This month we continue our exploration of interfacing technology by showing you how to wire up XLR connectors correctly.
In the accompanying series of photographs, we used musiflex cable (which has a conductive plastic screen, plus drain wire) because it's particularly easy to terminate. We've also used a two core ('paired') cable, as would be normal for balanced operation, but, of course, the more common single core screened wire involves the same procedures.
1. To begin with, unscrew the XLR, put the fixing screw safely to one side where applicable, and slacken off the cable clamping. (Photo 1) The cable can then be pushed through the XLR's body from the rubber bushing inwards. If it's a tight fit, apply a little vaseline to the inside of the bushing to lubricate things a little.
Next, prepare the cable by stripping back the outer sheath about 25mm (1"). If you're using a cable with a braid screen, comb the braiding with a small screwdriver, so all the strands run parallel. Then twist
the strands tightly and cut off any stray whiskers.
2. The conductors are prepared next: Cut these back to 20mm (¾") overall length, then strip the last ¼" and tightly twist the strands. At this point the drain (or screen) wire isn't insulated, and might come into contact with the other terminals, causing a short, when the plug is assembled. To prevent this, add a ½" length of PVC - or better still, rubber sleeving. After you've insulated the screen/drain wire, tin each conductor in turn. Hold the iron on for a few seconds, then
add a small quantity of solder, taking care not to melt the insulation (Photo 2).
3. Moving on to the XLR insert, we now need to tin the solder buckets (Photo 3). This can prove quite fiddly if you don't have the luxury of a vice or a pair of grips. However, you can always plug the insert into a spare socket; we used a lead tester in the photograph, but don't forget to turn off
equipment before using it as a soldering jig!
The tinning process demands a little practice, as it's important not to fill the buckets or terminals with solder. The best way is to hold the iron on for 10 to 20 seconds, and then apply the solder very sparingly. If the terminal 'floods', tap the insert sharply on the bench to clear it out, and repeat the process until only a thin layer of fresh solder adheres to the inside of the terminal. Some XLR inserts have soft plastic insulation, which melts easily, so it's best to practice first of all on the relatively indestructible Cannon or Switchcraft types.
4. With the insert still in place (in the lead tester's socket in this instance), push the first of the wires into its solder bucket. Until you've developed an instinct for the opposing pinouts on male and female plugs, double check the pin. Are you sure it's the correct one? if in doubt, the pin numbers are usually given on the outside of the insert.
With the wire in place, heat the bucket for a few seconds - taking care not to melt the insulation - then feed in some solder and allow the joint to cool. Take care not to cause a dry joint by shaking the wire before the solder has solidified - you can test for this by tugging on the wire sharply once the joint is cool to touch. If it falls apart, heat up the bucket, tap out the excess solder and begin afresh. Photo 4 shows the last terminal (pin 3) being heated, with the solder standing by.
5. With all the wires in place, the plug is now ready to be assembled. First slide the insulator over the insert. At this point, you may wish to add some Loctite 'screwlock' to the fixing thread to prevent the XLR coming undone at a later date (Photo 5). Incidentally, cheap nail varnish is also excellent for locking small screws in place!
6. Next, slide the parts together. Don't tug on the lead to do this: rather, hold the lead and push
the XLR body over the insert. Again, you may want to use a lubricant on the rubber bushing to make the parts slide easily together. And before the screwlock dries, add the fixing screw. Lastly, tighten up the cable clamping.
If you're using an ITT-Cannon XLR, you may find the clamping inadequate if the cable's thin. In which case, the best course is to build up the cable's diameter with some sleeving, or even with a few turns of a good quality PVC tape.
7. With one end of the cable assembled, it's now time to think about identification, such as colour coding or numbering. The photograph shows a red Hellerman sleeve being applied; these are available in all the colours of the rainbow, plus black and white, from leading studio suppliers. Alternatively, you can use a cable lettering or numbering system - one company, Canford Audio even offers heatshrink tubing with your name printed on it. Whichever system you choose, remember to add the identification before
you solder a plug to the other end!
8. If you follow the guidelines laid down here, and with practice the end result should look like the connector in Photo 8.
Canford Audio Ltd can be contacted at: (Contact Details).