262 Chapter 8
8.4.8.1 Direct Current Management
With direct-coupled circuitry, unwanted DC “ offset voltages ” will be amplifi ed by the
power amplifi er’s respective stage gains. Excess DC is of great concern and must be
avoided. It can be (i) produced internally, by mismatches in resistor or semiconductor
values or by intrinsic topological asymmetry or (ii) introduced externally, from preceding
DC-coupled signal sources.
Internally produced DC offsets may be kept to safe levels by precision in design and
component selection. This requires matching of two or three apposite parameters of the
differential pair at the front end of each stage, assuming some version of the conventional
high NFB “ op-amp ” type of architecture. The “ pair ” might be BJTs, FETs, or valves.
And to ensure that the source resistances (at DC) seen by each input leg are the same,
or close, and not too high either, depending on bias current. If the resistor values then
confl ict with CMR, the latter should have priority, in view of EMC requirements, and the
nonrecoverability of the CMR opportunity. Direct current balance may be restored by
other means, for example, current injection.
Externally applied DC, appearing on the inputs, because of essentially healthy but
imperfect preceding equipment, will usually be in the range of 0.1 to 100 mV. More than
/–100 mV would suggest a DC fault in the preceding source equipment. Assuming a
gain of 30 , this would result in 3 V at the amplifi er’s output. Because such a steady
offset will eat up headroom on one-half of the signal swing, the clip level is lowered
asymmetrically. A direct coupled power amplifi er should not be harmed by this and
should also protect the speakers it is driving, but equally it is entitled to shut down to
draw attention to such an unsatisfactory situation. In the most advanced designs of
analogue path yet published,^9 DC coupling is adaptive: if DC above a problem level
persists at the input, DC blocking capacitors are automatically installed and the user is
informed by LED.
Some low-budget domestic power amplifi ers have long offered part and manual direct
coupling. The power stage may not be wholly direct coupled, but at least the DC blocking
capacitor(s) at the input can be bypassed via a second “ direct ” or “ laboratory ” input.
The user is expected to try this but revert to the ordinary ac-coupled inputs if DC on the
source signal is enough to cause zits and plops. A blocking capacitor(s) at the input can
be bypassed via a second “ direct ” or “ laboratory ” input.