273Chapter 11
Audio Transformer Basics
by Bill Whitlock
11.1 Audio Transformer Basics................................................................ 275
11.1.1 Basic Principles and Terminology....................................................... 275
11.1.1.1 Magnetic Fields and Induction...................................................... 275
11.1.1.2 Windings and Turns Ratio.......................................................... 276
11.1.1.3 Excitation Current................................................................ 276
11.1.2 Realities of Practical Transformers....................................................... 277
11.1.2.1 Core Materials and Construction..................................................... 278
11.1.2.2 Winding Resistances and Auto-Transformers........................................... 280
11.1.2.3 Leakage Inductance and Winding Techniques.......................................... 281
11.1.2.4 Winding Capacitances and Faraday Shields............................................ 282
11.1.2.5 Magnetic Shielding............................................................... 283
11.1.3 General Application Considerations...................................................... 283
11.1.3.1 Maximum Signal Level, Distortion, and Source Impedance................................ 283
11.1.3.2 Frequency Response.............................................................. 285
11.1.3.3 Insertion Loss................................................................... 286
11.1.3.4 Sources with Zero Impedance....................................................... 287
11.1.3.5 Bi-Directional Reflection of Impedances.............................................. 287
11.1.3.6 Transformer Noise Figure.......................................................... 288
11.1.3.7 Basic Classification by Application.................................................. 289
11.2 Audio Transformers for Specific Applications................................................. 290
11.2.1 Equipment-Level Applications.......................................................... 290
11.2.1.1 Microphone Input................................................................ 290
11.2.1.2 Line Input...................................................................... 291
11.2.1.3 Moving-Coil Phono Input.......................................................... 291
11.2.1.4 Line Output..................................................................... 292
11.2.1.5 Inter-Stage and Power Output....................................................... 293
11.2.1.6 Microphone Output............................................................... 294
11.2.2 System-Level Applications............................................................. 295
11.2.2.1 Microphone Isolation or Splitter..................................................... 295
11.2.2.2 Microphone Impedance Conversion.................................................. 295
11.2.2.3 Line to Microphone Input or Direct Box............................................... 296
11.2.2.4 Line Isolation or Hum Eliminators................................................... 296
11.2.2.5 Loudspeaker Distribution or Constant Voltage.......................................... 298
11.2.2.6 Telephone Isolation or Repeat Coil................................................... 300
11.2.2.7 Telephone Directional Coupling or Hybrid............................................. 300
11.2.2.8 Moving-Coil Phono Step-Up........................................................ 301
11.3 Measurements and Data Sheets............................................................. 302
11.3.1 Testing and Measurements............................................................. 302
11.3.1.1 Transmission Characteristics........................................................ 302