Polk Cobra: Amplifier killer? (Part 2) Updated 25 Aug 2021

 

Polk Cobra: The Amplifier Killer? Part 2.

Polk Cobra cables have garnered a reputation as "Amplifier killers". In part 1, we looked at the original long leads that were easily shorted as one possible cause. In this post we will look at the original lengths these cables were supplied with.

I own/have owned 12, 15, 20, 25 and 30 feet (3.6, 4.5, 6 & 7.5 m) long pairs of Polk/Monitor Audio/Super speaker/OTK. All of these original lengths are longer than the usual 3, 5, 6 or 8 feet (1, 1.5, 2 or 2.5m) that most modern cables are supplied in. To make matters worse, Polk cables are VERY difficult to be cut and terminat. Understandably, they were never supplied in rolls like many modern cables, to be cut and custom terminated.

Taking the extreme example, the 30 ft Cobra cable will have a Capacitance of 15nF! The unlucky amplifier connected with this cable is driving a variable impedance (speaker) in series on both sides with 15nF capacitors. Hence the joke: “you want to try a Polk Cobra cable? Just buy a lamp cord and solder 10nF capacitors.”

While this capacitance does not have much impact on the tube amplifiers, Solid State amplifiers with their low output impedance have a tougher time. This explains some of the amplifier failures associated with Polk Cobra cables. Early (60-70’s) SS amplifiers with lots of feedback, oddball designs with no output protection circuits (like NVA), and amplifiers designed for specific values of Cable Capacitance and Impedance (like early Naim) are susceptible to catastrophic failures with long runs of Polk Cobra cables.

The intriguing question is why did the designer specify cables long enough for concert halls and palaces? The answer is found (partly) in the following phrase taken from the labels of my 25 ft pair.

“Low inductance speaker cables improved substantially sound quality both of high and low frequency ranges and their 9.15 Ω impedance helped optimistic transfer of electrical energy from amplifier to loudspeakers.”

Of course, the first part is what makes for the Cobra sound like it does. The “optimistic” (optimal) in the second part is interesting. I think what the designer tried to do is to create cables that match speaker impedance so that there is no signal bounced back due to the difference in impedance at the cable-speaker interface. So, one should use a 30ft Cobra cable for a Rogers LS3/5 15 Ω speakers. Obviously, shorter lengths are more suitable with 6 or 8 Ω speakers. The great Nelson Pass touches on this in his analysis “Speaker cables: Snake oil or Science” that I have referenced in one of my posts. (Mind you, we are not talking about ordinary snake oil, but Cobra oil, Hahaha).

Solution:

1.   Most tube amplifiers (OTL, SET & PP) are safe even with longer Cobra cables. I used a 25ft pair with my Audio Innovations series 500 (EL34, Push-pull Tube amplifier) for a year without any problems till I learned how to terminate the Cobra cables.

2.   Cut and terminate the cables to ‘normal’ lengths of not more than 2.5m (capacitance of 3.7nF max) with SS amplification. When it comes to speaker cables, especially Cobra cables, shorter runs are always better. Shorter lengths of Cobra cables are safe with most moderns amplifiers (see exceptions in my post on Polk Cobra-Amplifier Compatibility).

3. If you are planning to use long runs of Cobra cable with SS amplifiers;

1. Get Polk branded round version with AIM (Audio Interface Module). 

or

2. Check the circuit/design of the amplifier.

To give you some peace of mind, I have tried 25ft (7.5m) pair without AIM with my beloved Accuphase E206 and Linn Classik without problems for a few hours. But this was only on experimental basis. Eventually I learned to cut and terminate the cables and now use nothing longer than 2.4m (8ft.

Ideal solution:

·         In my thinking the ideal cable is a “Zero cable”: zero inductance, capacitance and resistance ( resulting in zero impedance). Of course, this  is impossible in the real world. One can get closer to the ‘zero cable’ ideal by using Polk Cobra cables in the shorted parallel configuration. Please see read my post on Polk Parallel. Ideal, but expensive.