Interconnect Inductance vs. Capacitance

Thanks all for the great comments. I understand that the capacitance and the inductance are not the whole picture for most high-end audio gears. Like what I said earlier, I have a preamp that uses a 100K attenuator at the output and I need 10 ft of ICs between preamp and power amp.

Let me use an example to illustrate my point. Take Cardas Golden Cross ICs as an example. According to its website, the capacitance is 25.5 pF/ft. For 10 ft ICs, it yields 255pF.

Since the preamp attenuator has 100K max resistance, let's assume 50K ohm output impedance. With these two numbers, let's compute the 3dB roll-off freq, Fc.

Fc = 1/(2*Pi*R*C) = 12.5 KHz

As you can see, the highs are significantly rolled in this configuration! Hope it all makes more sense now.

Can you please chart the numbers you used, my calculations are different then yours. I used 1/2*3.14*100 or 50*255), neither one of the impdencance vlaues match your solution. I got 20017.5

Using the same forumula, and for my system, I have 14 foot cables, but a low 14pf per foot for a total 196Pf, Neotech UPOCC cables, preamp has 100ohm output impedance and the amps are 210 kOhm input impedance.

The numbers read: 1/(2*3.14*100*196)= 30772

I think my math is off some, Thanks.
Audioquest4life

No, as far as I am aware no one sells interconnects with capacitance high enough to "grossly" affect high end frequency response. But under extreme circumstances (high component output impedance, long cable length, high cable capacitance per unit length), it could become marginally significant. So in that sense it is potentially a system-level issue, that the consumer should be aware of.

For less than 6 foot IC's no worries but once you get to 12 feet or more then you would be surprised at how poorly some RCA line level gear will perform. I don't think the high end roll off is the most noticeable - to me there often seems to be a loss of dynamics when using very long runs - I'd avoid long RCA runs and go with XLR balanced if that is your need.

I agree with Vett's calculations, and the 12.5kHz answer. When I said that interconnect capacitance could become "marginally significant" under extreme circumstances, I was thinking of source components with active output stages. For passive preamps, or preamps with unbuffered resistive attenuators at their output, the effect can obviously be more than "marginal."

Audioquest4life, not sure where you are going wrong with your math, but for 100 ohm output impedance and 196pf capacitance, the 3db bandwidth would be:

1/(2*3.14*(100)*(196exp-12)) = 8,124,269 Hz (i.e., 8.1 MHz)

For Vett's example, it would be:

1/(2*3.14*(50000)*(255exp-12)) = 12,489 Hz

Although of course the 50K assumption is something of an oversimplification, and in practice I think the answer might not be quite that bad. The 50K output impedance assumes the control is set for 6db attenuation, and is the total impedance looking back into the output. But, first, I would think the control typically would be set for greater than 6db attenuation. Let's call it 12db, which would mean 25K between the output terminal and ground, and 75K between the output terminal and the preamp's internal voltage source which drives the attenuator. The high frequency rolloff would be determined, in this example, by the voltage divider ratio formed by the parallel combination of the 255 pf and the 25K, and the 75K. I'm not going to bother trying to figure that out, but my suspicion is that the net result would be a somewhat wider bandwidth than what would be provided by the 50K assumption. In any event, the 50K assumption does seem like a reasonable rough ballpark, which makes the point that the treble rolloff can be significant.

Regards,
-- Al

Shadorne -- Thanks for your observations. I just want to make sure it's clear to everyone that my previous post was written before I saw your last post, and was in response to the prior posts, not to your good observations.

Regards,
-- Al