Rupert Neve Portico 5012 - Duo Mic Pre-Amp - Manual (Page 8)

7
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
but had not been standardized as they are today. Condenser microphones, before the days of
semiconductors, used tube head amplifiers that were coupled to the outgoing line with a
transformer.
Microphone amplifiers, such as in a mixing console, also used tubes and these typically have a high
input impedance.
Microphones are Voltage generators, not Power generators. 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 head 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. This of course, 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 RND “Portico”
TM 5012 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 with the result that microphone distortion is very low adding up to a noticeable
improvement in “transparency”.
5012 DUO MIC PRE