Rupert Neve Portico 5012 - Duo Mic Pre-Amp - Manual (Page 4)

5012 DUO MIC PRE
3
The PORTICO TM Series
The new RND “Portico” TM modular
preamplifiers and analogue audio
processors constitute a range of
building blocks that may be used
independently or in combination to
provide key elements that were
traditionally included in large format
Sound Control Consoles.
Your 5012 Duo Mic Pre
Microphone Input
The microphone input is balanced but not floating, being a variant of an instrumentation amplifer
using a “Transformer-Like-Amplifier” (T.L.A.) configuration with a toroidal Common Mode
Rejection Low Pass Filter that excludes frequencies above 150 kHz.
The T.L.A. is followed by an actual input transformer permitting a full +26 dBu input signal to
be handled at unity gain without an input pad over the whole audio spectrum.
This combines the advantages of both an “Electronically Balanced” and true Transformer input.
When the Phantom voltage is switched off, this input serves as a very high quality Line Input.
A microphone is a voltage generator, not a power amplifier. Most microphones give their most
accurate performance when they are not loaded by the input impedance of a traditional
preamplifier.
Years ago transformer inputs with tubes were used for microphone preamps. It was convenient
to design the input for an impedance of 1,000 0r 1,200 ohms. Some microphones are still designed
to work well into a low load impedance. If the microphone has an inductive source (such as would
be the case if it has a transformer output) a low input impedance would cause the high frequencies
to roll off. This can be an advantage with some microphones!
If the microphone has an electronic circuit output, loading this with a low impedance will stress
the mic amplifier, causing slew rate and compression. A high input impedance allows the
microphone to “breathe” and give of its best, particularly advantageous with very high level
percussive sounds.
If it is desired, nevertheless, to present a lower input impedance to the microphone, we suggest
that an XLRF and XLRM assembly be made up with the desired resistor value connected between
pins 2 and 3. (If there is sufficient demand we may be offering this as a catalog item later)