326 Chapter 10
maximum symmetrical voltage swing; a large DC-blocking capacitor is therefore used in
series with the output. The second kind have positive and negative supply rails, and the
output is biased to be at 0 V, so no output DC blocking is required in normal operation.
10.5.1 Advantages of AC Coupling
- The output DC offset is always zero (unless the output capacitor is leaky).
- It is very simple to prevent turn-on thump by purely electronic means. The
amplifi er output must rise up to half the supply voltage at turn on, but providing
this occurs slowly, there is no audible transient. Note that in many designs, this
is not simply a matter of making the input bias voltage rise slowly, as it also
takes time for the DC feedback to establish itself, and it tends to do this with a
snap action when a threshold is reached. - No protection against DC faults is required, providing that the output capacitor
is voltage rated to withstand the full supply rail. A DC-coupled amplifi er
requires an expensive and possibly unreliable output relay for dependable
speaker protection. - The amplifi er should be easier to make short-circuit proof, as the output
capacitor limits the amount of electric charge that can be transferred each cycle,
no matter how low the load impedance. This is speculative; I have no data as to
how much it really helps in practice. - AC-coupled amplifi ers do not, in general, appear to require output inductors
for stability. Large electrolytics have signifi cant equivalent series resistance
(ESR) and a little series inductance. For typical amplifi er output sizes the ESR
will be of the order of 100 m Ω ; this resistance is probably the reason why
AC-coupled amplifi ers rarely had output inductors, as it is enough resistance
to provide isolation from capacitative loading and so gives stability. Capacitor
series inductance is very low and probably irrelevant, being quoted by one
manufacturer as a few tens of nanoHenrys ’. The output capacitor was often
condemned in the past for reducing the low-frequency damping factor (DF),
for its ESR alone is usually enough to limit the DF to 80 or so. As explained
earlier, this is not a technical problem because “ damping factor ” means virtually
nothing.