Rupert Neve 511 - 500 Series Mic Pre with Silk - Manual (Page 5)

5
DYNAMIC RANGE
Traditionally, high quality microphones such as ribbons, had very low source
impedances – as low as 30 ohms at the output of a ribbon matching transformer.
Moving coil microphones were higher impedance, but had not been standardized
as they are today. Condenser microphones, before the days of semiconductors,
used tube pre-amplifiers that were coupled to the outgoing line with an impedance
matching transformer. Microphone amplifiers, such as in a mixing console, also
used tubes, which typically have a high input impedance.
Microphones are voltage generators, not power generators. At the low operating
level of a microphone, it is always desirable to deliver the maximum possible
signal voltage into the amplifier. It was traditional to provide an amplifier
input impedance of about 1,000 or 1,200 ohms; about 5 or 6 times the source
impedance of the microphone. This provided relatively low loading on the
microphone – whatever its type – and went a long way to avoid voltage loss.
In the early 1960’s when the “Pop” music scene was exploding and sound levels
in the Studio became very high, there was concern that the capsule amplifiers in
condenser microphones would overload if the console input impedance was too
low. In the early days of consoles I was asked to provide higher input impedance
than the normal 1,000 ohms. Because the microphone preamplifiers in the early
recording consoles used transformer gain in the input stage , this resulted in less
“step-up” in the console input transformer and there were then fears that we would
lose out at the other end of the scale; noise. The fact that microphones were less
heavily loaded allowed an increased microphone signal. The reduced loading also
resulted in less deviation of frequency response due to variation of microphone
impedance and consequently less distortion at high levels.
The Portico 511 microphone amplifier provides an input impedance of 10,000
ohms which means that variations in microphone source impedance with
frequency, have only a very small effect on the sonic quality. This high input
impedance has minimal effect on microphone output and loading which results
in mic distortion which is very low, adding up to a noticeable improvement in
“transparency”.
A NOTE ON DISTORTION
The human hearing system is a remarkably complex mechanism and we
seem to be learning more details about its workings all the time. For example,
Oohashi demonstrated that arbitrarily filtering out ultrasonic information that
is generally considered above our hearing range had a measurable effect on
listener’s electroencephalo-grams. Kunchur describes several demonstrations
that have shown that our hearing is capable of approximately twice the timing
resolution than a limit of 20 kHz might imply (F=1/T or T=1/F). His peer reviewed
papers demonstrated that we can hear timing resolution at approximately with 5
microsecond resolution (20 kHz implies a 9 microsecond temporal resolution,
while a CD at 44.1k sample rate has a best-case temporal resolution of 23
microseconds).
It is also well understood that we can perceive steady tones even when buried