Audio Transformers-Contents Page
Handbook for Sound Engineers, 3rd Edition
Audio Transformers
Audio Transformers
by
Bill Whitlock
Jensen Transformers, Inc.
9304 Deering Avenue
Chatsworth, CA 91311
This work first published by Focal Press in 2001 as
Chapter 11
Handbook for Sound Engineers, Third Edition
Glen Ballou, Editor
Copyright © 2001, 2006 Bill Whitlock
All rights reserved
Bill Whitlock Audio Transformers Page 1
Handbook for Sound Engineers, 3rd Edition
Bill Whitlock Audio Transformers Page 1
Handbook for Sound Engineers, 3rd Edition
Bill Whitlock Audio Transformers Page 1
Handbook for Sound Engineers, 3rd Edition
1 Audio Transformer Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Basic Principles and Terminology . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 Magnetic Fields and Induction. . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Windings and Turns Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 Excitation Current. . . . . . . . . . . . . . . . . . . . . . . . . .3
1.2 Realities of Practical Transformers. . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Core Materials and Construction . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Winding Resistances and Auto-Transformers . . . . . . . . . . . . . . . . 6
1.2.3 Leakage Inductance and Winding Techniques . . . . . . . . . . . . . . . . .7
1.2.4 Winding Capacitances and Faraday Shields . . . . . . . . . . . . .. . . . 8
1.2.5 Magnetic Shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 General Application Considerations . . . . . . . . . . . . . . . . . . . . . 9
1.3.1 Maximum Signal Level, Distortion, and Source Impedance . . . . . . . . . . 9
1.3.2 Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
1.3.3 Insertion Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11
1.3.4 Sources with Zero Impedance . . . . . . . . . . . . . . . . . . . . . . . 12
1.3.5 Bi-Directional Reflection of Impedances . . . . . . . . . . . . . . . . . 12
1.3.6 Transformer Noise Figure . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.7 Basic Classification by Application . . . . . . . . . . . . . . . . . . . 14
2 Audio Transformers for Specific Applications . . . . . . . . . . . . . . . . 14
2.1 Equipment-Level Applications . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.1 Microphone Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
2.1.2 Line Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.3 Moving Coil Phono Input . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.4 Line Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.5 Inter-Stage and Power Output . . . . . . . . . . . . . . . . . . . . . . .17
2.1.6 Microphone Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2 System-Level Applications . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Microphone Isolation or "Splitter" . . . . . . . . . . . . . . . . . . . 18
2.2.2 Microphone Impedance Conversion . . . . . . . . . . . . . . . . . . . . . 18
2.2.3 Line to Microphone Input or "Direct Box" . . . . . . . . . . . . . . . . .18
2.2.4 Line Isolation or "Hum Eliminators" . . . . . . . . . . . . . . . . . . . 19
2.2.5 Speaker Distribution or "Constant Voltage" . . . . . . . . . . . . . . . .21
2.2.6 Telephone Isolation or "Repeat Coil" . . . . . . . . . . . . . . . . . . 22
2.2.7 Telephone Directional Coupling or "Hybrid" . . . . . . . . . . . . . . . .23
2.2.8 Moving Coil Phono Step-Up . . . . . . . . . . . . . . . . . . . . . . . . 23
3 Measurements and Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . .24
3.1 Testing and Measurements . . . . . . . . . . . . . . . . . . . . . . . . . .24
3.1.1 Transmission Characteristics . . . . . . . . . . . . . . . . . . . . . . .24
3.1.2 Balance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.3 Resistances, Capacitances, and Other Data . . . . . . . . . . . . . . . . 25
3.2 Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.1 Data to Impress or to Inform? . . . . . . . . . . . . . . . . . . . . . . 25
3.2.2 Comprehensive Data Sheet Example . . . . . . . . . . . . . . . . . . . . .25
4 Installation and Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .28
4.1 A Few Installation Tips . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2 De-Magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
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