Polk Cobra Cable

inspiredsports

I've been in the hunt for 100' of the round (versus flat) version of braided green and copper colored Polk "Cobra Cables".

Due to its braided design it is reported to have the absolute lowest inductance of any cable ever made. I guess this means it is the "fastest" cable ever made and yields the most extended high frequency response.

The wire also has incredibly high resistance which caused problems with solid state receivers/amps of that era. Cobra Cable was marketed in very long single run lengths and there are many stories about how lots of receivers and amps were "blown up" by this "horrible" cable.

The solution invented was to run 6 (or even 8) runs of the cable in parallel over the shortest distance possible to lower the resistance. The strands must be soldered green to green and copper to copper with care taken to never allow even 1 wild filament to stray. The insulation is supposedly toxic when burned so that presents another challenge.

At any rate, a fellow Polkie has been looking out for me and located 100' that I bought a few minutes ago.

There is one particular internet posting I've seen that compares paralleled Polk Cobras with $15K cables !!! Well, I'm going to find out. http://www.high-endaudio.com/RC-SpkrCab.html

I will be posting the project images and progress here.

mhardy6647

umm, no... the problem with Cobra Cable was high capacitance.
Paralleling a bunch of them would be... interesting.

http://www.diyaudio.com/forums/multi-way/50724-polk-cobra-speaker-cables.html
http://db.audioasylum.com/cgi/m.mpl?forum=cables&n=127692&highlight=CRAZYDIAMOND
http://www.roger-russell.com/wire/wire.htm
http://www.audiokarma.org/forums/showthread.php?t=49018

inspiredsports

I guess I should have written that BOTH resistance and capacitance were high and that the theory is that paralleling is said to decrease both.

Face

The Goertz silver cable mentioned by him(AG series) is still readily available. I ran it in my system before MIT and was very pleased.

inspiredsports

Face wrote: »

The Goertz silver cable mentioned by him(AG series) is still readily available. I ran it in my system before MIT and was very pleased.

Sweet! 100' of the Goertz AG 3 "Divinity" will only set me back about 12K instead of the 2 Benjamin's I dropped for my Cobra experiment

Face

Why do you need 100'?

dorokusai

It's not that great but good for you. For most old time Polkies it's a nostalgic cable. I've had 50ft sitting in a box for years with no intention of using it again.

mhardy6647

Paralleling will decrease resistance (according to Ohm's Law) and increase capacitance additively.
Resistance won't hurt an amplifier (except at the limit of infinite resistance in a transformer coupled amplifier with fairly high B+).

inspiredsports

Face wrote: »

Why do you need 100'?

I would think the similar design of the Goertz cable would have the same problems/limitations as the Cobra cable and would also have to be paralleled? No?

At any rate, the Cobra Cable experiment will be exactly that; an experiment with a nostalgic link to Polk's past and a conversation piece at a future Polkfest.

Face

inspiredsports wrote: »

I would think the similar design of the Goertz cable would have the same problems/limitations as the Cobra cable and would also have to be paralleled? No?

Not that I'm aware of.

If you're interested, I do have some 15-20' runs of their copper cable that aren't being used ATM.

inspiredsports

mhardy6647 wrote: »

Paralleling will decrease resistance (according to Ohm's Law) and increase capacitance additively.

Resistance won't hurt an amplifier (except at the limit of infinite resistance in a transformer coupled amplifier with fairly high B+).

The Cobra Cable is slated to arrive tomorrow.

As I study the characteristics of this cable (both high resistance and high capacitance) and apply Ohm's Law as you shared, the the work-around of using multiple runs in parallel to lower the cable's high resistance makes the high capacitance even WORSE.

I guess this idea was suggested 25 years ago by Peter Moncrieff of International Audio Review in IAR Issue #9 as a way to improve bass transmission.

I now see this makes it a cable to be used only in conjunction with tube amps, as all of that capacitance will not sit well with most solid state gear.

TECHNOKID

inspiredsports wrote: »

The Cobra Cable is slated to arrive tomorrow.

As I study the characteristics of this cable (both high resistance and high capacitance) and apply Ohm's Law as you shared, the the work-around of using multiple runs in parallel to lower the cable's high resistance makes the high capacitance even WORSE.

I guess this idea was suggested 25 years ago by Peter Moncrieff of International Audio Review in IAR Issue #9 as a way to improve bass transmission.

I now see this makes it a cable to be used only in conjunction with tube amps, as all of that capacitance will not sit well with most solid state gear.

Maybe adding an isolation transfo at the amp end would help?

inspiredsports

TECHNOKID wrote: »

Maybe adding an isolation transfo at the amp end would help?

At some point the work-arounds to make this work with solid state gear probably defeat the whole idea.

I was intrigued that for 25 years the man who wrote the article reached from the link in post #1 felt Polk Cobras in a parallel configuration were the best cables on earth, even when compared to $15K stuff!

I think my Cobra project will be slated for use with my first tube amp whenever I am blessed with the ability to arrange for one to appear hear .

dorokusai

It has been used with all amplifiers....whether your amplifier likes it is another issue. It's not tube amplifier specific.

mhardy6647

The issue with some (perhaps most) wideband, direct-coupled (ss) amplifiers can be (in)stability into such a capacitive load. Transformer-coupled vacuum tube amps are less likely to be driven into self-destructive oscillation (but there are tube amps that might succumb, too).

inspiredsports

Does anyone know/remember the capacitance of this cable per foot or meter? It's usually presented as picofarads (pF/Ft or pF/M).

I just got to thinking that the capacitance the amp sees would remain the same whether using a single 50' run, or that 50' run cut and paralled into 8- 6' runs. It's the resistance that woud be cut.

inspiredsports

GOOD GRIEF !! . . . $360.00 + S&H.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200624406242

Toolfan66

Still cheaper then MIT..