The design of DC servos
In the section of this book dealing with input stages I have gone to some lengths to demonstrate that a plain unassisted amplifier – if designed with care – can provide DC offset voltages at the output which are low enough for most practical purposes, without needing either an offset-nulling preset or a DC servo system. For example, the Trimodal amplifier can be expected not to exceed ±15mV at the output. However, there may be premium applications where this is not good enough. In this case the choice is between manual adjustment and DC servo technology.
DC offset trimming
Preset adjustment to null the offset voltage has the advantage that it is simple in principle and most unlikely to cause any degradation of audio ...