There is ONE way to ensure that your SL-1176-KIT
will be as noise resistant as possible and that's to take advantage of
the chassis and make it the "firewall." Whether you choose XLR or quarter-inch
connectors, making sure the signal ground and the chassis ground are ONE
at the point of entry will keep noise power-, radio- and television-related
out of the electronic ground where it can be amplified.
With the exception of speaker cabling,
mic- and line-level audio cabling has a shield, a protective barrier against
stray noises. Those noises need to be dumped at the door on to the chassis
and NOT inside the unit where the noise can contaminate circuit board
While we recommend using XLR connectors,
all-metal quarter-inch connectors make this point more visually obvious.
By mounting them with a toothed locking washer, positive contact with a
metal chassis is ensured even if painted, the washer teeth will break
On a standard 3-pin XLR connector, pin-1
is ground, pin-2 is "signal-hi" and pin-3 is "signal-lo." There's also
a "fourth lug" tied to the XLR body that is grounded when the connector
is fastened to a metal chassis. Again, use toothed locking washers with
mounting hardware (screws and nuts) to insure a positive connection.
Another point is to have low-impedance
ground paths wiring and PCB traces should be as short and as thick (in
terms of gauge) as possible. Even a soldered connection can add resistance
and hurt noise immunity. Keep the wire continuous no breaks to maintain
a low-impedance ground distribution system. For example, the trip from
pin-1 of the XLR input connector, through the 4th lug, all the
way to your chassis star ground point should be a continuous wire run to
the central or "star" ground point.
A good place to make the central "star"
ground is as close to the AC power receptacle as possible. Connections
to this point will be:
CENTRAL GROUND POINT
The XLR ground wires
The AC power receptacles safety ground wire
Power supply ground via an open header on
the power Supply PCB.
Each of the boards with the exception of the
power supply has two pins dedicated to ground at the power header. Pick
an unused power header and run a wire from pin-1 to the central ground
Use a green wire for all ground connections
Drill a hole in the chassis, near the
IEC power input connector, to mount a 4-40 bolt and nut for the solder
lugs as shown in Figure-1a and Figure-1b. Solder Lugs are available everywhere,
the sample images and their part numbers are from www.unimaxsupply.com/.
If you haven't already learned
the trick explained for wiring the power supply transformer, simply mark
the multiple points of contact along the length of the XLR ground wire
an inch wide maximum roll a sharp knife or razor blade around the circumference
of the wire in those two places. Then, cut in a straight line between these
two cuts and peel off the insulation.
If successful, you'll be able to run a
continuous piece of wire to all four XLR connectors tying pin-1 and pin-4
together in the process the end of the wire terminating at the chassis
solder lug near the power input connector. If you find this too difficult,
it's ok to run a separate wire from each XLR pin-1 / pin-4 combo to the
central ground point.