266 Chapter 8
The input stage’s supply rail voltage usually has a large bearing on the maximum safe
DM and CM input voltages. Here, low supply voltages may do no favors.
8.5.5 Off-State Vulnerability
When an amplifi er using BJTs is switched off, both differential and common-mode
voltages as low as /–0.5 V may be damaging. Users are advised to always power-up
preceding equipment before the power amps. This is universal practice among informed
users, both domestic and professional. However, if the prepowering of the source involves
the passage of signals above 0.Sv peak to amplifi er inputs, then unless the transistors
behind the sockets are protected before the amp is powered-up, they may well be
damaged. This mode of subtle, progressive damage and sonic degradation to analogue
electronics has yet to be widely recognized. It can be overcome without changing
otherwise sensible practices, by suitably designed input protection.
8.5.6 Occurrence Modes
Damage to input devices may be catastrophic if the overvoltage causes high currents to
fl ow. This is rare.
Otherwise, with BJT inputs, damage may be subtle. Transistor parameters are degraded
but NFB action initially hides the worst. Telltale signs would be changed or, reducing
sonic quality, raised, increasing and/or intermittent noise, higher %THD, and possibly
increased DC offset at the amp’s output.
With ICs, damage may be cumulative, caused by peculiar metal migration effects
occurring in ICs ’ microscopically thin conductors. This means an input stage can appear
to handle abuse repeatedly until eventually the catastrophic failure occurs when all the
conductor has migrated away!
8.5.7 Protection Circuitry
Power amps have been designed to survive likely levels of both CM and DM overvoltages
by the use of some combination of the following.
- Series input resistors, which may already be part of the input stage’s RF fi ltering,
will limit the current fl owing into inputs. If the resistance between the input
socket and the active device is 5 k Ω , then above 25 V DC or peak signal would be
needed to get more than 51xA to fl ow.