This is a recording of the hum present on my G9 mic pre.

The recording starts with the unit turned off, 
it is turned on and warms up,
the Gain control is increased from the lowest setting to the highest,
Output is cranked from lowest to highest,
Gain is turned back to the lowest setting,
Output is reduced to the lowest setting, 
the input, phase and lowcut settings are changed, 
then the connections to the front panel are disconnected and re-connected one at a time in this order: 
TO OUTAMP (at this time the hum increases and noise also increases)
TRANSFO- (at this time the hum almost disappears, signal also disappears) 
TRANSFO+(at this time the hum almost disappears, signal also disappears) 
OUT- (at this time the hum almost disappears, signal also disappears) 
OUT+ (at this time the hum almost disappears, signal also disappears) 
FROM 48V (no change)
then the unit is turned off and cools down.

 

masoste