278 Chapter 8
8.6.7 Soft Clip
“ Soft clip ” is a feature that aims to defeat the suddenness of the onset of hard distortion
above the clip level in conventional, high NFB power amplifi ers. It may be provided as a
fi xed or switchable option. Unlike compression and limiting, there are no time constants,
no settings, and no attempt to avert serious distortion of a sine wave. However, the
clipped waveform does not readily square off and retains some curvature (dV/dt) even
with heavy overdrive (e.g., at 10 dBvr). This greatly reduces the massed production of
unpleasant, high harmonics and intermodulation products of hard clipping. One apparent
(but not necessarily actual) snag is that because hard clipping is a real limit, soft clipping
has to begin to occur up to –10 dB below full output (–10 dBvr). This is tantamount to
saying that distortion (%THD say) with a 100-W amplifi er begins rising from above
about 10 W, as opposed to rising very abruptly above exactly 100 W, while remaining
extremely low up to this point. Here is one difference between low and high global
feedback amplifi er behavior.
Soft clipping restores the more forgiving behavior of low feedback to a high NFB amplifi er.
The extent to which it undoes all the high feedback’s other benefi ts is unqualifi ed. At least
the high NFB is in operation for most of the time, for with proper headroom allowance,
most of the musical content should lie below the –10-dB threshold or so, whence the soft
clip is inactive. Usually soft clipping is arranged to be symmetrical. This may not create the
most consonant harmonic structure. Figure 8.12 shows a classic circuit.
8.7 Computer Control .................................................................................................
Computer control of audio power amplifi ers has been slow to develop. This is because
amplifi ers have not been a useful place, in most instances, for physical control surfaces.
Table 8.6 : Manufacturer and their products
ARX systems Anticlip
Carver Clipping eliminator
Crest Audio IGM (Instantaneous Gain Modulation)
Crown (Amcron) AGC in PSA2 (Automatic Gain Control)
Malcolm Hill Headlok