Audio Engineering

Interfacing and Processing 261

be an acceptable point for the bass response low cutoff frequency (f3 L ). The result is
considerable HP fi ltering, permanently engaged. Subsonic signals may then rarely pose a
problem, but sonic quality may be degraded up into midfrequencies, while a great deal of
the music’s ambient cues is lost.

8.4.8 Direct Coupling

When all HP fi ltering is removed, a power amplifi er becomes direct—or ‘ DC ’ (direct
current)—coupled. ‘ DCC ’ would have been better, but that now means something else.
Extending the response to zero frequency, that is, “ down to DC, ” is achieved readily at the
design stage with most transistor topologies. The advantages are sonic, and substantial,
due to the excision of intrinsically imperfect parts and the removal of an intrinsically
unnatural fi ltration, and the signal-delay and the possible charge accumulation on
asymmetric music signals it brings. For this is the truth of all signal path HPF capacitors,
both those in series and in NFB arms. Whether DC coupling is safe or workable in a
particular amplifi er is a separate design question.

With conventional valve amp topologies, the response to DC is not fully achievable,
except in the few workable ‘ OTL ’ designs. However, it is still possible to direct couple the
remainder of a valve amplifi er, with global DC NFB taken before the transformer. In fact,
the fi rst precision DC amplifi ers were valve op-amps.

Gain control

(if fitted)

Power

stage

(if employed)





Power

supply

reservoir

caps

or

E

F

HT F

HT

C

Lower arm

of global NFB

(if employed)

D





Input stage (if fitted)

A

A B

Input





Figure 8.7 : High-pass fi lter capacitor positions. The potential locations of DC blocking/HPF

capacitors in the signal path of conventional transistor power amplifi ers, assuming that gain

blocks (the triangles) are internally direct coupled.