Aurora Audio GTM822-2GN - Manual (Page 6)

In every case, the headphone level control should be set to zero before the
headphones are plugged in and then gradually increased to a
comfortable listening level. High levels can permanently damage your
hearing (and wreck your headphones)!
The Sifam VU meters are electronically buffered so that their internal rectifiers
do not affect the distortion specification of the GTM. The 0VU is +4dBu =
1.228 volts ac. All three analogue meters have internal LED illumination so no
bulb changing is required.
The phase meter provides constant confirmation of the mono compatibility of
the stereo mix signals. Out of phase components in the mix signal can spoil the
sound of the stereo image as well as causing cancellations in the mono
derivatives. In practice the meter will normally swing from center to about two
thirds full-scale deflection on the right. The deflection is proportional to the
phase difference between the two outputs. A hard left deflection indicates a
signal 180 degrees out of phase.
The phase meter circuit comprises of two very high gain pre-amplifiers that
“square” the input signal and drive a sum and difference network. Individual
output amplifiers buffer the two mixes and propel the center zero needle to left
or right of center depending on which amplifier has the stronger signal. If the
signal is comprised substantially of in-phase components the indicating needle
will be driven to the right (“+” region) and will ride on top of the signal,
fluctuating constantly as it checks phase at the varying input levels.
The phase meter will only measure the phase integrity of a stereo signal.
If one or both signal paths are not present the meter may indicate either
in or out of phase depending on whether the sum or difference meter
amplifier is the one fed with the arbitrary signal. Generally, unless there
is a signal on both outputs, one should not be concerned if the meter
reads a small degree of “―” reading or full “+” reading.
In the event that the phase meter indicates a significant anti-phase reading
(substantially to the left of center) then the user should use the channel solo and
mute buttons to locate which channel is carrying the error signal.
Try either muting individual channels or soloing pairs of channels until the
“bad phase” channel is identified.
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