Three Types of Cobra Cables (that I know of) updated 4/Sep/2021

Three versions of Polk/Monitor Cobra cables

The following table summarizes physical and electrical properties of the three versions of Polk Audio, Monitor Audio branded and non-branded (SHF-SP/Super Speaker Cables).

Photo 1: At the Top Monitor Audio (flat ribbon), middle Oval (like USB type C) and bottom Round (Polk Audio) versions. 

 

Names/Labels	Polk Audio Cobra or None	Monitor Audio Super or None	SHF SP Super Speaker or None
Picture	Photo 1- bottom	Photo 1-top	Photo 1- middle
Cross section	Round	Flat ribbon	Oval (like USB Type C)
Sectional diagram	Fig 1	Fig 2	Fig 3
Dimensions (mm) (Outer)	Diameter 7.3 mm	11 x 2.2 mm	9.8 x 3.2 mm
No of strands	12x6 (72) Copper and 12x6 (72) Green
Damper	Black foam or Clear Silicone rubber cylinder	None	Thin layers of nylon/polyester film
Inductance per m (micro H)	0.1 (measured);  0.15 (specification) (this maybe due to low accuracy of my DIY grade meter)
Capacitance per m (nF) (measured)	1.55 without AIM (see note below)	1.84	1.6
	43 nF with AIM
Current flow	Spiral (solenoid)	Zig-zag (strands double back at the edges	Spiral (flattened) Like a flattened solenoid
Original lengths (ft)	12, 15, 20, 25,30	12,15,20,25,30	12,15,20,25,30
Packaging	Polk Audio box packing	Plastic sleeve	Plastic sleeve
Distinguishing mark	Polk audio Analogue Interface Module	Monitor Audio marking on the cable.	No marking or SHF-SP Super at ends
Reviews available on-line	Arthur Salvatore's http://www.high-endaudio.com/RC-SpkrCab.html	None	Audiofi review of Accuphase E-470 using Polk Cobra Cable E-470 review
Technical parameters & analysis	Nelson Pass analysis from 1980 Science or snake oil?
Sound	Theoretically the best (symmetrical geometry)	Very slight loss of clarity (Verified by professional musician test subject)	Same as round version. Verified by professional musician and reviewer test subjects

 

 

 

Note on cross section diagram: This is a simplified view. In reality, the Green and Copper colored strands cross each other at right angles, all down the cable.

Note: As you can see, the Round and Oval versions are quite similar but the Flat (Monitor Audio) version is different.

Note: I have included only cables that I have seen/bought personally. Other labels/markings and lengths do pop up on auction sites. Dear readers, please add comments if you have additional information. I will update my post and acknowledge your contribution.

Note: Ironically the only modern professional review available on the internet that mentions Polk Cobra is that of an Accuphase E470 Solid State amplifier, not a tube or SET amplifier (https://audiofi.net/2018/09/accuphase-e-470-integrated-amplifier-one-for-a-lifetime)! Cobra cables (Oval version) are mentioned in the last paragraph before conclusion. (BTW, I supplied the cables for that review. Eventually, the reviewer acquired a pair of Round version of the cable. He also found no sonic difference between the Round and the Oval versions. The other reviews of Cobra cables must be before the time of the internet! 

Note: Credit is due to Arthur Salvatore for keeping the flag flying for Polk Cobra cables. If it was not for a casual mention is a DIY forum discussing Litz winding (that went something like "if you are interested in the ultimate Litz cable, forget about winding it yourself, just get Polk Cobra") and Arthur Salvatore's blog I would still probably be trying to wind my own DIY Litz cables. Come to think of it, I still have a roll of magnet wire in my DIY cabinet.

Note: The Analogue Interface Module (AIM) is a clever filter designed by Mathew Polk to prevent the high capacitance/low inductance of the Cobra cable from damaging the amplifier. A 47nF capacitor and a resistor connected in parallel between the positive and negative terminals to prevent oscillation (according to the article by the great Nelson Pass).

Note: The information on this blog is in addition to what is available at Salvatore’s high-endaudio blog.

Manufacturer: I have not been able to ascertain name of the designer and manufacturer that made these cables in the 70-80s. Here is an excerpt from The Absolute Sound article by famed reviewer Dick Olsher.
''The Japanese were apparently first to commercialize a Litz-wire speaker cable, possibly based on the work of Kaneda and Egawa. Imported by Polk Audio circa 1977, it became recognized as the first high-end cable design and was commonly referred to as “Cobra cable” due to its distinctive appearance. It was constructed of two bundles of Litz wire (colored green and copper), one for each cable polarity, which were intimately woven together around a plastic core. Such a geometry drastically minimized cable inductance since the induced magnetic fields around the negative and positive conductors were in the same direction but in opposite polarity and largely canceled each other out. The resultant inductance was only 0.026µH/ft.—an order of magnitude lower than that of 18-gauge zip cord. The downside was a massive increase in cable capacitance to 500pF/ft. or almost 20 times that of 18-gauge zip cord. That didn’t sit well with marginally stable solid-state amps, which simply blew up when smitten by the Cobra’s capacitive venom."