ABSTRACT
The input impedance of a preamplifier must be high to allow for interfacing to anachronistic valve equipment, whose output may be taken from a valve anode. Even light loading compromises distortion and available output swing. The tape-monitor switch allows the replay signal from the tape deck to be compared with the source signal. Line input buffering stage has to provide a high input impedance and variable gain for the balance control. The tone control stage acts in separate bands for bass and treble, so there are two parallel selective paths in the side-chain. These are simple RC time-constants, the bass path being a variable-frequency first-order low-pass filter, and the associated bass control only acting on the frequencies this lets through. The preamplifier includes relay muting on the main outputs. This is to prevent thuds and bangs from upstream parts of the audio system from reaching the power amplifiers and speakers at power-up and power down.
