Polk Cobra, the amplifier killer? Part 1 (updated Mar 2022)

 Oh my, these Cobras can kill!

I just typed 'Polk Cobra Speaker cables' in my google search (31/1/2021) and the got the following results in order of ranking. Arthur Salvatore's high-endaudio blog followed by two (eBay/hifishark) ads and the fourth result was "I hooked up the dreaded Polk Cobra Cables" (my emphasis). Forum comments on Polk Cobra cables are replete with phrases like "amplifier killer", "only suitable for Tube amplifier", "fries your SS amplifier", "be warned", "high capacitance" etc.

I will try to debunk this myth that the high capacitance of these cables caused all those failures. I write this post with a foreboding that I am going to get flamed by some 'Polk Cobra experts' and past users. Well, bring it on, my Hifi-Brothers.

Cobras did not kill my SS amplifiers, even six of them together!

When I got my first 25ft pair of Monitor Audio Super Speaker cables (Polk Cobra, UK version), I was very careful to use it only with my Audio Innovations Series 500 tube amplifier. You see, I had read and believed the 'amplifier killer' story. After I learned how to terminate the cable, I tried my first 2.5m pair with my Tripath test amplifier (cheap enough to take risks) and was surprised that nothing was damaged even after 3 hours of operation. Then I gingerly tried it with my 1989 Accuphase  Integrated amplifier. I went slowly and carefully, and again was surprised that nothing was damaged. The amplifier sounded wonderful. So I made another pair for my Accuphase. Couple of months later, I got braver and tried the Cobra with my Linn Classic all in one player and again was pleasantly surprised by the good sound and lack of misbehavior from the amplifier. That got me thinking.

 

Then I read the detailed high-end cable analysis done by the great Nelson Pass (I will publish that article in another post. See the wonderfully flat impedance curve of the Polk Cobra in that analysis!). "Papa" Pass shares his experience that his Threshold Stasis amplifier kept blowing its fuse with Cobra cables. He traced it to the high capacitance causing oscillations and solved it by adding a small capacitor across the cables. He notes that Mathew Polk found the same problem with an Electro-research amplifier and solved it by adding Zobel damper circuit (mafe of 0.047uF capacitor and 6Ohm resistor) in parallel with the speaker. By the way, the Polk Cobra cable did not 'kill' or 'fry' either amplifier. It just blew the amplifier fuse. It's interesting to note that Mathew Polk took the trouble to modify the Cobra cable to make it safer, rather than reject it at the first sign of trouble. Remember that Polk makes speakers, NOT cables. The Cobra cables (Super Speaker cables as labelled) must have sounded really good for him to take such a risk!. It is also interesting to note that the combination of high capacitance and ultra low inductance is the real culprit rather than just high capacitance. And it was solved by adding capacitors!

After this, I really got bold. I tried it with my Vintage SAE A202 power amplifier with excellent results. An audiophile friend wanted to try it with his Plinius pre-power combo in spite of my dire warnings. He said, "if it works with your Accuphase, it should have no problem with my Plinius". He found that the Cobra was better than his expensive Siltech cable. In 2017 I purchased my best (and most expensive) ever amplifier, The Absolute Tune by Einstein Audio (Germany) that uses a tube preamp stage and an innovative transistor Circlotron power section. To be sure that this rare topology will be okay with my Cobra cables, I wrote to Einstein Audio and received assurance that the capacitance should not be a problem. True enough and the best sound ever in my humble home! 

When I made the first six parallel pair (thank you Sean), I gingerly tried it (with sky high capacitance) with my Accuphase and was surprised again, that nothing blew up even after an hour and was pleased to note that my Accuphase bass got even more emphatic (maybe that says more about Accuphase reliability rather than Cobra cable per se). And finally, I have been using Polk Cobra 2m pair with my vintage SAE model XXX1B (early 70's) for more than a year now!

I now use a 1.8m Shorted Six parallel pair in my main system to drive Spatial Hologram M4 Turbo speakers. I switch between three amplifier combos namely Mingda Piccolo KT90 Single Ended Integrated, GAS Thalia-SAE A202 and Opera Consonance Pre- DIY Firstwatt Clone.

So, the Cobras bite but don't kill?

You may have read in some forums "Polk Cobra cables fried my amplifier". But my experience is that Cobra cables (even 6 parallel Cobra) is safe with well-designed (both vintage and modern) SS amplifiers. How to make sense of this? Here is my conclusion (assumption is more accurate as I didn't witness this personally).

Yes, the Cobra cables killed many SS (and Tube) amplifiers. Some of those failures were caused by poor amplifier design interacting with high capacitance/low impedance of the cables. But many more were damaged by accidental shorting of the amplifier due to the poor design of the Cobra original leads. Bear with me and read on.

The length of the original exposed leads are more than 3cm (see picture below). The distance between positive and negative terminals of most amplifiers are 1-2 cm. Some of the Vintage amplifiers and modern cheap amplifiers have really close (designed for disaster, anyone?) speaker terminals. What this means is that it was easy to short out the positive and negative speaker terminals when the Polk Cable are used with its original leads.

Leads are longer than 3 cm.

 

Now, I would like you to think back to the 80-s when these cables were first marketed. The battle was between 'orange globes' making liquid, syrupy music and 'dancing green/orange' needles making gut-thumping musical vibrations being played out in dimly lit audio showrooms (Tubes vs SS in short). Just imagine the poor salesman switching to Polk Cobra cables in the semi-darkness (before the time of mini-flashlights and mobile phone lights). Some of them must have shorted the cables and killed amplifier to his own and the customer's shock. And what do you think the poor hapless salesman tells the Store Manager? "Peter, I accidentally shorted speaker terminals and killed the amplifier."?. Very unlikely! He probably said "Peter, these Cobra cables are no good. Everything was fine until I switched to these cables". And, needless to say, the customer who witnessed this disaster will remember this for ever and share it ad-infinitum. 

My Topping Tripath mini amplifier. Its so easy to short it with Cobra!

Even with my full size Yamaha amplifier, the leads can be easily shorted.

The other contributing factor was the many poorly designed SS amplifiers in the market during this time as designers were trying to get higher and higher power ratings and lower distortion by using copious amounts of negative feedback. Some were designed without adequate protection circuitry to improve sound quality (like Naim), to cut cost or due to plain ignorance. Some of these amplifiers did fail due to the Cobra cable capacitance/inductance but it is fair to think that they would have failed anyway.

So you see, the Polk Cobra cables can be "amplifier killers" but it may have been due to poorly designed Cobra leads and poor quality amplifiers rather than any electrical property of the cable!

I have traded these cables for years (I once had 400 feet of these cables in my stock) and the only known case of amplifier damage by Cobra cable sold by me was caused shorted cables on a vintage tube amplifier!

So, how careful must one be with Cobra cables? The following is for Cobra single run. For a discussion on Cobra parallel, please read my other post.

• Shorten or cut-off (and re-terminate for better sound) the long leads before use.
• With any kind (SET, PP etc) of Tube amplifiers, it is generally safe to use Cobra cables. 
• Cobra cables are great with OTL amplifiers (high impedance of the cable helps the performance of the amplifier).
  
• Most modern (post 70's) Solid State amplifiers (except for some odd ball designs like LFD, NVA, older Naim) will be okay with single runs.
• Do not try Polk Cobra cables with the following amplifiers:
• Naim: especially early Naim that specifies NACA4/NACA5 speaker cables. I am not sure how modern Naim amplifiers will behave with Cobra cables. I do not know of anyone who successfully used Cobra speaker cables in a Naim system. So, best advice is 'Naim-fans, stay away'.
  
• NVA & LFD and similar amplifiers.
• Early Threshold Stasis and Electro-research amplifiers.
  
• If your amplifier manufacturer specifically discourages Polk Cobra cables. Take it with a pinch of salt though. Most manufacturers will 'automatically' discourage use of Cobra cables due its bad reputation and due to the fact that anything 'DIY' is anathema.  
  
• Well designed SS amplifiers even from 70's (classics now) will be OK.
• If you are a technically minded DIY-er, study the circuit diagram of your (vintage) amplifier before using Cobra cables. The problem usually is high-frequency signal bouncing back from the cable-speaker interface and feeding the negative feedback circuit which causes oscillation.
  
• If you are a non-technical audiophile here is how to test whether the Cobra cables are safe with your amplifier. But proceed at your own risk.
  
1. Get a full frequency sweep (20Hz-20KHz) track (I use the Full glide from Cardas-Ayre Irrational But Efficacious CD).
2. Play the frequency sweep through your system with your usual speaker cables. Listen to ensure no abnormalities.
   
3. Install Cobra speaker cable in the system.
   
4. Turn on the system. Listen for any abnormality at zero volume. Increase volume (with no input) to 12 position and listen for any abnormality for at least 30 s.
5. Play the frequency sweep at very low volume. Sit close to the amplifier and listen/watch for any abnormality. If some thing does not sound right (especially high frequency sound), reduce volume and turn off amplifier immediately.
   
6. Repeat with progressively higher volumes (in small steps). If you hear any abnormal sound, stop immediately. Be patient and go slowly.
7. Continue till you listen to at least two positions higher than your normal listening volume. If you normally listen at 10 position, got to 11 and 12 0'clock positions.
   
8. If no abnormality, delight in the Cobra cable.
9. If something does not sound right, at any point in the above steps STOP and do NOT use Cobra cable.
   

Now folks, bring on the flamethrowers.