Solid-state Guitar Amplifiers

419 Pages·1696·31.03 MB·English

by  Teemu Kyttälä

Teemu Kyttälä

Solid-State Guitar
Amplifiers
Copyright © 2008 Teemu Kyttälä. All rights reserved. No part of this publication may
be reproduced in any form or by any means without the prior written permission of the
author.

All trademarks are trademarks of their respective owners. The names are used in
editorial fashion only, and to the benefit of the trademark owner, with no intention of
infringement of the trademark.

Solid-State Guitar Amplifiers

1st Edition
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Acknowledgements

A heartfelt thanks goes out to Nigel Goodwin, Don Booth and Joseph Raymond who
assisted in proofreading this book. They should not however be held responsible for
any errors or oversights. Another heartfelt thanks goes out to Hugh Dean, whose
contributions to diyAudio forum have more than once proven priceless. I also like to
thank everybody who showed interest in this project.

I also like to express my gratitude to enthusiastic individuals such as Rod Elliott, R.G.
Keen, Paul Kemble, Randall Aiken, John Broskie, Steve Bench, Duncan Munro, M.J.
Renardson and Malcolm Hawksford. Your dedication and effort to present such vast
databases of audio electronics-related information in the Internet has not been left
unnoticed. My heartfelt thanks also goes out to the legion of people hosting audio- and
electronics -related forums or schematic sharing sites.

…

I apologize that in the current state the grammatical accuracy of this book’s content is
still somewhat “lacking”. English is not my native language and all proofreading has
been done on voluntarily basis, which, unfortunately means that parts of the book
have not been under the scope of the people who master the language better than I do.
So, please, try to bear my “Ingrish”. If you have comments, suggestions and
corrections feel free to sent them to teemu[spamfilter][email protected]. They will
be appreciated.
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1 INTRODUCTION......................................................................................................................9
1.1 FOREWORD...........................................................................................................................9
1.2 ABOUT THE SCHEMATIC NOTATION.....................................................................................10
1.3 COMMON TERMINOLOGY....................................................................................................11
1.4 ABOUT INTEGRATED CIRCUIT AMPLIFIERS...........................................................................18
1.5 TUBES VERSUS TRANSISTORS..............................................................................................18
1.6 AMPLIFIER’S TONE – OR IS IT?.............................................................................................26
1.7 OPERATIONAL AMPLIFIERS VERSUS DISCRETE CIRCUITS......................................................27
1.8 USE OF DSP IN AMPLIFIERS................................................................................................28
1.9 DECIBEL REFERENCES.........................................................................................................30
1.10 ABOUT VARIOUS TYPES OF DISTORTION..............................................................................30
1.11 SOME HISTORY OF (TRANSISTORIZED) GUITAR AMPLIFIERS.................................................31
1.12 THE DILEMMA OF “CLASSIC” AMPLIFIERS...........................................................................40
1.13 THE IDEAL OF SIMPLICITY...................................................................................................41
2 A BASIC INTRODUCTORY TO POWER AMPLIFIERS................................................46
2.1 A QUICK OVERVIEW............................................................................................................46
2.2 THE EFFECT OF OUTPUT IMPEDANCE AND DAMPING FACTOR...............................................48
2.3 THE BASIC BUILDING BLOCKS OF A POWER AMPLIFIER........................................................49
2.4 BI-AMPING.........................................................................................................................51
2.5 REVEALING PERFORMANCE.................................................................................................52
3 A DETAILED EXAMINATION OF POWER AMPLIFIERS...........................................55
3.1 VOLTAGE AMPLIFIER STAGES..............................................................................................55
3.2 OUTPUT STAGES..................................................................................................................57
3.2.1 Class-A.......................................................................................................................57
3.2.2 Complementary pairs and push-pull operation.........................................................58
3.2.3 Quasi complementary................................................................................................61
3.2.4 Compound follower....................................................................................................62
3.2.5 Totem pole and topologies using output transformers...............................................64
3.3 INPUT STAGES.....................................................................................................................66
3.3.1 Single transistor.........................................................................................................66
3.3.2 Combined VAS and input...........................................................................................68
3.3.3 Current feedback or “singleton”...............................................................................68
3.3.4 Long-tailed pair.........................................................................................................70
3.4 TECHNIQUES TO IMPROVE LINEARITY..................................................................................73
3.4.1 Balancing the long-tailed pair...................................................................................73
3.4.2 Symmetric designs: Cascoded complementary LTP..................................................76
3.4.3 Symmetric designs: Cascoded complementary CFB and “Semi-symmetry”.............79
3.4.4 Balanced voltage amplifier stages.............................................................................80
3.5 OPERATIONAL AMPLIFIERS AS DRIVERS..............................................................................81
3.6 NEGATIVE FEEDBACK LOOPS...............................................................................................85
3.6.1 Basics.........................................................................................................................85
3.6.2 Current sensing and mixed-mode feedback...............................................................87
3.6.3 Resonance, Presence and damping controls..............................................................90
3.6.4 Feed-forward.............................................................................................................92
3.7 CROSSOVER DISTORTION COMPENSATION...........................................................................94
3.8 CONSTANT CURRENT SOURCE LOADS................................................................................100
3.9 BOOTSTRAP CONSTANT CURRENT SOURCES......................................................................102
3.10 CASCODING AND RAIL SWITCHING....................................................................................106
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3.11 BRIDGED, PARALLEL AND GROUNDED OUTPUT CIRCUITS..................................................109
3.12 CLASS-A+ AMPLIFIERS.....................................................................................................112
3.13 AMPLIFIER STABILITY – PREVENTING OSCILLATION.........................................................113
3.14 PROTECTIVE CIRCUITS.......................................................................................................117
3.14.1 Protecting input stage from static voltage spikes....................................................117
3.14.2 Turn-on delay...........................................................................................................118
3.14.3 Current limiting and flyback protection...................................................................119
3.14.4 More about short circuit protection.........................................................................124
3.14.5 Baker Clamps...........................................................................................................128
3.14.6 Thermal switches......................................................................................................130
3.14.7 Thyristor latches and crowbars...............................................................................130
3.14.8 Protecting speakers from DC...................................................................................133
3.14.9 About fusing.............................................................................................................135
3.15 MOSFETS AND IGBTS.....................................................................................................136
3.16 SIGNAL LIMITING WITHIN POWER AMPLIFIER CIRCUIT.......................................................139
3.17 DC SERVOS......................................................................................................................142
3.18 IMPEDANCE MATCHING TRANSFORMERS AND 70-VOLT SYSTEMS......................................144
3.19 EXAMPLES OF HYBRID POWER AMPLIFIERS .......................................................................146
3.19.1 MosValve and its heirs.............................................................................................146
3.19.2 Peavey......................................................................................................................149
3.19.3 Music Man................................................................................................................150
3.19.4 ADA..........................................................................................................................151
3.19.5 Ampeg SVT...............................................................................................................154
4 HEAT SINKING....................................................................................................................156
4.1 BASICS OF HEAT GENERATION IN A SEMICONDUCTOR.......................................................156
4.2 HEAT SINK MATH..............................................................................................................157
4.3 INTERFACE PROPERTIES ....................................................................................................160
4.4 HEAT SINKS AND COOLING SOLUTIONS..............................................................................161
5 POWER SUPPLIES..............................................................................................................167
5.1 TRANSFORMERS................................................................................................................167
5.2 RECTIFIERS AND SUPPLY POLARITY...................................................................................171
5.3 SUPPLY FILTERING............................................................................................................177
5.4 REGULATED SUPPLIES.......................................................................................................181
5.5 INRUSH CURRENT AND HOW TO LIMIT IT............................................................................184
5.6 SURGE LIMITING WITH MOVS...........................................................................................185
5.7 POWER SUPPLY OVERVIEW................................................................................................185
5.8 QUICK INTRODUCTORY TO SWITCHED MODE POWER SUPPLIES..........................................187
5.9 SAFETY PRECAUTIONS.......................................................................................................189
6 PREAMPLIFIER CIRCUITRY...........................................................................................194
6.1 WHAT IS A PREAMPLIFIER?................................................................................................194
6.2 ABOUT TRANSFER CURVES AND CLIPPING CHARACTERISTICS............................................194
6.3 BASIC GAIN STAGES..........................................................................................................196
6.3.1 Common emitter.......................................................................................................196
6.3.2 Stages using multiple transistors.............................................................................199
6.3.3 Common source........................................................................................................201
6.3.4 SRPP........................................................................................................................206
6.3.5 Discrete buffers........................................................................................................207
6.3.6 Operational amplifiers.............................................................................................209
6.4 MAGNETIC PICKUPS AND INPUT STAGE..............................................................................216
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6.5 PIEZOELECTRIC PICKUPS...................................................................................................220
6.6 THE EFFECTS OF FREQUENCY RESPONSE............................................................................221
6.7 TONE CONTROLS...............................................................................................................223
6.7.1 Quick introduction to tone control terminology......................................................223
6.7.2 Active tone controls..................................................................................................224
6.7.3 Scaling impedance and frequency............................................................................224
6.7.4 “Pass”-filters and other crude tone controls..........................................................225
6.7.5 Bingham...................................................................................................................227
6.7.6 Baxandall circuit and its variations.........................................................................228
6.7.7 “FMV” – tonestack: Fender, Marshall and Vox.....................................................230
6.7.8 “Big Muff” tone control and other ideas.................................................................232
6.7.9 Utilizing gyrators.....................................................................................................235
6.7.10 T-Filters...................................................................................................................238
6.7.11 About placement of the tone controls.......................................................................238
6.7.12 Automatic frequency-compensated volume control.................................................239
6.8 VOLUME- AND GAIN CONTROLS........................................................................................240
6.9 SPRING REVERBS...............................................................................................................242
6.10 EFFECT LOOPS AND LINEOUT CIRCUITS .............................................................................246
6.11 CLIPPING CIRCUITS............................................................................................................250
6.12 SOFT LIMITERS..................................................................................................................257
6.13 TREMOLO CIRCUITS...........................................................................................................264
6.14 CABINET SIMULATORS......................................................................................................267
6.15 PHASE SPLITTERS..............................................................................................................268
6.16 EXAMPLES OF TUBE EMULATION CIRCUITS........................................................................268
6.16.1 TransTube................................................................................................................268
6.16.2 Carvin SX series’ Classic Tube Emulation..............................................................274
6.16.3 FlexWave..................................................................................................................275
6.16.4 Valve Reactor...........................................................................................................276
6.16.5 Valvestate.................................................................................................................277
6.16.6 SansAmp...................................................................................................................279
6.17 WHAT IS AN “ACOUSTIC AMPLIFIER”?..............................................................................283
6.18 PRACTICAL DESIGN EXAMPLE...........................................................................................286
7 MISCELLANEOUS CIRCUITRY......................................................................................293
7.1 LIGHT BULB CURRENT LIMITER.........................................................................................293
7.2 FANS.................................................................................................................................294
7.3 CLIPPING INDICATORS.......................................................................................................296
7.4 SWITCHING CIRCUITS........................................................................................................298
7.4.1 Jack switches............................................................................................................298
7.4.2 Switches in remote boxes.........................................................................................299
7.4.3 LDR switches............................................................................................................299
7.4.4 Bipolar transistor and FET-switches.......................................................................301
7.4.5 Switching OpAmps...................................................................................................305
7.4.6 CMOS-switches, logic circuits and microcontrollers..............................................306
7.4.7 Remote control circuits............................................................................................309
7.5 A LITTLE BIT ABOUT MIDI................................................................................................311
7.6 SOME USEFUL INTERFACES FOR MICROPROCESSORS..........................................................312
7.7 RELAYS.............................................................................................................................315
8 EARTHING AND COMMON CIRCUITS.........................................................................317
8.1 INTRODUCTORY TO GROUNDING AND ITS TERMINOLOGY..................................................317
8.2 COMMON RETURN CURRENT ROUTING AND GROUNDING TOPOLOGIES ..............................318
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8.3 SAFETY GROUND CONSIDERATIONS...................................................................................322
8.4 EARTHING AND GROUND LOOPS........................................................................................324
8.5 “LOOP BREAKER” CIRCUITS..............................................................................................324
9 PRACTICAL COMPONENT CHOICES AND DESIGN ASPECTS..............................326
9.1 RESISTORS........................................................................................................................326
9.2 CAPACITORS.....................................................................................................................327
9.3 POTENTIOMETERS AND TRIMMERS....................................................................................330
9.4 SWITCHES AND JACKS.......................................................................................................332
9.5 SEMICONDUCTORS............................................................................................................333
9.6 POWER SUPPLY PARTS.......................................................................................................335
9.7 FUSING..............................................................................................................................336
9.8 PRINTED CIRCUIT BOARD DESIGN......................................................................................337
9.9 COMPONENT LAYOUT AND HEAT DISSIPATION..................................................................344
9.10 ABOUT EPOXY...................................................................................................................345
9.11 SPICE SIMULATIONS........................................................................................................346
9.12 HOW TO LEARN MORE?.....................................................................................................348
10 SPEAKERS.........................................................................................................................350
10.1 POWER RATINGS AND OTHER MISUNDERSTANDINGS RELATED TO SPEAKERS....................350
10.2 EFFICIENCY AND SPL RATINGS.........................................................................................350
10.3 IMPEDANCE AND FREQUENCY RESPONSE...........................................................................353
10.4 DIRECTIVITY, DIFFRACTION AND DISPERSION...................................................................357
10.5 SPEAKER STRUCTURES......................................................................................................358
10.5.1 Dynamic speakers....................................................................................................358
10.5.2 Field coil speakers...................................................................................................360
10.5.3 Piezoelectric and electrostatic speakers..................................................................361
10.5.4 Ribbon and planar speakers....................................................................................361
10.5.5 Flat panel and NXT..................................................................................................362
10.6 HOW A SPEAKER “DIES”....................................................................................................362
10.7 ELECTRICAL SPEAKER MODEL AND THIELE-SMALL PARAMETERS.....................................364
11 CABINET DESIGN...........................................................................................................368
11.1 MATERIALS.......................................................................................................................368
11.2 CONSTRUCTION TECHNIQUES............................................................................................371
11.3 ABOUT ACOUSTIC DESIGN OF GUITAR CABINETS...............................................................373
11.4 RESONANCE......................................................................................................................374
11.5 CABINET SHAPE: DIRECTIVITY, DIFFRACTION, BEAM BLOCKING ETC................................376
11.6 OPEN-BACKED CABINETS..................................................................................................378
11.7 SEALED CABINETS.............................................................................................................379
11.8 DUCTED/VENTED CABINETS..............................................................................................380
11.9 PASSIVE RADIATORS.........................................................................................................386
11.10 SEALED ENCLOSURES REVISITED...................................................................................386
11.11 HORN STRUCTURES.......................................................................................................387
11.12 TRANSMISSION LINE ENCLOSURES.................................................................................388
11.13 COMPOUND OR BAND-PASS CABINETS...........................................................................389
11.14 ISOBARIC LOADING.......................................................................................................390
11.15 ISOLATION CABINETS....................................................................................................390
11.16 EXAMPLES OF CABINET DIMENSIONS.............................................................................391
11.17 TYPICAL SPEAKER WIRING CONFIGURATIONS................................................................393
12 CHASSIS CONSTRUCTION...........................................................................................397
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12.1 COMMON CHASSIS MATERIALS AND TYPES........................................................................397
12.2 BASICS OF METALWORKING..............................................................................................399
12.2.1 Cutting metal............................................................................................................399
12.2.2 Bending....................................................................................................................399
12.2.3 Drilling.....................................................................................................................400
12.2.4 Polishing and painting.............................................................................................401
12.3 VENTILATION AND OTHER THERMAL-RELATED CONSIDERATIONS.....................................402
12.4 EFFECTS OF MECHANICAL VIBRATION...............................................................................404
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Introduction
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1.1 Foreword

Even these days a shroud of mystery covers many aspects of guitar amplifier design.
Plenty of the material written about the topic is controversial or bluntly inaccurate.
Even larger amounts of the material is focusing on rather unimportant topics – such
as, which tube, transistor, diode, resistor material or capacitor brand provides the best
tone. A huge amount of the material out there is written - and later interpreted and
rewritten - by people who have little or no knowledge at all about even common
electronics. On top of that, marketing departments of various companies are
constantly and deliberately spreading a huge amount of propaganda in order to
increase their sales and to convince the people that their new product is somehow
exceptional, innovative and worth buying. Almost every design out there claims to be
either “pristinely clean” or “capturing the tube tone and warmth” – whatever that is.
There are so many reasons why so much discussion evolves around “mojo” topics that
it would be vain to list them all here; unfortunately these topics also create room for
false beliefs that give an impression that guitar amplifier designing is a black art of
some kind. It is not. Amplifier designing is - and always will be – pure science based
on electronics theory.

While there is plenty of material written about tube amplifier design solid-state
amplifiers are mostly left ignored. The case is even worse when it comes to solid-state
guitar amplifiers. Fortunately, solid-state and tube amplifier circuits share a lot of
similarities. The theory is – for the most part – interchangeable. However, many
principles used in designing tube amplifiers will not work with transistor circuitry and
if they do, they might provide unsatisfactory results. Claims that a solid-state amplifier
cannot sound as good as a tube amplifier is utterly untrue and mostly heard from the
mouths of three kinds of people: 1) people that have very little or no knowledge about
electronics, 2) tube snobs who persistently claim that all transistor amplifiers must
sound bad, yet would stick a Fuzz Face (or similar) in front of their tube amplifier
without even blinking an eye and 3) experts of tube amplifier circuitry that
unfortunately possess a limited knowledge about solid-state design. While someone
might master tube technology, the truth is that tube- and solid-state circuitry differ a
lot from each other. One should not unreservedly believe statements about transistor
circuitry presented by a tube expert – as one should not unreservedly believe
statements about tube circuitry presented by a solid-state circuitry expert either.
Though the knowledge about both technologies should go hand-in-hand, too often it
doesn’t. Very regrettable thing is that a great deal of highly talented tube guitar
amplifier experts deliberately avoid educating themselves about solid-state as they see
it as an inferior technology.

Yet, the fact remains that plenty of people start their electronics hobby by building
solid-state audio circuits. Most of these people are repelled by the lethal voltages of
tube circuits – or simply by the sheer cost of them. Many people would like to build a
solid-state power amplifier but do not know how to do it. The popularity of chip
amplifiers is a very good example of this phenomenon since IC-based circuits are easy
to assemble even with little or no knowledge of circuit theory at all, what happens
when these hobbyists wish to expand their knowledge is that they bump into utterly
technical books filled with complex circuit designs and math formulas. While these
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books have their purpose they certainly are not very helpful for a beginner. The
amount of books that offer an insight to discrete power amplifier design - an insight
that would be intelligible even for a complete novice - is certainly too small. Even
more minimal is the amount of books that focuses into solid-state guitar amplifier
electronics. This book is intended to fill this niche.

In this book I do not wish to present things with excessive technicality – there are
better sources for that kind of information. Nor do I wish to make biased comparisons
between solid state and tube technology. However, since the latter is an eternally hot
topic (which always comes up when transistor amplifiers are mentioned) it is
discussed shortly in section 1.5. My aim is to present various forms of basic circuitry
used in guitar signal amplification - and the theory related to it – preferably in a form,
which hopefully is understandable with common knowledge of circuit theory. One can
then use this information as a base for learning more about concerned matter. If you
don’t know what a resistor or a capacitor is - or if you do not understand the basics of
common circuit theory (such as Ohm’s and Kirchhoff’s laws) - I suggest you start
with reading something other than this book.

Unlike some books, this one will not present a complete design example. I feel that
this kind of approach would steal the base from the book since my idea essentially is
to teach one to create such a design by him/herself as well as to make the designer
ponder the aspects and choices related to the design process. Anyone can copy a
design without learning anything in the process and even easier is to criticize a design
and mark all theoretical content either good or bad according to it. I have spent a great
deal of time in writing this book and gathering and assembling the information
required to do so. If on top of that I’d had to create a design (bear in mind that a
design is not just a schematic) the process would have become overwhelming and
even more time consuming. Besides, I do not wish to give an impression that I favor
some topologies over other – which is what people tend to think too easily if they are
presented with something that should be “the ultimate design”.

Also, I feel obligated to include a compulsory warning here: Electricity can be
dangerous. Electricity can cause injury or death! The circuits shown in this book are,
to the best of the author’s knowledge and belief, correct and safe. However, one must
take great care when assembling electronic circuits – especially those that carry mains
voltages. The author cannot accept any responsibility for any accidents that may
occur. The use, application and transmission of electricity are subject to rules,
regulations and guidance lain down in numerous regulations and standards. Most of
these can be obtained for reference in your local library.
1.2 About the schematic notation

Some of the schematic symbol notation used in this book can be considered outdated
and non-standard these days. However, I have stuck to notation procedures that are
clear to decipher and natural to me. I will not apologize for it. Some may argue that a
US style resistor symbol can be misinterpreted as a symbol of inductor and therefore
using the European “block” symbol should be preferred. I agree that standards are
good, but as my excuse, let it be mentioned that if I ever draw schematics by hand I
simply find the US symbol faster and easier to draw. I have seen plenty of schematics
where an inductor or transformer coil is actually drawn like the US symbol of resistor;
this was actually very common in power electric notation of 1940’s and 1950’s. Yet
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