TABLE OF CONTENTS 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.
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 performance. In servicing the offset should not need renulling unless one of relatively few components are changed; the input devices have the most effect, because the new parts are unlikely to have exactly the same beta, but the feedback resistors also have some influence as the input stage base currents flow through them.
The disadvantages are that an extra adjustment is required in production, and since this is a set-and-forget preset, it can have no effect on DC offsets that may accumulate due to input stage thermal drift or component ageing.
Figure 14.1 shows one simple way to add a DC trim control to an amplifier, by injecting a small current of whatever polarity is required into the feedback point. Since the trim circuit is powered...
