Rupert Neve Portico 5012 - Duo Mic Pre-Amp - Manual (Page 6)

5012 DUO MIC PRE
SPECIFYING THE NOISE PERFORMANCE OF THE RND 5012
Noise: is specified in 3 ways:
With Gain at Unity Better than –100 dBu
With Gain at 66 dB Better than –62 dBu
Equivalent Input Noise Better than –128 dBu
Decibels are used to express relationships and to make comparisons. One could state that amplifier
“a” has 3 dB less noise than amplifier “b” but unless you have a point of reference for one of the
two amplifiers, you would not know how good they were by comparison with other amplifiers. “a”
and “b” might both be pretty bad amplifiers but you would only know that “a” was less bad than
“b” by 3 dB (dBu is derived from the original telephone standard, dBm indicating 1 mW of power
in a 600 ohm circuit. This worked out at 0.775 Volts. However, today we use many different
impedances in Pro Audio. The term dBu indicates the same 0.775 Volts RMS as a voltage level, not
a power level.)
There are several types of electrical Noise but here we are mainly concerned with Noise from the
devices used in amplifiers plus Noise from circuit resistance that is heard, ideally at an extremely
low level, as a “hiss” accompanying the music signal.
Thermal or Johnson noise results from the Brownian motion of ionized molecules within a
resistance. Thermal noise is entirely fundamental and cannot be eliminated.
Present day microphones are designed to have a source resistance (more correctly, impedance) of
between 150 and 300 ohms. So even if there is no sound input to the microphone, its own source
resistance will be generating Noise that depends on its resistance.
The amplifier to which the microphone is connected also generates noise from its amplifying
devices and circuit resistances. When these two noises – the external and the internal noises are
added together, we have an “Equivalent Input Noise”. A microphone signal is often at a very low
voltage level and needs a considerable amount of amplification.
Assume that we provide 66 dB of gain:
Noise from the input, measured at the output of the amplifier is now, obviously 66 dB greater than
at the input. This output noise is relatively easy to measure but, as we’ve seen, it consists of the
microphone source resistance noise plus the amplifier noise. Taking gain into account, this is the
Equivalent Input Noise (or E.I.N.). But, as I have tried to show, it does not really tell us how good
the amplifier is.
Fortunately if we know all the factors that govern Thermal noise, such as Temperature, Bandwidth
and, of course the value of the Resistance itself, we can calculate Thermal noise very accurately.
So with this figure calculated, any difference between our measured noise and the Thermal noise
must be amplifier noise.
Most designers of good microphone amplifiers just quote the E.I.N. because this figure is often so
close to the calculated resistor noise that amplifier noise must obviously be very small indeed.
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