Interfacing and Processing
Ben Duncan8.1 The Input
For the user, “ the input ” is often just a socket—often one groped for amidst a tangle of leads.
This chapter untangles the details of the rarely recounted considerations that lie behind audio
power amplifi er input sockets that enable the signal source to connect to the amplifi er (and
maybe to many amps) with the least loss of fi delity and without introducing unwanted noise.
The amplifi er is treated as a whole without considering the power capability or type of the
output section.
8.1.1 Input Sensitivity and Gain Requirements
- 1 D e fi nition
Input sensitivity is the signal level at the input needed to drive an amplifi er up to its full
capability, to just before clip, into a stated, nominal impedance, often 8 ohms. Clip may
be defi ned as the onset of visible waveform fl attening or as a certain percentage THD N
distortion factor.
An older, less used defi nition (favored in the 1978 IHF standard) is the signal level
needed to deliver I watt into a given nominal load, say 8f2. This is fi ne for comparing
or normalizing drive levels between amps having different power ratings, but as input
sensitivity per se has no particular merit, the usefulness, for real amplifi ers and speakers
of widely varying power capabilities and sensitivities, ends there.
8.1.1.2 Description
Sensitivity is usually expressed as a voltage, either directly in volts or millivolts
(1/1000ths of a volt), or in dBu. Mostly, sensitivity fi gures are assumed to be rms values
CHAPTER 8