Interfacing and Processing 2718.6.1.6 Position
As in other analogue audio circuits, the placement of any gain control device requires
careful considerations in regard to considering trade-offs in headroom and SNR. But in
power amplifi ers having a minimum path, there is not much choice for location. They all
end up after the input is unbalanced but before it is raised far.
Placement couldn’t be contemplated after the point of signal passing to the input of the
power stage, for example, as pots having fi lm tracks (cf. wire wound) that are suitable for
audio by virtue of low rotation noise are unsuited to high dissipation. In any event, most
power stage topologies don’t have a place for inserting a single-ended, passive voltage
divider, don’t like having their gain widely changed, and are moreover wrapped around
by NFB.
Adequate CMR (at the amplifi er’s input) demands good balancing, which in turn relies on
resistance matching to better thanat least 0.5%, and since even makers of very expensive,
high specifi cation pots have problems maintaining matching between two or more
sections to even 2%, over the entire travel, pots passing audio have to be placed after the
input signal has been converted to single ended, that is, after the debalancer (DTSEC).
Virtually all power amplifi er gain pots (or whatever other gain control devices) end up
thusly sandwiched. A few are used in active mode, where the pot is used in the NFB loop,
of either an added line-level stage or even a gain-change tolerant power stage. This seems
smart but it has its own problems.
8.6.1.7 Fixed Install
In amps principally intended for fi xed installation, whether for a home cinema or public
venues, and where power amplifi er gain trims are needed or helpful for setting up,
“ knobless ” gain controls are welcomed. Here, shafts are normally recessed and can
Figure 8.10 : Gain pot settings. Shown are six ways of looking at any power amplifi er’s gain
control; in this instance the simplest and most familiar “ volume” control type. The fi nal
knob labeled “ input clip volts (pk) ” scale is for peak levels and is correct only for an
amplifi er that clips at 900 mV rms. In reality, the point would depend on speaker loading,
mains voltage, the program, etc. The constant 9.6-V peak reached at lower levels shows
where the input stage clips or where zener-based input-protection clamping is operating.
Courtesy of Citronic Ltd.