NBSAudio: DIY interconnect cables


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Part 1. “Nordost” type construction.

We all went the same road: integrated –> component equipment –> more expensive components; stock interconnects –> upgraded interconnects –> another interconnects. I would like to share here the latest state of my quest for the optimum interconnects.

Between thousand offered cables in the range from $2 to $2000 there are a few with posted specifications and test results. And there are only three parameters available: Resistance, Capacitance, and Inductance measured by foot of cable. Comparison is quite simple; the less is better. To minimize inductance the wire should be straight,   to minimize resistance the wire should be thick (or a few thin ones connected in parallel), to minimize capacitance between wires two wires should be as far as possible from each other. You’ll combine those requirements with modern technology and you’ll get flat Nordost cable like Solar Wind or BLUE HEAVEN.

Look at that cable, does it remind you something? Yes, you are right, that is the plain flat speaker cable! So the recipe for the perfect interconnect is simple: take 3 ft of regular speaker cable (16 GA would work just fine), solder to two not rusted RCA connectors (joke) and you’ll get a high performance interconnect.      

Now, question #1: how about pure copper, long crystals, silver wire etc. The answer: the only integrated parameter for all that stuff is a wire resistance. If you think resistance is too big for you, take a thicker wire. The rest of BS like skin effect, speed of electrical current in the wire and so on, is too boring to discuss.

Question #2: insulation tubing material. It is important when you need to lower capacitance of the coaxial cable. For the flat pare of wires the distance between wires is more important than material itself. Simple PVC jacket would work just fine.

Question #3: shielding of the cord, possibility of EMI/RFI. The answer: there are three possible sources:

 - RF radiation from the outside of the house (radio waves). It is so week in the normal condition that you need a special device (called a FM/AM Tuner) to make it audible. If it is much stronger (you live under the transmitter antenna), you need to run from that house, it is not safe to live here.

- EM radiation from the inside of the house. If you have a dimmer or a water pump next to your preamplifier, you need to quit that hobby of building the home theater and listening music.

- EMI/RFI from your own equipment. If you use ham radio power supply from the  WW2 era to power your CD player, than, well, there is nothing I can say.

To summarize: in normal conditions EMI/RFI does not exist and there is no need for shielding. Example: Nordost Solar Wind interconnects.

Now, the test. It took me about 10 min to assemble 3 ft test cord.

Measurement of capacitance gave 7.0 pF/foot. Couldn’t measure the inductance and resistance b/c it was below the sensitivity of my multimeters.  For audition I attached it between Denon 3910 universal player and Rotel 1068 preamplifier (followed by Bryston 4B ST and Polk RTi150 speakers).

As you may guess, cable performed just fine, exactly the same as the higher grade interconnect I used before.  

Part 2. Testing procedure

Let me start with the bold statement that it is impossible in theory to A/B test interconnects by switching them consequently. Our visual short-term memory sustains for about tens seconds, and echoic, the auditory (hearing) version of short-term memory is even shorter, less than four seconds. (http://en.wikipedia.org/wiki/Echoic_memory)

It means there is no way to remember how that musical fragment sounded in details after you stopped the music, changed cables and start that fragment again. It is like you want to compare two pictures, and you open one photo album, looked at the picture for a 10 seconds, closed the album, ran into the second room, picked up another album, opened it, took a 10 sec look at the second picture, closed the album, and so on. It is technologically possible to build an electronic switch that would switch cables each two seconds and run a loop of the same musical fragment, but I don’t know anybody who build it and actually used it in the home cable test.

But there is a possibility to test two sounds in the same time the same way you compare two pictures placing them on the table one next to another. To do it you have to run two sounds simultaneously.  In practice it means you will use cable A in the one channel (left), and cable B in another (right). To insure identical signal running through two cables we need a mono sound signal. For example, you feed both channels of preamp with the left channel from CD player through the Y-splitter, and your A and B cables connected preamp and power amplifier. 

Don’t rush to start testing, we are not ready yet. What are the pre-conditions for this test? The channels (which include electronics, speakers and room itself) should be identical. It means that mono signal has to be precisely localized exactly in one point between speakers from the chosen listening position.  No “advanced” imaging between left and right, partially “forwarded” or “laid-back” sounds of particular instruments. If we have precise localization, we are ready; if not then we need to fix our system.

What to look for? If two cables are identical there wouldn’t be any changes in the sound imaging. It’ll center precise between speakers for all frequencies, all instruments. In case of different resistivity of two cables, especially at different frequencies, there would be easy to identify the shift of the sound image from the center, different for different instruments. Phase shift (I guess) could be heard as the shift of the different instruments front-back in the right and left parts of the soundstage. Plus, you can imaging some other peculiar changes in the originally coherent single pointed sound.

Typical mistake would be to make a judgment based on the first test. As we said, two channel have to be identical on the path after cable insertion to the listener. It is never a case, the listening room itself could be asymmetrical. So to eliminate that factor you need to switch cables from the left to the right and check, if some nonlinearity of the audible signal would move along with the cable in question.        

Part 3. Now, let’s get it serious

 Bases on the considerations discussed above, I made “ultimate” interconnects.

 

3.1. Choice of the wire.

The optimal would be 20 GA Silver-Coated Copper wire with Teflon insulation. (19 single wires 32 AWG each stranded together).

AWG

Diameter, inch

Area, (Sq. mm)

Diameter, mm

Copper
resistance, (Ω/km)

1 m cable resistance (Ω)

18

0.0403

0.823

1.024

20.95

0.021

20

0.0320

0.518

0.812

33.31

0.033

30

0.0100

0.051

0.255

338.6

0.339

32

0.00795

0.032

0.202

538.3

0.534

33

0.00708

0.025

0.180

678.8

0.679

20 GA wire is still manageable construction wise, but has at least 10 times less resistance than typical 30 or 33 GA wire used in the interconnects. Lower diameter of the wire will lower capacitance of the cable, but there is a limit how low it could be for the cable as a whole, defined by the capacitance of the RCA connectors only.

3.2. Construction considerations

The distance between two wires will specify the diameter of the cable, and for most RCA connectors it’s limited to the 10 mm max. So 1 cm it is. The best dielectric would be the air, which leaves us with PVC pipe with conductors attached to the outer surface.

3.3. Testing and calculations

Capacitance of two long wires of radius r separated by distance d: C =  2πε0 /ln((d – r)/r) ≈ 55.6/ ln(d/r) pF/m when r << d.

19 wires together have a diameter about 1 mm, or r =0.5 mm, distance between wires d = 10 mm, so C = 18.5 pF/m

Measured capacitance of my cable is 20 pF/m, which is close to the theoretical minimum.

Mutual inductance of two wires considered as two coils with N1 = 3 and N2 = 3 with the area 0.785 cm2 and length = 1 m, M = μ0AN1N2/l ≈ 0.001 μH

Such small value is not measurable by any available to me meters, and can be considered as negligible.

3.4. Other components

Connectors

5/8 BRAIDED EXPANDABLE WIRE SLEEVING 25ft

 

 

20 AWG Silver Coated Teflon Wire (military specs).

20-AWG Silver-Coated Wire, Black Teflon

Core Tube

Heat shrink tubes

 

Construction

 

Construction type

Testing cables

DIY Construction

Measurements

Components

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