Aurora Audio GTM822-2GN - Manual (Page 9)

If all the studio equipment is bonded to a clean technical earth there is less
likelihood of any difference in potential = ground hum, occurring. In the event
that a loop exists between two items of equipment one should disconnect the
ground on one end of the interconnecting XLR or TRS cable… not disconnect
the units from the ac ground. Ground/earth is your friend… it helps eliminate
radio frequency issues… and disconnecting grounds is a sure path to creating
circulating ground currents and hum issues!
If a buzz is detected in the noise floor of the GTM it is important to check
external influences before suspecting a fault with the GTM. The most likely
cause of an angry buzz noise, more harsh and aggravating than a normal ac
hum, is the presence in the building of thyristor/triac light dimming devices.
These devices chop the ac current to change lamp brightness and can put
serious interference spikes into the ac supply. I have found this effect to also
occur with expensive Neve
TM
and SSL
TM
consoles and the fix is to replace the
light dimmers with either regular switches or variable transformer (Variac
TM
)
type power controllers. The effect is especially noticeable in buildings that are
shared with other businesses/occupants and the studio equipment is located
close to the main power cable into the building.
If other studio devices are not bonded to a studio technical earth, there may be
issues of ground loops between the GTM and other studio equipment. This can
usually be detected by unplugging all the audio cables and plugging them back
in, one at a time, until the culprit that creates the hum is located.
If this is the case, don’t lift ac grounds, as this is a dangerous electrical safety
practice. Try lifting the shields (pin #1 on XLRs or the sleeve on TRS jacks) at
ONE END ONLY of interconnecting cables and see if this fixes the problem.
Please feel free to contact me for advice if you have issues with mains
hum or RF interference in relation with the GTM.
For the best noise performance a golden rule is strong signal levels and switch
off any sources to the mix busses that you aren’t using. Typical examples might
be channels or effects returns assigned to the busses but not carrying any
signal… the unterminated cables to these channels might well pick up ac fields
into the high impedance inputs.
Always deselect bus switches on channels that are not currently in use to
minimize external noise pick up being added to the mix bus noise floor.
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