Polk Cobra Speaker Cable User Guide (Updated August 2021)

Introduction:

These cables are rather special because of the design and manufacturing method that is unlikely to be replicated.

·         So called ‘Litz” speaker cables are easy to find. But unlike some so-called ‘Litz’ cables, Cobra is a true ‘Litz cable’ (not just Litz woven cable). The positive and negative strands are braided together without any Teflon (or similar) insulators. The individual strands are electrically isolated only by enamel (magnet wire or Litz wire).

·         The enameling process is world-class. There is no oxidation almost 40 years after manufacture!

·         A single run of Cobra contains 144 woven strands, 72 strands each for positive and negative!

·         The strands are made of extremely thin high purity copper. This seems contrary to what a speaker cable should be (more suitable for interconnects if one really thinks about it). They look thick due to the enamel.

·         One of the weaknesses (in my humble opinion) of the Cobra design is the use of PVC for sheathing rather than Teflon (which has better dielectric properties and is more durable). But the very clever designer might have his/her own clever reasons for using PVC (other than cost). The PVC plastic sheath become sticky and discolour as it ages.

·         The other weaknesses are the long (> 3cm) Nickel plated copper lead terminations. Just like a cheap resistor!

·         Contrary to common belief (and the great blog by Mr Arthur Salvatore) there are three versions of this cable, not two. The round (Polk branded) version is supposedly better (at least theoretically due to the fact that all the cables are equidistant from each other). Please read the post about the three types of Cobra cables. To my ears (and my friends including a professional reviewer) round version and semi-round version sound exactly same. The flat ribbon version (Monitor Audio branded)  has slightly less clarity. But the sound of all three Cobra version can be easily differentiated from other cables.

·         One sought after electrical property of Litz configuration (weave) is its ability to reject external electromagnetic fields compared to most other designs. Just observe the tangle of cables behind a typical HiFi system to realize why this is important.

·         Another reason the Cobra sound so clear may be because they minimize the skin effect due to hair thin conductors.

·         The strands are weaved in groups of 6 that cross each other at right angles. This reduces inductance drastically. They are claimed to be one of the lowest inductance speaker cables ever manufactured.

·         Unfortunately, the grouping of the cables and their closeness makes Cobra capacitance rather high for a speaker cable.

·         Whatever the reason, they sound wonderful (to my ears, anyway), which is amazing considering they were one of the first high end speaker cables manufactured. It is like finding that the Ford Model T manufactured in 1930 is better than the latest Volkswagen Golf (the first versus the most modern ‘People’s Car’)!

·         The Cobra sound? Clarity and transparency are the key words. It preserves the transients and gives one the feeling of listening to live music with real instruments. It is not a warm or smooth sounding cable but a ‘live’ sounding cable. For a much better description of the Cobra sound, may I direct you to ‘Recommended Components’ section of Arthur Salvatore High-end audio column (where I learned about these cables). I am no silver-tongued HiFi reviewer.

 

Amplifier matching:

·         Some electrical properties of Cobra cables are listed below (compared to typical speaker cables)

o    Very low inductance (although theoretically ideal, it reacts with some low output impedance amplifiers)

o    Flat impedance curve (ideal).

o    High Capacitance (not good).

o    Higher than normal resistance (not good).

·         Tube amplifiers: no problems as they have high input impedance. Cobra cables will make tube amplifiers sing like never before. They must have been tested using tube amplifiers of the 70’ and 80’s.

·         Cobra cables in parallel (recommended 6) are breath-taking with well-designed SET Tube amplifiers (especially low powered amplifiers driving High Efficiency speakers).They are pure magic with OTL amplifiers.

·         Vintage SS amplifiers:

o    Poorly designed vintage SS amplifiers cannot handle the low impedance/inductance/high capacitance of the Cobra cables. They may cause the amplifier feedback loop to go into oscillation. You get a blown fuse if you are lucky. Worse, if you are not.

·         Typical modern (post 80’s) SS amplifiers: no problems. I have used single run of Cobra cables with my 1989 Accuphase E206, 1997 Linn amplifier, 1984 SAE model A202, 1977 SAE Model 31B, 2017 Yamaha, 2020 Yamaha and other SS integrated and power amplifiers with excellent results with no problems. Please see my post on all the amplifiers that I have used with Cobra cables.

·         My friends and customers have used Polk Single runs with many SS amplifiers such as Krell, Plinius, NAD, Accuphase, Harman Kardon etc without any problems. Please see my post on all the amplifiers my friends and customers have used with Cobra cables.

·         Warning: Some modern SS amplifiers such as Naim (80-00's), NVA & LFD have low input impedance and/or no protection circuitry in their output stages. DO NOT use Cobras with these amplifiers. Do not use these cables with Vintage Threshold Statis and similar amplifiers.

Cobra parallel runs (standard configuration):

Parallel runs are the ideal Cobra speaker cable solution because it overcomes the problem of higher than normal resistance. It really makes the music ebb and flow by improving bass definition and slam. They are amazingly (and brutally) transparent and neutral. But remember that capacitance in parallel adds up. Cobra parallel while ideal for tube amplifiers are not recommended for Solid State amplifiers. I have tested my Accuphase E206 with 6 parallel of 2.5m for many hours without any problems but I do not dare to make 6-parallel resident in my Accuphase system.

Latest information on Parallel run (Shorted configuration):

See my post on parallel configuration. I have innovated a new way of paralleling Cobra cables that reduces capacitance, making it suitable for both tube and SS amplifiers.

Caring for the Cobra cables:

·         Warning: Any damage to the enamel can short out the cables without warning. The damage may not be visible to the naked eye. If the cables are shorted, your amplifier protection circuit may be activated or your amplifier fuse will blow, if you are lucky. If not, you are looking at extensive damage.

·         Be gentle. Give them some tender loving care and you will find that they are really good and can outlast many system upgrades.

·         Do not pull on the cables when you are removing the cables from amplifier or speaker connections. Hold the banana or spade or soldered section (for bare solder termination) and pull. Pulling on the cables may damage the enamel or dislodge the connectors or worse, damage the enamel. Remember that the weakest part of the Polk Cobra cable is the part just above the soldered terminations (due to heat during de-enameling process).

·         Never step on them, especially with your shoes on.

·         Bend or twist them all you wish but never fold them in two. Sharp folds/kinks can cause enamel damage.

·         If you accidentally cause any kink or visible damage to the cable, take them out and test for shorts before use.

·         Never spray/apply contact cleaners or other solvents directly on the cable ends/ terminations. Due to tight winding, the cable strands act as capillary and will suck in any solvent. Some solvents may dissolve the enamel. If it is really necessary, use Caig Deoxit or similar contact enhancers in liquid form (NOT spray). I once destroyed 1 m length of cable by trying to strip the enamel with chemical stripper and mineral spirit. The cable worked for a week and then suddenly shorted.

·         Do not try to terminate them on your own unless you have done your research and you know what you are doing. Terminating them is tedious, time consuming and hazardous (the enamel fumes is toxic when burned off during the stripping process).

·         When cleaning amplifier or speaker contacts with solvents/spray, first remove the Cobra cables. Wait till the solvent/cleaner is completely dry before reinstalling the cable.

·         It is best to use bare solder terminations with Cobra. But not all binding posts are bare wire/solder friendly. If really needed, use good quality connectors with Cobras. You do not want to clean or handle these cables too often.

Prepared by: Fellow music lover, Muhsin bin Ahamed, Malaysia       email: polkcobrafan@gmail.com

Dated 16 August 2021

“Enjoy the music, forget the cable!”