Building a shielded XLR cable

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Recently I did a shoot in a studio for some interviews as online content for a magazine. I had some issues with electrical interference which I think was caused by the lights inducing interference on my microphone cables. In the event it wasn’t a problem, as the shots in use were too wide to boom, so my wireless mics came to the rescue.  However, I wanted to have a solution for this problem in future, and after a little research I wanted a 5 meter Starquad cable with Neutrik EMC XLR connectors.  I couldn’t find one of these readymade, so I went about making one.

Neutrik state that the EMC connectors were designed to give “enhanced RF screening for critical applications” this is achieved through the use of clever internal shielding, a circular capacitor based LC filter linking the connector shell to the cable shield, and a ferrite bead on pin-1 (ground). The signal conductors are not treated, and therefore there is no effect on signal quality. More info can be found in this PDF.

Starquad cable is an improvement on standard 2 conductor twisted pair cable for balanced signals. By using four conductors tightly wrapped, and wiring the opposite cores in parallel, interference can be reduced by up to 20dB.  More info about starquad can be found in this Sound-on-Sound article.

Starquad Cable

I purchased 5m Klotz Starquad cable and male and female neutrik EMC XLR connectors.

Neutrik provide this useful PDF providing assembly instructions

This is what comes in the packets for the two connectors:

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As in the instructions, the packet also suggests cutting the sheath back 15mm, the braided shield back 5mm and the to strip the individual conductors to 3mm:

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However, I discovered with pairing two conductors from the starquad it was easier to strip the cable longer and twist together along the length. It also helped to leave the shield uncut until later in the process:

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The paired conductors could then be tinned with solder, cut to the required 3mm and soldered into the connector (remembering first to slide the plastic connector bushing onto the cable)

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One interesting point to note is that the klotz starquad has an unconventional colour coding. Starquad is usually either a blue pair and a white pair, or a green-white pair and a red blue pair (it is important to pair opposite conductors in the cable lay, otherwise the interference reducing properties will be compromised). The Klotz cable however, pairs black/blue and red/green conductors.

Once the soldeirng is done, a two-piece metal shield is attached over them.  You can see in this photo how I pulled the uncut braided shield down over the jacket.

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The braid can then be pulled down over the metal:

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and the plastic chuck placed over it (there will be excess shield wire poking out between the metal shield and the chuck and this must be trimmed off):

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The whole assembly can then be inserted into the housing, and the bushing screwed on holding it all together.

Hopefully this new cable will make induced EMI hash from lighting ballasts a thing of the past!

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