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 1 Audio Transformer Basics . . . . . . . . . . . . . 2 1.1 Basic Principles and Terminology. . . . . . . . 2 1.1.1 Magnetic Fields and Induction . . . . . . . . .2 1.1.2 W indings 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 W inding Resistances and Auto-Transformers . . . 6 1.2.3 Leakage Inductance and W inding Techniques . . . 7 1.2.4 W inding 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 Equipm ent-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 M easurements 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 M aintenance . . . . . . . . . . . . . 28 4.1 A Few Installation Tips . . . . . . . . . . . . . . . . 28 4.2 De-Magnetization . . . . . . . . . . . . . . . . . . . 28 References . . . . . . . . . . . . . . . . . . . . . . . . 29