Aurora Audio GTQC User Manual

AURORA AUDIO INTERNATIONAL

1520 NORTH CAHUENGA BOULEVARD

HOLLYWOOD, CA90028

EMAIL: [e-mail] Telephone: [phone]

Web Site: http://www.auroraaudio.net Fax : [phone]

GTQC Instruction Sheet

Introduction:

Well done for purchasing a hand-built, all discrete class A circuitry GTQC channel strip!
It is designed to give you years of superb sounds and service.

Unpacking Instructions:

Carefully remove the unit from the custom foam packing and fit the fuse into the fuse-holder. For best
noise performance always use a grounded 3 core / 3 pin a.c. cord. If you connect the GTQC to an
unbalanced source or destination, either use the ¼” jacks or wire to the XLR pins 2 and 3 only. Leave pin
1 high. This will help prevent ground loops.

The rear jack socket behind the pre-amplifier is mono and should be connected with a mono jack. If a
balanced TRS jack is plugged in, there will be a 6dB loss as there would be no signal on the ring contact.

The rear jack socket behind the compressor is also a mono jack. This is the side chain linking connector to
enable the GTQC to link to other GTQC compressors and link their side chains together for multi-channel
compression.

Wiring of this connector is tip = dc control volts, and sleeve = 0v (ground) common.

Operating Guide:

1. Preamplifier and Equalizer

The GTQC 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.

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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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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!

Depending on the reference level recommendations of the A to D manufacturer, the analogue levels on its
input should be typically around –18dBf. This will optimize both the signal to noise ratio and the
headroom of the analogue signal. A degree of variance, say –16dBf, is acceptable but higher levels will
begin to degrade the analogue performance with no improvement to the quality of the sound.

When using condenser microphones requiring +48v, be sure to turn the volume control down before
pressing the 48v switch as applying this voltage may involve a switch on thump, especially if set to high
gains.

2. Compressor Details

The compressor can either be used on its own or in conjunction with the preamplifier + equalizer

The drawing above shows the GTQC compressor section.

The five control potentiometers function as follows :-

NB There is one further small detent at either end of the pot rotation.

Threshold (Grey knob) :- A 17 detented potentiometer with a total range of 40dB, i.e from -10dBu to
+30dBu.

Ratio (Grey knob) :- A 17 detented potentiometer covering a range of ratios from 1.5:1 to 10:1.

Attack (Blue knob) :- A 17 detented potentiometer covering attack times from 500 microseconds to 5

milliseconds.

Release (Blue knob) :- A 17 detented potentiometer covering release times from 100 milliseconds to 3
seconds.

Gain (Red knob) :- A non-detented potentiometer covering gains from 0dB to +20dB. The silk screened
dB incremented scale is accurate and allows for fine level adjustments.

The eight illuminated switches are as follows :-

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