Audio Engineering

(Barry) #1
Preamplifi ers and Input Signals 199

Practical audio circuit designs will therefore regard headroom simply as one of a group
of desirable parameters in a working system whose design will be based on careful
consideration of the maximum input signal levels likely to be found in practice.


Nevertheless, improved transistor or IC types, and new developments in circuit
architecture, are welcomed as they occur and have eased the task of the audio design
engineer, for whom the advent of new program sources, in particular the compact disc,
and now digital audio tape systems, has greatly extended the likely dynamic range of the
output signal.


7.12.1 Signal Characteristics


The practical implications of this can be seen from a consideration of the signal
characteristics of existing program sources. Of these, in the past, the standard vinyl
( “ black ” ) disc has been the major determining factor. In this, practical considerations of
groove tracking have limited the recorded needle tip velocity to about 40 cm/s, and typical
high-quality PU cartridges capable of tracking this recorded velocity will have a voltage
output of some 3 mV at a standard 5-cm/s recording level.


If the preamplifi er specifi cation calls for maximum output to be obtainable at a 5-cm/s
input, then the design should be chosen so that there is a “ headroom factor ” of at least 8
in such stages preceding the gain controls.


In general, neither FM broadcasts, where the dynamic range of the transmitted signal is
limited by the economics of transmitter power, nor cassette recorders, where the dynamic
range is constrained by the limited tape overload characteristics, have offered such a high
practicable dynamic range.


It is undeniable that the analogue tape recorder, when used at 15 in/s, twin-track, will
exceed the LP record in dynamic range. After all, such recorders were originally used
for mastering the discs. But such program sources are rarely found except among “ live
recording ” enthusiasts. However, the compact disc, which is becoming increasingly
common among purely domestic hi-fi systems, presents a new challenge, as the
practicable dynamic range of this system exceeds 80 dB (10,000:1), and the likely range
from mean (average listening level) to peak may well be as high as 35 dB (56:1) in
comparison with the 18-dB (8:1) range likely with the vinyl disc.

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