The truth about audio interconnect cables
Last updated: Dec. 22, 2017
Although it is not related directly to speaker building, it is worth to check out the misconceptions about the audio interconnect ('RCA') cables. As with speaker cables , there are a lot of myths and pseudo-science in their marketing.
In order to understand why the big fuss about the RCA cables is meaningless, we need to take a closer look at the electric model of the cable (equivalent circuit, RC model).
Asymmetrical line out and line in with interconnect cable (without grounding)
In the model, the voltage source Ug and the Rg series resistance represent the line out or headphone output. Rin is the input resistance of the amplifier, Uin is the voltage on the input resistor (which will be amplified by the amplifier). Rs is the resistance of the wire (back and forth), Cp is the capacitance between wire and shielding. The effect of inductance in audio interconnect cables is negligible, and since Rg>>Rs and Rin>>Rs therefore wire resistance and skin effect is negligible too.
The cable's capacitance together with the output resistance of the previous stage and the input resistance of the next stage forms a low-pass filter. So the most important parameters are the capacitance of the cable and the output resistance of the driving source (only these affects the frequency response). The higher the capacitance of the cable (the longer the cable) and the higher the output resistance of the driving source, the lower the cut-off frequency. Thus, the frequency response of the cable (and any electronic device) "on its own" is meaningless. Attributing characters like dynamics or soundstage to an interconnect cable is just fantasy.
I made a frequency response simulation of a 10 meter long RCA cable in TINA-TI (the figure above). The cable is driven by a voltage source that has an 1 kOhm output resistance. The input resistance is 10 kOhms, the cable's capacitance per meter is 300 pF/m (so 3 nF is the total capacitance). This is essentially a "worst-case" simulation, as both the 300 pF/meter and the 1 kOhm output resistance can be taken as an upper limit.
The deviation from the linear response at 20 kHz is only 0.48 dB for this line driver and RCA cable! A 2 meter long RCA cable with a capacitance of 300 pF/m feed from a line out of 1 kOhm output resistance leads to a deviation from the linear line of only 0.02 dB at 20 kHz.
Maximum length values for RCA cables of 300 pF/m capacitance, if the maximum deviation from linear response is 0.5 dB at 20 kHz
So if we buy an interconnect cable, we have to answer three questions:
- How far should the signal go? (length)
- How much is the output resistance of the source?
- What is the capacitance per meter (pF/meter or picofarad/meter) of the interconnect cable?
In general, the first is known, the second - and rarely the third - can be found in datasheets. However, cable capacitance and the output resistance of the line driver don't really matter, if the cable is shorter than 5 meters. If the output resistance of the source is not known and the cable is longer than 5 meters, then it is worth using an RCA cable with small capacitance or a 75 Ohm coaxial cable (RG-59). (The only problem with coax cables that they are can not be bent in a small radius.)
Some other useful info:
- Connector's capacitance is negligible (up to 10 pF per terminals)
- The capacitance between the wires is much smaller than the capacitance between wiring and shield.
- Capacitance per meter of RCA cables vary by 50-300 pF/m. 75 Ohm (RG-59) coaxial cables have 70 pF/m capacitance.
- Line outs have output resistance in the range of 500-1000 Ohms. Headphone outputs output resistance ranges from 50 to 200 Ohms.
There are usually two problems with RCA cables. One is the inadequate shielding, though it only affects cables longer than 2 meters. The other is that they tend to break at the connector. While the DVD or CD player or turntable are being cleaned, the cables are moving and, in the case of cheaper cables, there will be a wire break in a couple of years. One solution is to replace them, the other one is to buy a cable with more massive connector. At the same time, wasting a lot of money for exotic, hi-end and audiophile interconnect cables are unnecessary.