

Scaling

Quantisation

Number Systems

Precision

The Fast Fourier Transform (FFT)

The Periodicity Assumption

Windowing

How the Author’s Distortion Measurements Were Made

Designing for Low Distortion

Signal Amplitude

Cascodes and Distortion

Grid Current

Distortion due to Grid Current at Contact Potential

Distortion due to Grid Current and Volume Controls

Operating with Grid Current (Class A2)

Distortion Reduction by Parameter Restriction

Distortion Reduction by Cancellation

Differential Pair Distortion Cancellation

Push–Pull Distortion Cancellation

The Western Electric Harmonic Equaliser

Side-Effects of the Harmonic Equaliser

DC Bias Problems

Cathode Resistor Bias

Grid Bias (Rk=0)

Rechargeable Battery Cathode Bias (rk=0)

Diode Cathode Bias (rk˜0)

Constant Current Sink Bias

Individual Valve Choice

Which Valves Were Explicitly Designed to be Low Distortion?

Carbonising of Envelopes

Deflecting Electrons

Testing to Find Low-Distortion Valves

The Test Circuit

Audio Test Level and Frequency

Test Results

Interpretation

A Convention

Alternative Medium-µ Valves

Weighted-Distortion Results

Overall Conclusions

Coupling from One Stage to the Next

Blocking

Transformer Coupling

Low Frequency Step Networks

Level Shifting and DC Coupling

A DC Coupled Class A Electromagnetic Headphone Amplifier

Using a Norton Level Shifter

Distortion and Negative Feedback

Carbon Resistors and Distortion

Noise

Noise from Resistances

Noise from Resistive Volume Controls

Noise from Amplifying Devices

Grid Current Noise and the Poisson Distribution

Electrometers and Grid Current

Noise in DC References

How the Author’s DC Reference Noise Measurements Were Made

Gas Reference Noise Measurements

Variation of Gas Reference Noise with Operating Current

Semiconductor Reference Noise Measurements and Statistical Summation

Variation of Zener Reference Noise with Operating Current

Noise of the Composite Zener Compared to a 317

Red LED Noise

References

Recommended Further Reading

4. Component Technology

Resistors

Preferred Values

Heat

Metal Film Resistors

Power (Wirewound) Resistors

Ageing Wirewound Resistors

Noise and Inductance of Wirewound Resistors

Non-Inductive Thick Film Power Resistors

General Considerations on Choosing Resistors

Tolerance

Heat

Voltage Rating

Power Rating

Capacitors

The Parallel Plate Capacitor

Reducing the Gap Between the Plates and Adding Plates

The Dielectric

Different Types of Capacitors

Air Dielectric, Metal Plate (εr˜1)

Plastic Film, Foil Plate Capacitors (2<εr<4)

Metallised Plastic Film Capacitors

Metallised Paper Capacitors (1.8<εr<6)

Silvered Mica Capacitors (Muscovite Mica, εr=7.0)

Ceramic Capacitors

Electrolytic Capacitors

Aluminium Electrolytic Capacitors (εr˜8.5)

Tantalum Electrolytic Capacitors (εr˜25)

Variation of Capacitance with Frequency

Imaginary Capacitance

General Considerations in Choosing Capacitors

Voltage Rating

Capacitance Value

Heat

ESR

Leakage and ‘d’

Microphony

Bypassing

Magnetic Components

Inductors

Air-Cored Inductors

Gapped Cores for AC Only

Gapped Cores for AC and DC (Power Supply Chokes)

Self-Capacitance

Transformers

Iron Losses

DC Magnetisation

Copper Losses

Electrostatic Screens

Magnetostriction

Output Transformers, Feedback and Loudspeakers

Transformer Models

Input Transformer Loading

Why Should I Use a Transformer?

General Considerations in Choosing Transformers

Uses and Abuses of Audio Transformers

Guitar Amplifiers and Arcs

Other Modes of Destruction

Magnetic Screening Cans

Magnetic Core Deterioration

Thermionic Valves

History

Emission

Electron Velocity

Transit Time

Individual Elements of the Valve Structure

The Cathode

Thoriated Tungsten Filament Fragility

Direct Versus Indirectly Heated Cathodes

The Thermal Problem

The Electrostatic Problem

The Electromagnetic Problem

The Indirectly Heated Cathode Solution

Heater/Cathode Insulation

Cathode Temperature Considerations

Heaters and their Supplies

Current Hogging and Heater Power

Heater Voltage and Current

The Control Grid

Grid Current

Thermal Runaway due to Grid Current

Grid Emission

Frame-Grid Valves

Variable-µ Grids and Distortion

Other Grids

The Anode

The Vacuum and Ionisation Noise

The Getter

The Mica Wafers and Envelope Temperature

Valve Sockets – Losses and Noise

Valve Bases and the Loktal™ Base

The Glass Envelope and the Pins

PCB Materials

References

Recommended Further Reading

5. Power Supplies

The Major Blocks

Rectification and Smoothing

Choice of Rectifiers/Diodes

Rectifiers To Be Avoided (Gas)

Rectifiers To Be Avoided (Selenium)

Rectifiers To Be Avoided (Copper Oxide)

RF Interference/Spikes

The Single Reservoir Capacitor Approach

Ripple Voltage

The Effect of Ripple Voltage on Output Voltage

Ripple Current and Conduction Angle

Transformer Core Saturation

Choosing the Reservoir Capacitor and Transformer

Back-to-Back Mains Transformers for HT Supplies

Voltage Multipliers

The Choke Input Power Supply

Minimum Load Current for a Choke Input Supply

Current Rating of the Choke

Mains Transformer Current Rating for a Choke Input Supply

Current Spikes and Snubbers

Intermediate Mode: The Region Between Choke Input and Capacitor Input

PSUD2

Broadband Response of Practical LC Filters

Region 1

Region 2

Region 3

Region 4

Estimation of Wide-Band LC Response

Sectioned RC Filters

Regulators

The Fundamental Series Regulator

The Two-Transistor Series Regulator

The Speed-Up Capacitor

Compensating for Regulator Output Inductance

A Variable Bias Voltage Regulator

The 317 IC Voltage Regulator

The 317 as an HT Regulator

Valve Voltage Regulators

Optimised Valve Voltage Regulators

Using a Pentode’s g2 as an Input for Hum Cancellation

Increasing Output Current Cheaply

Regulator Sound

Power Supply Output Resistance and Stereo Crosstalk

Power Supply Output Resistance and Amplifier Stability

The Statistical Regulator

Bypassing the Composite Zener

Optimising the Statistical Regulator

References for Elevated Heater Supplies – the THINGY

Common-Mode Interference

Heaters and History

How Common-Mode Heater Interference Enters the Audio Signal

Mains Transformers and Inter-Winding Capacitance

Reducing Transformer Inter-Winding Capacitance

Post-Transformer Filtering

Practical Issues

Transformer Regulation

HT Capacitors and Voltage Ratings

Can Potentials and Undischarged HT Capacitors

The Switch-On Surge

Mains Fusing

Mains Switching

A Practical Design

HT Regulation

HT Rectification and Smoothing (a PSUD2 Exercise)

Heater Rectification and Smoothing (a Manual Exercise)

Heater Regulation

Mains Filtering

Adapting the Power Supply to the EC8010 RIAA Stage

HT Regulation

Reference Voltages

HT Rectification and Smoothing (a PSUD2 Exercise)

Heater Regulation

Heater Rectification and Smoothing (a Manual Exercise)

References

Recommended Further Reading

6. The Power Amplifier

The Output Stage

The Single-Ended Class A Output Stage

The Significance of High Output Resistance

Transformer Imperfections

Classes of Amplifiers

Class A

Class B

Class C

Class *1

Class *2

The Push–Pull Output Stage and the Output Transformer

Modifying the Connection of the Output Transformer

Output Transformer-Less (OTL) Amplifiers

The Entire Amplifier

The Driver Stage

The Phase Splitter

The Differential Pair and Its Derivatives

The Input Stage

Stability

Slugging the Dominant Pole

Low Frequency Instability, or Motorboating

Parasitic Oscillation and Control Grid-Stoppers

Parasitic Oscillation of Ultra-Linear Output Stages, and g2 Stoppers

Parasitic Oscillation and Anode Stoppers

High Frequency Stability and the 0V Chassis Bond

Stability Margin

Classic Power Amplifiers

The Williamson

The Mullard 5-20

The Quad II

New Designs

Single-Ended Madness

The Scrapbox Challenge Single-Ended Amplifier

Choice of Output Valve

Choice of Output Class

Choosing the DC Operating Point by Considering Output Power and Distortion

Specifying the Output Transformer

Biassing the Valve

The Cathode Bypass Capacitor

Finding the Required HT Voltage

HT Smoothing

HT Rectification

The HT Transformer

HT Choke Suitability

The HT Regulator Option

Estimating Amplifier Output Resistance

What are the Driver Stage Requirements?

Driver Stage Topology

Choice of Valve for the Driver Stage

Determining the Driver Stage Operating Point

Setting Driver Stage Bias

Is the Output Resistance and Gain of the Proposed Driver Stage Adequate?

But What About Global Feedback?

Summing Up

Teething Problems

Listening Tests

Designer’s Observations

Conclusions

Obtaining more than Single Digit Output Power

Sex, Lies and Output Power

Loudspeaker Efficiency and Power Compression

Active Crossovers and Zobel Networks

Parallel Output Valves and Transformer Design

Driving Higher Power Output Stages

The Crystal Palace Amplifier

13E1 Conditions

Driver Requirements

Finding a Topology that Satisfies the Driver Requirements

(1) Minimal Measured Distortion

(2) Distortion to be Composed of Low Order Harmonics

(3) Push–pull Output with Good Balance

(4) Large Undistorted Voltage Swing

(5) Sufficient Gain to Enable Global Negative Feedback if Required

(6) Low DC Output Resistance to Avoid Problems with DC Grid Current

(7) Low AC Output Resistance to Drive Load Capacitance

(8) Tolerance of Output Stage Conduction Angle Changes from 360° to 0°

(9) Instantaneous Recovery Even After Gross Overload

Circuit Topology: Power Supplies and Their Effect on Constant Current Sinks

Va(max) and the Positive HT Supply

Symmetry and the Negative HT Supply

The Second Differential Pair and Output Stage Current

Why Not Have Tighter Stabilisation?

The First Differential Pair, Its HT Supply, and Linearity

Valve Matching

The Essential Twiddly Bits

The Cascode Constant Current Sink and Stabilisation Against Mains Variation

The 334Z Constant Current Sink and Thermal Stability

High Frequency Stability

HT Regulators

Stereo versus Mass

Power Supply Design

Designer’s Observations

Exceeding Vg2

GM70

Measuring Ik

Global Negative Feedback

Conclusions

The Bulwer-Lytton Scalable Parallel Push–Pull Amplifier

Background

Designing the Followers to Drive the Output Valves

Comparing Cathode and FET Source Followers

Output Stage Bias, Balance and Coupling

Providing Gain

Gain Stage CCS and Gain Balance

Balanced Inputs on Power Amplifiers

The Volume Control and Baffle Step Compensation

Audio Circuit Comments

Power Supplies

Global Negative Feedback

References

Further reading

7. The Pre-Amplifier

Input Selection

Disparate Levels between Sources

Adjacent Contact Capacitance (Crosstalk Between Sources)

Contact and Leakage Resistance (Noise)

Solutions and Problems Peculiar to Electromechanical Switches (Relays)

Volume Control

Limitations on the Control’s Value (Disturbing Frequency Response)

Logarithmic Law (Perceived Volume Not Changing Smoothly with Rotation)

Switched Attenuators (Disturbing Channel Matching)

Switched Attenuator Design

Spreadsheets and Volume Controls

Volume Controls for Digital Active Crossovers

Volume Control Values and Their Effect on Noise

Grid-Leak Resistors and Volume Controls

Balanced Volume Controls

Light-Sensitive Resistors as Volume Controls

Transformer Volume Controls

Balance Control

Law Faking

Cable Driver

Determination of Required Quiescent Current

Choice of Follower Valve

Practical Considerations

Adding Gain

Polarity Inversion

Tone Control

Obtaining a Clean Signal from Analogue Disc

Comparison of Analogue Levels between Vinyl and Digital Sources

RIAA and Replay Rumble

The Mechanical Problem

Arm Wiring and Moving Coil Cartridge DC Resistance

Hum Loops and Unbalanced Interfaces

Balanced Working and Pick-Up Arm Wiring

RIAA Stage Design

Determination of Requirements

Implementing RIAA Equalisation

‘All in One Go’ Equalisation

Split RIAA Equalisation

The Final Choice

A Simplified Example RIAA Stage

Noise and Input Capacitance of the Input Stage

Valve Noise

1/f Noise

Connecting Devices in Parallel to Reduce noise

Valve Noise Summary

Noise Advantage due to RIAA Equalisation

Stray Capacitances

Calculation of Component Values for 75μs

180μs, 318μs Equalisation and the Problem of Interaction

3180μs and 318μs Equalisation

Awkward Values and Tolerances

The EC8010 RIAA Stage

The Input Stage

Optimising the Input Transformer

The Second Stage

The Output Stage

Refining Valve Choice by Heaters

Choosing the Implementation of RIAA Equalisation

Grid Current Distortion and RIAA Equaliser Series Resistances

3180μs, 318μs Pairing Errors due to Miller Capacitance

The 75μs Problem

The Computer Aided Design (CAD) Solution

3180μs, 318μs Pairing Manipulation

75μs/3.18μs Manipulation

Practical RIAA Considerations

RIAA Direct Measurement Problems

Production Tolerances and Component Selection

RIAA Equalisation Errors due to Valve Tolerances

The Balanced Hybrid RIAA Stage

No Step-Up Transformers

Semiconductors to the Rescue

Miller Capacitance

DC Stabilisation and Consequent Gain Reduction

JFET Noise

BJT Noise

Choosing between the BJT and JFET: Equalisation, Distortion and HT Power

Reconciling the Balanced Decision with Practicalities

Implications of the Block Diagram

The Unity-Gain Cable Drivers

Deciding the HT Voltage

Input Stage BJT Miller Capacitance

VCE and BJT Linearity

Input Resistance and Bias Current

Input Stage Noise

RIAA Calculations

The Source Followers

The Constant Current Sinks

The HT Supply

Total Gain and Channel Balance

Summary

References

Recommended Further Reading

Appendix

Valve Data

Standard Component Values

Resistor Colour Code

Plastic Capacitor Coding

Cable

Square Wave Sag and Low Frequency f–3 dB

Playing 78s

Equalisation

CD

Sourcing Components: Bargains and Dealing Directly

References

Index