SCHOEPS
GmbH · Spitalstr. 20 · D-76227 Karlsruhe (Durlach) · Tel: +49 (0)721 943 20-0 · Fax: +49 (0)721 495 750
www.schoeps.de · [email protected]
10
”Ground-Lift” Switch
Unit without
ground-lift switch
Conductive chassis
Conductive chassis
Circuit
ground
Hum current
Connection by
way of the house
AC safety ground
VSR 5 U
1
2
2
3
3
Connector
housing
Circuit
ground
I
Cable shield
The red dashed line is a ground
loop. As in the windings of a
transformer, magnetic fields--
either from power supply trans-
formers or due to differences in
chassis potential among con-
nected items of equipment –
cause current to flow at 50 or
60 Hz.
C*
Ground-lift
switch
”GND”
”LIFT”
”HI-Z”
1
* Bypass capacitor for RF; the
SCHOEPS
K EMC _U cable also has bypass capa-
citors in its XLR plug
connector housing must be interrupted, and
this can again lead to problems with RFI.
To avoid both RFI and ground loops at the
same time, we recommend the use of the
SCHOEPS
K EMC _ U cable. In the connectors
of this cable there is no DC connection
between pin 1 and the housing; thus no
ground loops can occur. RFI on the other
hand is conducted to chassis ground through
condensers built into the plug connector.
This provides RF protection despite the lack
of a DC connection from pin 1 to the chassis.
So that the ”Ground-Lift” switch does not
create a ground loop, do not set it to the
”GND” position when this type of cable is
being used. The ”HI-Z” position of the
Ground-Lift switch is designed to bridge the
difference in ground potential among pieces
of equipment that are connected to power
outlets some distance apart, without causing
hum problems.
Safety is maintained by keeping the chassis
of the VSR 5 connected to AC safety ground
regardless of the ”Ground-Lift” switch setting.
Avoiding ground loops
The greater the size of the loop encompassed
by the relevant equipment, the greater the
effective AC magnetic field radiated by trans-
formers or electrical conductors, and the
stronger the induced hum is likely to be. Thus
to the extent possible, all interconnected
equipment that uses three-wire AC power
cords should be powered from the same out-
let. This will minimize the ground loop area
and thus the resulting hum.
The origin of ground loops
