Polk Cobra, the amplifier killer? Part 1 (updated Mar 2022)
Oh my, these Cobras can kill!
Cobras did not kill my SS amplifiers, even six of them together!
So, the Cobras bite but don't kill?
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.
- Get a full frequency sweep (20Hz-20KHz) track (I use the Full glide from Cardas-Ayre Irrational But Efficacious CD).
- Play the frequency sweep through your system with your usual speaker cables. Listen to ensure no abnormalities.
- Install Cobra speaker cable in the system.
- 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.
- 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.
- Repeat with progressively higher volumes (in small steps). If you hear any abnormal sound, stop immediately. Be patient and go slowly.
- 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.
- If no abnormality, delight in the Cobra cable.
- 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.
My system: Cambridge Audio CXN music streamer (music on Western Digital HDD), Musical Fidelity A308 CD player, Krell KCT pre-amp, 2x Krell 350MCx Monoblocks, Revel Ultima Salon 2 speakers. Initially I got the author to make me a 2m pair of Polk Cobras to connect my power amps to the speakers. Worked perfectly. We used spade connectors, so the worry about shorting was minimized. Next he made me a 1m pair to connect the Pre amp to the power amps. These were CAST equivalent cables using LEMO connectors. Again, worked well. Finally got a really short one made to connect the streamer to the pre-amp with XLR connectors. Bottom line, I am VERY happy with the cables. They replaced Transparent Musiclink cables and interconnects. I actually believe the Polk Cobra's have opened up the soundstage, I hear the separation between the highs and mids so much better. There is more clarity in the music. The bass has rounded up very nicely, with no rumbling vibrations which were common with the Transparent cables. Lastly I started to listen to some of my older CD's which used to sound harsh with the previous set-up, but now sounded easy on the ear!!
ReplyDeleteI did make comments in a previous post. But just to add to this article, I changed all my Transparent musiclink cables and interconnects to Polk Cobra cables. My system includes Musical Fidelity A308 CD player, Cambridge Audio CXN streamer, Krell KCT pre-amp and Krell 350Mcx mono Block power amps (2x). The Polk cables worked really well and I am absolutely happy with the switch. Music sounds better with clearer highs and mids, well rounded Bass and overall sounding more open to listen to. I even started listening to some old cd's which I found a little harsh on the previous cables.
ReplyDeleteThank you for your positive comments.
ReplyDelete