Aurora Audio GTP8 - Manual (Page 2)

The D.I. input has 10 Megohm input impedance and around 10dB gain. It can be used (to great effect)
with musical instrument pickups, but works equally well with high-level signals like a D.A.T. or CD
player. If you insert a jack into the front jack socket it will automatically disable the signal from the rear
panel XLR.
Illuminated switches are fitted to select phase reverse and 48v phantom power.
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 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. A microphone normally needing 40dB gain would need 60dB
gain and any peaks would drive both the pre-amplifier and the A to D into clipping. Not good!