Preamplifi ers and Input Signals 217arise in such tests. The most common of these is that of the intrusion of noise and alien
signals, either from the supply line or by direct radio pick up.
This latter case is a random and capricious problem that can only be solved by steps
appropriate to the circuit design in question. However, supply line intrusions, whether
because of unwanted signals from the power supply or from the other channel in a stereo
system, may be reduced greatly by the use of circuit designs offering a high immunity to
voltage fl uctuations on the DC supply.
Other steps, such as the use of electronically stabilized DC supplies or the use of separate
power supplies in a stereo amplifi er, are helpful, but the required high level of supply line
signal rejection should be sought as a design feature before other palliatives are applied.
Modern IC op-amps offer a typical supply voltage rejection ratio of 90 dB (30,000:1).
Good discrete component designs should offer at least 80 dB (10,000:1).
This fi gure tends to degrade at higher frequencies, which has led to the growing use
of supply line bypass capacitors having a low effective series resistance. This feature
is either a result of the capacitor design or is achieved in the circuit by the designer’s
adoption of groups of parallel connected capacitors chosen so that the AC impedance
remains low over a wide range of frequencies.
A particular problem in respect to spurious signals, which occurs in audio power
amplifi ers, is a consequence of the loudspeaker acting as a voltage generator, when
stimulated by pressure waves within the cabinet, and injecting unwanted audio
components directly into the negative feedback loop of the amplifi er. This specifi c
problem is unlikely to arise in small signal circuitry, but the designer must consider what
effect output/line load characteristics may have, particularly in respect to reduced stability
margin in a feedback amplifi er.
In all amplifi er systems there is a likelihood of microphonic effects due to vibration of
the components. This is likely to be of increasing importance at the input of “ low-level, ”
high-sensitivity preamplifi er stages and can lead to coloration of the signal when the
equipment is in use, which is overlooked in the laboratory in a quiet environment.
7.16.4 Mains-Borne Interference
Mains-borne interference, as evidenced by noise pulses on switching electrical loads, is
most commonly due to radio pick up problems and is soluble by the techniques (attention