[jamesgarvin]

I've read opinions from some engineers and cable manufacturers that low capacitance in interconnect cables is generally preferred to high capacitance cables, particularly for longer runs, and particularly relative to tube equipment. In some comments I've read statements relative to the RM200, here, the Stereophile review, and the manufacturer's comment following the review, to the effect that the RM200 can survive a low capacitance cable so prevalent in the high end industry. I interpret from these comments that perhaps low capacitance interconnects are not a good thing.

I've just purchased an RM200, with the balance configuration, and expect, hopefully, to take delivery from the dealer in the next week or so, and am looking at a balanced interconnect in the neighborhood of 16-20 feet. I've got a PS Audio 4.6 pre-amp, which I'll be upgrading next, likely to a tube unit.

I guess the general question is whether a high or low capacitance cable is preferable, advantages or disadvantages of each, whether the same answer would apply to tube and solid state equipment equally, and do these change based upon cable length. Thoughts?

[Ericus Rex]

Hi Roger,

I've been using the DH Labs Q-10 speaker cable with my RM-9 for a while now.  Your discussion of cable specs lead me to ask the company their specs for this cable.  They replied that the resistance is "30 pf per foot" and the capacitance is "0.002 ohms per foot of loop resistance".  How do these numbers sound and what does 'loop resistance' mean?

Thanks!
Eric

[Speedskater]

Loop Resistance is:
If you short the signal conductors together at one end of the cable and measure the resistance of one conductor to the other conductor at the other end of the cable. It will take a real good meter (with a 4 terminal ohm meter ) to read low values like this.

[Speedskater]

That is a big cable!  0.002 ohms per foot of loop resistance is about equal to AWG 5 wire. (5mm diameter)

[geezer]

They replied that the resistance is "30 pf per foot" and the capacitance is "0.002 ohms per foot of loop resistance".

Just to be clear: presumably you meant to write "the capacitance is 30 pf per foot, and the resistance is .002 ohms per foot."

And, to respond to the original question: The lower the capacitance the better.

[Ericus Rex]

They replied that the resistance is "30 pf per foot" and the capacitance is "0.002 ohms per foot of loop resistance".

Just to be clear: presumably you meant to write "the capacitance is 30 pf per foot, and the resistance is .002 ohms per foot."

And, to respond to the original question: The lower the capacitance the better.

Ooooops!     Yes, of course.

Is 30 pf per foot considered low?

[geezer]

Is 30 pf per foot considered low?

If you just want the simple answer, just scroll down to the bottom. If you want he explanation, keep reading.

The potential problem with cable capacitance is that shunts away part of the signal from the circuit you're trying to send it to. This normally would not be a serious problem if the effect were frequency independent. But as shown below, it IS frequency dependent, affecting high frequencies more than low which leads to distortion of the signal. Here's how it works:

The effect depends on the length of the cable you're using. The capacitance will represent an impedance in parallel with the input impedance of the circuit the cable is feeding. The impedance of a capacitor is given by:

                           Z = 1/[2*pi*C*f]

Where  pi = 3.14159, etc., C is the capacitance (in Farads, not pico Farads) and f is the frequency of the signal being transmitted. In your case the capacitance will be 30pF times the length of the cable. To give you a concrete example, I'll consider a 10 foot cable carrying a 10 K Hertz signal into a circuit with 10 K ohms input impedance.

Then in the above equation, f = 10000, and C = 300 pF, which is 300*10^-12 Farad. This gives a capacitive impedance of 159 K ohms. The parallel combination of this capacitance with the 10 K ohm input impedance reduces the total impedance to 9.4 K ohms; a 6% change, or less than 0.3 dB, which is a difference well below the ability of the ear to detect.

The change of impedance for a 100 Hertz signal would be, by a similar calculation, less than 0.1%; even less detectable.

So unless you have an unusual situation, such as a 100 foot or more cable, you surely are OK with 30pF per foot.

[mark funk]

Blue Jeans Cable has some balanced interconnect cables (Belden 1800f 13pF/ft). Belden has been making cables for over 100 years. The cable it self does not cost much, it's how they are terminated that could run in to more cash. These are terminated with Neutrik XLR. That would be for a 19 foot run $43.50 per cable, not too bad.