While I'm on the subject of increasing apparent loudness, I don't know whether it is as widely appreciated as it
should be that compression is only half the answer. Compression is a long-term type of gain reduction, working at
the very least over periods of tens of milliseconds. If you try to achieve very fast acting compression by using
very short attack and release times, you may well end up with distortion of low frequencies where the compressor
actually changes the shape of the waveform. There comes a point in maximizing apparent loudness where the
compressor has given all it has got to give. Clipping, on the other hand, works on a very short timescale.
Transistorized circuitry reacts within microseconds to any level that is too great for the power supply to cope
with and cuts it short, creating harsh harmonics, but at the same time extra loudness. The soft clipping of valve
and valve-emulating designs rounds rather than clips the peaks but, once again, operates on a short time scale.
The problem with soft clipping, if used alone, is that it only works on high-level signals. Clip-worthy peaks only
occur in quantity in high-level signals and low-level signals, although they may indeed have the occasional
clippable peak, are largely unaffected. The answer is to use a compressor and a soft clipper in series.
The compressor evens out the general level of the signal but, since it works over a comparatively long time scale,
the peaks are not clipped but simply brought to a more uniform level. The clipper then has more material to work
on. A useful alternative is to use a series-parallel configuration as shown in Figure 4. Here, the compressor
smooths out the levels, the valve emulation device soft clips the peaks, and the result of that whole process is
added to the uncompressed signal. The result is controllable enhancement over a wide range of levels. If you
want to go further then you might add an equalizer after the compressor so that you can choose the frequency
range that will be affected to add just the right hint of distortion without going over the top, particularly in the
mid-range.