Audio Preamplifiers

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 10 mV 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 ?1 mV, 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 1 V and the input signal voltage 1 mV, the best signal-to-noise ratio possible is just 60 dB.

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 that...