Amp History 1 Op Amp Basics 2 Specialty Amplifiers 3 Using Op Amps with Data Converters 4 Sensor Signal Conditioning 5 Analog Filters 6 Signal Amplifiers 1 Audio Amplifiers

Audio signal preamplifiers (preamps) represent the low-level end of the dynamic range of practical audio circuits using modern IC devices. In general, amplifying stages with input signal levels of 10mV or less fall into the preamp category. This section discusses some basic types of audio preamps, which are: ■ Microphone— including preamps for dynamic, electret and phantom powered microphones, using transformer input circuits, operating from dual and single supplies. ■ Phonograph— including preamps for moving magnet and moving coil phono cartridges in various topologies, with detailed response analysis and discussion. In general, when working signals drop to a level of ≈1mV, the input noise generated by the first system amplifying stage becomes critical for wide dynamic range and good signal-to-noise ratio. For example, if internally generated noise of an input stage is 1µV and the input signal voltage 1mV, the best signal-to-noise ratio possible is just 60dB. In a given application, both the input voltage level and impedance of a source are usually fixed. Thus, for best signal-to-noise ratio, the input noise generated by the first amplifying stage must be minimized when operated from the intended source. This factor has definite implications to the preamp designer, as a "low noise" circuit for low impedances is quite different from one with low noise operating from a high impedance. Successfully minimizing the input noise of an amplifier requires a full understanding of all the various factors which contribute to total noise. This includes the amplifier itself as well as the external circuit in which it is used, in fact the total circuit environment must be considered, both to minimize noise and maximize dynamic range and signal fidelity. A further design complication is the fact that not only is a basic gain or signal scaling function to be accomplished, but signal frequency response may also need to be altered in a predictable manner. Microphone preamps are an example of wideband, flat frequency response, low noise amplifiers. In contrast to this, phonograph preamp circuits not only scale the signal, they also impart a specific frequency response characteristic to it. A major part of the design for the RIAA phono preamps of this section is a systematic analysis process, which can be used to predictably select components for optimum performance in frequency response terms. This leads to very precise functioning, and excellent correlation between a computer based design and measured lab operation. The microphone preamplifier …