Aurora Audio GTM822-2GN - Manual (Page 10)

There are external adjustments accessible on each channel via multi-turn trim
pots above and below the stepped input gain control at the top of each channel.
The lower preset trim pot sets the 0VU reference level of the LED VU display.
It is factory set at 0VU = +4dBu = 1.228 volts ac when measured at the pre-
fade insertion return.
When setting the meter calibration point it should be noted that, in order to
produce a seamless bar of light, the LEDs go through stages of bright up. The
0VU LED will begin to light dimly at around -0.5VU = +3.5dBu
(approximately 1.159 volts ac) and will be brightest at 0VU = +4dBu
(approximately 1.228 volts ac).
The upper preset trim pot sets the channel gain and the 0dB reference point of
the channel fader. If the stepped gain control is set at 0dB and a suitable level
(e.g. +4dBu) is applied to the input XLR, the signal at the output XLR should
be the same when the fader is at 0dB… either at the 0dB mark on the rotary
fader or the 0dB point on the remote linear fader. The trim pot has around
1.5dB level in hand for gain and as much as 6dB loss available. The adjustment
should always be as close to unity gain as possible across all the channels to
provide for a well balanced mix.
In the event that you need to check an output level or calibration point and
have minimal test equipment, you can use a digital multimeter to good effect if
you apply the following formula…
To convert dBu to volts, volts = antilog (dB/20) * 0.775
e.g. volts = 10^ (4/20) = 1.584 * 0.775 = 1.228 volts
Generally the user can ignore the third decimal place as this will be beyond the
accuracy of the multimeter. Two decimal places will be accurate enough for
most purposes.
The GTM mixer is built on modular principles and the internal modules
can be removed and exchanged with minimal effort and no special tools
or soldering. In the unlikely event of a fault occurring our distributors
will have stocks of the spare circuit board assemblies and these can be
fitted by their service departments with minimal delay or turn-around
time .
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