SL-1176-KIT

GROUND SCHEME

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 ground traces. 

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 through. 

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:

  • The XLR ground wires
  • The AC power receptacle’s 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 point.
  • Use a green wire for all ground connections
CENTRAL GROUND POINT
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/

FIGURE-1a: 
Part number 10329-Short (toothless)

FIGURE-1b:
Part number 10329-Long (tooth)
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.