GENERAL
1) The 414A chassis should always be earth-grounded (through the third wire
in the power cord or through the rack) for maximum protection from
shock. Float this ground (to reduce hum) only as alast resort.
Because it is not always possible to determine if the equipment driving or being
driven by the 414A has its circuit ground internally connected to its chassis ground
(which is always connected to the ground prong of the AC line cord, if present),
and because the use of the AC power line ground often introduces noise or other
imperfections such as RFI, hum, clicks, and buzzes, the wiring techniques in the
diagram are not universally applicable.
If you follow the diagram and hum or noise appears, don't be afraid to
experiment. If the noise sounds like alow-level crackling buzz, then there probably
isn't enough grounding. Try connecting the (-) input of the 414A to achassis ground
terminal on the barrier strip and see if the buzz goes away. You can also try
strapping the 414A's chassis and circuit grounds together (on the barrier strip) to
see if this helps.
Aground loop usually causes asmooth, steady hum rather than acrackly buzz. If
you have aground loop, you can often break it by disconnecting the jumper
between circuit and chassis grounds on the 414A's rear-panel barrier strip. In either
case, think carefully about what is going on, and keep in mind the general
principle: one and only one circuit ground path should exist between each piece of
equipment! (Bear in mind that the circuit grounds of the two channels of the 414A
dual-channel unit are connected together internally, and could conceivably introduce
aground loop if you do not take this connection into account in planning your
wiring.)
Wiring The 414A With Single-Conductor Shielded Cable: Sometimes, particularly if
you are using the 414A with musical instruments or home-type equipment, single-
conductor shielded cable may be the only type immediately available (as opposed to
the preferred two-conductor cable). In this case, connect the inner conductors of
the shielded cables to the (+) sides of the 414A inputs and outputs. Connect the
shield of the 414A input cable to the (-) input, and connect the shield of the 414A
output cable to the (-) output on the rear-panel barrier strip.
The shield will ordinarily receive chassis ground from the external equipment
which it is connecting to the output of the 414A. The chassis ground/circuit ground
jumper on the rear barrier strip of the 414A should be left in whichever
configuration gives minimum hum or buzz. To minimize hum or buzz, it may be
necessary to jumper one or more shields to chassis ground. Because use of single-
conductor cables virtually eliminates any possibility of carefully controlling the
system grounding scheme, it is NOT RECOMMENDED! Even so, it often does work
adequately.
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