Polk Cobra: Amplifier Killer? (Part 3)

 Damaged Enamel & Shorted cable

Let's now explore another source of  trouble with  Cobra cables namely damaged enamel and shorted cable. 

Damaged Enamel :

As explained in the post on Cobra cable structure, there is no insulator separating the positive and negative strands other than the enamel. In the Litz winding, they crisscross each other all along the length of the cable. Enamel is reliable in static environment like inside a transformer winding where the cable is wound once and not flexed/moved again (other than vibrations). But a speaker cable faces an entirely different situation.

• Are connected and disconnected numerous times.
• Maybe flexed, moved, twisted, bent, pushed or pulled in order to fit equipment position/rack.
• May be stepped on (hopefully accidentally, not in a Hulk state of mind).
• Exposure to solvents such sprays, cleaners, enamel remover formula.

Personal experience: Circa 2016, when I purchased my first 25ft pair of Cobra, I tried various ways to de-enamel the cable to find the best way. One of the ways I tried was by dipping the stripped end of cable (about 2 cm) in a commercial chemical stripper. The process was slow as the stripper was slow to etch the parts of the strands that was in close contact with each other (with 72 strands of cable, most of them are. After the process was completed, I cleaned the stripped ends with mineral spirit and alcohol and tested the cable for short. It tested good. I left it over the week (because I ran out of time I did not install them in my system. Lucky me). The following weekend, I tested the cables again and found that it was shorted. Interestingly the enameled strands inside the jacket looked intact. I tried cutting off the ends by 10cm lengths (and test for short) and found that the enamel was compromised for about 20 cm above the stripped part. I think the tight winding of the cable acts as a capillary to draw liquids into the cable jacket and the fumes/vapor does the rest. In the end, I cut off another 80cm and scrapped it, just to be on the safe side. I learned two lesson from this debacle:

1. Always test Cobra cable for shorts just before use. 
2. Do not experiment with long cables. After this debacle, I made it a habit to cut short 10cm pieces for experimentation.
   

Shorted cable:

The weakest part of the cable after de-enamel and soldering is the part where the soldered part joins the enameled intact strands.

•  Soldered part is solid but the enameled strands are flexible. Needless to say, this type of  joint is always weak.
• The enamel just above the soldered part is partially damaged/cracked due to heat.
• The individual copper strands inside the enamel are hair thin and can easily break at the joint.

Some of the strands may break from the soldered end/connector end. These strands may make contact with the opposite connector. If this happens with copper colored strands it is noticeable but loose green strands are not easy to see. A single strand can create a short!

Personal experience: I made my first experimental Cobra Interconnect Cable in 2018. Sometime later, that cable caused humming problem. I dismantled the RCA connector and found that a single strand had broken loose and was touching the other terminal. This cable, was in and out of systems/ components  numerous times (it was experimental) and all that strain must have caused the strand to break loose.

If you are terminating Cobra cables, use some heat-shrink tube to protect this part. If your cable already has a heat-shrink/jacket to protect this part, do not remove it. 

May I refer to my post titled "Caring for Polk Cobra Cable" for further tips on preventing problems.

May you always find joy in music.