The GTQ2 can accommodate any signal you care to throw at it, including high-level line inputs. The
sensitivity switch provides gain adjustment from +80dB to –10dB and the level pot allows for fine
adjustments. The level pot has an audio taper and is –20dB at half rotation. I would recommend using
the level pot between ¾ and full rotation and never below ½ rotation unless part of a deliberate fade. If
you have to turn the pot below ½ way, the sensitivity switch can be adjusted at least 4 clicks less!
Operating the unit in this fashion ensures that you keep the headroom in the designed 26dB region.
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. The same gain structure rules apply.
If a keyboard or similar ac powered device is connected to the DI input and causes a ground loop, make
a cable with a male XLR connector such that signal goes to pin 2, shield to pin 3 and no connection to
pin 1. The GTQ2's transformer balanced input will resolve the ground loop issue.
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 reduces the headroom
.