Rupert Neve 511 - 500 Series Mic Pre with Silk - Manual (Page 6)

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under 20 to 30 dB of noise. And we know that most gain stages exhibit rising
distortion at higher frequencies, including more IM distortion. One common IM
test is to mix 19 kHz and 20 kHz sine waves, send them through a device and then
measure how much 1 kHz is generated (20-19=1). All this hints at the importance
of maintaining a sufficient bandwidth with minimal phase shift, while at the
same time minimizing high frequency artifacts and distortions. All of the above
and our experience listening and designing suggest that there are many subtle
aspects to hearing that are beyond the realm of simple traditional measurement
characterizations.
The way in which an analog amplifier handles very small signals is as important
as the way it behaves at high levels. For low distortion, an analog amplifier must
have a linear transfer characteristic, in other words, the output signal must be an
exact replica of the input signal, differing only in magnitude. The magnitude can be
controlled by a gain control or fader (consisting of a high quality variable resistor
that, by definition, has a linear transfer characteristic.) A dynamics controller - i.e.
a compressor, limiter or expander - is a gain control that can adjust gain of the
amplifier very rapidly in response to the fluctuating audio signal, ideally without
introducing significant distortion, i.e. it must have a linear transfer characteristic.
But, by definition, rapidly changing gain means that a signal “starting out” to be
linear and, therefore without distortion, gets changed on the way to produce a
different amplitude.
Inevitably our data bank of “natural” sound is built up on the basis of our personal
experience and this must surely emphasize the importance of listening to “natural”
sound, and high quality musical instruments within acoustic environments that
is subjectively pleasing so as to develop keen awareness that will contribute to a
reliable data bank. Humans who have not experienced enough “natural” sound
may well have a flawed data bank! Quality recording equipment should be capable
of retaining “natural” sound and this is indeed the traditional measuring stick. And
“creative” musical equipment should provide the tools to manipulate the sound
to enhance the emotional appeal of the music without destroying it. Memory
and knowledge of real acoustic and musical events may be the biggest tool and
advantage any recording engineer may possess.
One needs to be very careful when one hears traces of distortion prior to recording
because some flavors of distortion that might seem acceptable (or even stylish)
initially, may later prove to cause irreparable damage to parts of the sound (for
example, “warm lows” but “harsh sibilance”) or in louder or quieter sections of
the recording. Experience shows that mic preamps and basic console routing paths
should offer supreme fidelity otherwise the engineer has little control or choice of
recorded “color” and little recourse to undo after the fact. Devices or circuits that
can easily be bypassed are usually better choices when “color” is a consideration
and this particularly is an area where one might consider comparing several such
devices. Beware that usually deviations from linearity carry at least as much long-
term penalty as initial appeal, and that one should always be listening critically
when recording and generally “playing it safe” when introducing effects that cannot
be removed.