The impedance switch on the rear panel selects either 300Ω or 1,200Ω input impedances. Most of the time
you will find that the 1,200Ω input works best with dynamic and condenser microphones but very low
impedance microphones (e.g. ribbon type) may work better with the 300Ω input. The 300Ω input provides
6dB additional gain if sourced from a low impedance, but if a higher impedance microphone is used (e.g.
close to 300Ω), the series impedance will create a 6dB attenuator that negates the 6dB gain. The switch
enables the user to experiment with which input impedance best matches the microphone.
Using analogue equipment in a digital world!
E.G. Analogue versus Digital levels
In my technical/design background in analogue circuitry, spanning over 30 years, the levels of audio were
calibrated in dBm, a throwback from the telephone and communications era where 0dBm was 1mW
dissipated into a 600 ohm load = 0.775 volts. 0dBm was later changed for the more convenient 0dBu
which is a voltage into any specified impedance.
In a broadcast studio, Peak Program Meters were used that were calibrated from 2 to 7. Mark #4 equated
to 0dBu and Mark #6 equated to +4dBu. The level +4dBu is 1.228 volts a.c. and also the 0VU reference
point on a VU meter. This is, coincidentally, #6 on the PPM meter and a typical line up level for an
analogue tape machine.
Most consoles and pre-amplifiers have a maximum output level before clipping of around 26dBu. This
gives them 22dB headroom above 0VU = +4dBu. Driving the console and pre-amplifier “hotter” than
+4dBu output reduces the headroom proportionately.
At the other end of the scale, the consoles/pre-amplifiers usually have +80dB gain and produce noise
figures in the –45 to –48dBu region and an Equivalent Input Noise of -125 to -128dBu. The noise floor
from a 200 ohm source at 20 degrees C is –129dBu so the amplifier is adding 1dB of noise to the absolute
noise floor. As the gain is reduced, the difference between the signal and the noise floor widens as the
noise is pushed further down.
Reminder:-
Increasing the gain amplifies the signal AND raises the noise floor.
Running the device at hotter levels than usual also reduces the headroom
.
In the digital world measurement criteria differ. Instead of using a reference level that relates to a
particular power or voltage (like 0dBm) the 0dBf reference is the maximum signal that the analogue to
digital converter can accept before the onset of clipping.
The 0dBf level is usually somewhere in the region of +18dBu to +24dBu in the analogue world…. It is
NOT
the same as 0VU (+4dBu) on an analogue VU meter.
It’s very important to use an A to D input level that maximizes the headroom and minimizes the noise in
the analogue world.
E.G
. If an attempt was made to drive the console or preamplifier high enough to hit the 0dBf (+24dBu)
reference level on the A to D, the amplifier would be running at over 20dB greater than it’s normal
operating level.
This raises the noise floor by 20dB (ten times louder) and reduces the analogue headroom to around 2dB.
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