Interfacing and Processing 2638.4.8.2 Autonulling
Direct current offset may be continually forced to near zero volts by a servo , which is
another name for brute-force VLF and DC feedback, applied around an amplifi er overall,
or just the input or output stage. Servos have been de rigueur in U.S. and U.S.-infl uenced
high-end domestic power amplifi ers for some years. Alas, those who have designed
them into high-performance power amplifi ers have clearly not thought through the
consequences. Tellingly, servos are not usually nor likely to be found in amplifi ers with
truly accurate sounding bass.
The reasons are clear enough today: servos cause the same or even wilder distortions in
LF frequency and/or phase response, and/or signal delay vs. frequency (group delay).
Figure 8.8 shows this.
They also compromise the integrity of the circuitry they are wrapped around by
increasing noise susceptibility, while the capacitor imperfections that DC coupling is
supposed to overcome are reintroduced, as distortion-free DC servo action depends on an
expensive, bulky, high-performance capacitor for integration. In this way, the DC servo
returns us tobefore square one, with the added cost and complexity. Worse, the original
thinking behind servo’ing was to save money (!) on input transistor and part matching, as
a servo will “ fi x ” any DC in its range, often up to /–5 V, including DC appearing on the
equipment input. This is neat, but like so many “ smart ” options, DC servo’ing is not quite
suitable for audio.
8.5 Damage Protection ................................................................................................
The input stages of most audio equipment are unprotected. This approach appears to save
on parts cost, complexity, and sonic degradation; however, in reality, it may indeed cause
costs and degraded sonics. The inputs of power amplifi ers are certainly among those most
likely to sustain input voltages that may be damaging to the active parts inside.
8.5.1 Causes
Typical culprits include fi rst, large signals from line level sources, and from amplifi er
outputs, experienced through accidental connections (see Section 8.5.2). Here, excessive
signal voltages that could be applied could range from a few volts, up to 230 V rms, and
from below 10 Hz to above 30 kHz.