Building Krell CAST design cables with Polk Cobra

The first (and probably last) Krell CAST design cables made with Polk Cobra cables.

 

Genuine LEMO FGG.1B.304.CLAD52 connectors.

First things first. Here is the pin-out (wiring diagram) for the Krell CAST cables.

 

Pin no	Designation	Comments
1	Ground	Similar to XLR (Balanced cable)
2	Hot
3	Cold
4	Flag	Signal to the system that  CAST cable is connected

 

The function of Pin 4 connection is to signal to the Pre and Power amplifier that CAST cable is connected to activate relevant relays/circuitry. I have not opened up a Krell to check the circuitry but I think this is a simple continuity check circuit. That is why it works even when it is shorted to Ground (Pin 1).

 

No	Configurations	Pin 4 connection
1	4 Pins/cables connected	Pin 4- Pin 4 Dedicated Flag
2	4 Pins/cables connected	Pin 4 – Pin 1 Shorted, Flag is common with ground
3	3 Pins/cables connected	Pin 4 Open. This will NOT work.

When we (me and my Krell-lover friend) first mooted this idea for Polk Cobra CAST cables, this fourth pin was a puzzle. Three pins for balanced cables in standard (and Krell CAST is a balanced signal), so what's the function on the fourth pin? I considered these possibilities.

• Pin 4 is connected to shield. This is unlikely that the shield is connected to Pin 4 on both sides. Even slightest voltage difference between the two chases will induce hum. 
• Pin 4 is chassis common ground. Also unlikely for the same reason as above.
• Pin 4 is some kind of trigger circuit. But how to confirm this (short of opening up an amplifier)?
  
• Pin 4 is connected to shield on one side only. This will make the cable directional but Krell CAST are not directional (especially original Krell version). Nordost (and some other third party) CAST cables are marked as directional but that does not mean much.
• Krell added a fourth pin on purpose to mis-lead DIY-ers.... 😁
  

 The breakthrough for me was when I read in one of the DIY forums that Audio-Gd makes a similar system called ACSS. Sometime ago I had researched and auditioned some Audio-Gd DAC/Preamps as a possible replacement for my 'long in the tooth' California Audio Labs Sigma 2 Tube DAC (not because of sound quality but due to inability to decode Hi-res and DSD). At that time I did not pay much attention to ACSS as I was not planning to change my amplifier. So I reviewed Audio-Gd website again and, voila, found direct reference to Krell CAST.

Here is a screenshot from Audio-Gd website. You can just google Audio-Gd website for more information. Mr King Wa (thank you for quick response) confirmed by email that ACSS and CAST use similar pin-out configuration and Pin 4 is Flag. 

For my DIY  Krell CAST cable I used configuration 1 in table 2 above. It worked perfectly and replaced Transparent Musiclink cables in my friend's system. I did not connect any pins or the LEMO body to the shield (in my design, 96 out of the 144 strands from the Cobra cable acts as shield). 

For those DIYers interested to make DIY Krell CAST cables, don't worry too much. Use a good IC cable with three (like XLR) or four signal wires (like Mogami Neglex Quad microphone cable) to connect the four Pins correctly and you are good to go. As the currents and voltages are really small, there is little possibility of damaging the amplifier. The most likely problem you will face is noise and hum. Go easy on the volume control when you start, run a frequency sweep (I use Full Glide from Cardas-Ayre Irrational But Efficacious CD) at zero volume then increasingly higher volumes to test for any abnormality. As we say in Malaysia, "Selamat mencuba".

Polk Cobra for Krell CAST Design

 Please read the section on the Cobra Cables structure/design before you continue (if you have not done so). Otherwise the following section will be confusing.Building CAST interconnect cables with Cobra involves iterative and time-consuming procedure because we need to select the lowest Capacitance combination.

Goal: CAST cable with low Capacitance. The Cobra cable has inherently very low Impedance and acceptably low resistance.

 

Procedure: 

1. Measure and cut two pieces of cable.
2. Remove about 4 cm plastic insulation sheath from all ends.
3. Separate and twist together each group of 6 strands on one end of each cable. Number them for easy reference (just stick a piece of masking tape). You will have 12 groups each of Copper and Green colored strands. Do this carefully so that none of the strands gets mixed (shorted) with other groups. Leave the other end as it is for now.
   
4. De-enamel the tips (about 3mm) all the 24 groups (12+12). Keep the groups separate. Test for shorts between groups. Even a single mixed strand can short two groups and mess up the next steps.
   
5. Select any two groups of Green strands that is wound in opposing directions and short them together (by tape). This is the 'Ground' Pin 1 cable.
6. Select any two of the other Green strands (also wound in opposing directions), short them together and measure Capacitance between the 'Ground' and these two cables. Record the Capacitance readings and cable numbers systematically. Repeat this experiment with all ten remaining Green groups (taking care only to pair strands wound in opposite directions). Choose the pair with the lowest capacitance. This is our "Flag' Pin 4 cable.
• It is important to pair groups of strands wound in opposing directions. This keeps the impedance low (and low impedance is part of magic of Cobra).
  
7. Select a pair of Copper strands (wound in opposite directions) and repeat procedure above. Measure capacitance between Copper pair and the Ground (Green pair). Choose two the Copper pairs with the lowest Capacitance. These will be the 'Hot' Pin 2 and 'Cold' Pin 3. 
8. Measure capacitance between the all the four pairs (two Green and two Copper). They should be uniformly low and close values. If not, redo steps 6 & 7.
9. Repeat the same steps with the second cable.
10. Review Capacitance measurements between the two cables to ensure they are close. 
11. Please note that Capacitance measurements for Trigger (Pin 4) is not critical. Capacitance between Pins 1, 2 and 3 are critical.
    
12. Cut-off all the non-selected groups of strands at end of plastic sheath. These cables (96 strands in 16 groups) with act as shield in this design.
13. Repeat steps 2 - 4 for the other end of the two cables.
14. Trace the other end of the the selected pairs of strands in steps 6 - 8 (just test for short) and mark them carefully.
15. Cut off the shield strands as in step 12.
16. Repeat for the other cable.
17. Now solder the pair of cables together and do final round of confirmation measurements (Capacitance and Short). Cable is now ready.
18. Use heat-shrink ( or other) insulators at exposed cable ends.
19. Solder LEMO (or compatible) connectors.
• LEMO connectors are really small and cramped. Do this very carefully.
20. Test again for good connections and Short.
21. Congratulations. Your cable is now ready. Enjoy.

Note for the technically minded (are not we DIY-ers all technically minded?):

One of these days I will post detailed information on my decision to use 12 strands. But the gist of the matter is that I have tried 72, 36, 18 and 12 strands for (+ve and GND each) for RCA cables and found that 12 and 18 strands are best sounding (indistinguishable between the two). As the LEMO connectors are so cramped I decided to use 12 strand (2 groups) configuration. For my RCA and XLR I use 18 strands (3 groups) for the sake of long term reliability. Even if one group of strands get damaged or disconnected, one will have no deterioration in sound quality.

 

LEMO connectors used for CAST

 Finally, the most important bit. How do they sound like? I will just quote my friend's evaluation verbatim (with his permission). Please note that the following comments are based on a system Polk Cobra Interconnect (CAST connectors and XLR) and Speaker cables. This is not a review of CAST cables in isolation. Quote:

I have used Transparent Balanced (XLR) interconnects and speaker cables for a long time. However after reading about the Polk Cobra cables, I wanted to see if they really made any difference. The Polk Cobra cables opened up the mid-range very nicely, making it sound more natural. Separation also improved, I could hear instruments which I did not hear with my previous cables. The bass which used to have a boom, became tighter and much more rounded. For a long time, I was toying with treating the room with acoustic boards/panels and the like, but after switching to the Polk Cobras, I have given up the idea! That in itself was a huge cost savings. 

In fact, I am now listening to a whole bunch of CD's I own, which used to sound harsh or flat before. Its amazing that the Polk Cobra's can make such a difference.

My music system:

Music Fidelity A308 CD player

Cambridge Audio CXN streamer + Western Digital HDD (music file format WAV)

Krell KCT Pre Amplifier

Krell 350MCx Monoblock power amps (pair)

Revel Ultima Salon 2 Speakers

APC UPS for source and pre amps

Anaconda power cables

Source to Preamp IC - previous - Transparent Balanced Musiclink XLR now  - Polk Cobra XLR

Pre to Power amps IC - previous - Transparent Balanced Musiclink, now - Polk Cobra with CAST ends

Power Amp to speakers cables - previous - Transparent Musiclink Super 6841XL, now - Polk Cobras (ends are soldered, bare wire)

Unquote.