733Chapter 21
Preamplifiers and Mixers
by Bill Whitlock and Michael Pettersen
21.1 Microphone Preamplifier Fundamentals...................................................... 735
21.1.1 The Microphone as a Signal Source...................................................... 735
21.1.1.1 Electrical Model of the Microphone.................................................. 735
21.1.1.2 Interactions with Preamplifier and Cable.............................................. 735
21.1.2 Some Considerations in Practical Preamplifiers............................................. 737
21.1.2.1 Gain and Headroom............................................................... 737
21.1.2.2 Input Impedance................................................................. 737
21.1.2.3 Noise.......................................................................... 737
21.1.2.4 Bandwidth and Phase Distortion..................................................... 738
21.1.2.5 Common-Mode Rejection, Phantom Power, and RF Immunity............................. 739
21.2 Real-World Preamp and Mixer Designs...................................................... 739
21.2.1 Transformers........................................................................ 739
21.2.2 Class A Circuitry..................................................................... 740
21.2.3 Shure SCM268 Four-Channel Mixer..................................................... 740
21.2.4 Cooper Sound CS 104 Four-Channel ENG Mixer........................................... 740
21.2.5 Jensen-Hardy Twin-Servo® 990 Microphone Preamplifier.................................... 741
21.3 21.3 Automatic Microphone Mixers........................................................ 741
21.3.1 The Audio Problems Caused by Multiple Open Microphones.................................. 742
21.3.1.1 Buildup of Background Noise and Reverberation........................................ 743
21.3.1.2 Reduced Gain Before Feedback..................................................... 743
21.3.1.3 Comb Filtering................................................................... 743
21.3.1.4 Summary....................................................................... 744
21.3.2 Design Objectives for Automatic Microphone Mixers........................................ 744
21.3.2.1 Examples of Design Objectives for an Automatic Microphone Mixer........................ 744
21.3.3 Controls and Features of Automatic Microphone Mixers...................................... 745
21.3.3.1 Controls and Features Unique to Automatic Microphone Mixers........................... 745
21.3.3.2 External Control Capability and Status Indication of Automatic Microphone Mixers............ 745
21.3.3.3 Examples of Communication Protocols Used in Automatic Microphone Mixers............... 746
21.3.3.4 Number of Open Microphones Attenuation (NOMA).................................... 746
21.3.3.5 Restricting the Number of Open Microphones (NOM).................................... 747
21.3.3.6 Input Channel Attenuation.......................................................... 747
21.3.3.7 Automatic Gain Control........................................................... 747
21.3.4 Types of Automatic Microphone Mixers.................................................. 748
21.3.4.1 Fixed Threshold Automatic Microphone Mixers........................................ 748
21.3.4.2 Variable Threshold Automatic Microphone Mixers...................................... 748
21.3.4.3 Gain-Sharing Automatic Microphone Mixers........................................... 748
21.3.4.4 Direction-Dependent Automatic Microphone Mixers..................................... 750
21.3.4.5 Noise-Adaptive Threshold Automatic Microphone Mixers................................ 750
21.3.4.6 Multivariable-Dependent Automatic Microphone Mixers................................. 751