Amplifiers etc.

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Speaker cables and interconnects

Q37 - What cables and interconnects do you recommend?

A37 - I prefer not to recommend any specific product. Cables can have audible effects and some manufacturers make sure they will, either through unusual electrical parameters and/or by suggestion. Weaknesses in the design of the output-to-input interface are exploited. Sounding different does not mean it is also a more accurate transfer from electrical to acoustic domain.
My guideline for speaker cables is to keep their resistance to less than 0.1 ohm for the roundtrip path of the current. This defines the maximum length of a 2-conductor copper cable for different wire gauges.

I measured the 16 gauge Megacable from Radio Shack (278-1270) that I use. A 10 foot length has 0.07 ohm resistance, 714 pF of capacitance and 1.9 uH of inductance. The line impedance is 51 ohm. A typical tweeter has a voice coil resistance of 4.7 ohm and 50 uH inductance. At 20 kHz this yields an impedance of about  |4.7 + j6.3| = 7.9 ohm. Add to this the cable inductance of j0.24 ohm, and 0.07 ohm resistance for 10 feet, and the impedance becomes 8.09 ohm. This causes a 7.9/8.09 = 0.98 or 0.17 dB reduction in tweeter output at 20 kHz which is insignificant.  The cable effect is even less at lower frequencies.

Speaker cables can act as antennas in the AM frequency band and may cause distortion in the output stage of a solid-state amplifier, if strong radio frequency signals are present. In particular, the cable capacitance in conjunction with the inductance of a driver voice coil may form a resonant circuit for these frequencies. The resonance can be suppressed by placing a series R-C circuit of 10 ohm/2 W and 0.33 uF/100 V across the cable terminals at the speaker end. 

Coaxial interconnects with phono (RCA) plugs tend to pick up radio frequencies in the FM band. The currents that are induced in the cable shield must not be allowed to enter the inside of the coax. This requires a very low resistance connection between the outer conductor of the phono connector and the chassis (signal ground) of the equipment that it plugs into. The continuity and low resistance of the shield is also very important for hum and buzz currents, so that they will not induce a voltage on the center conductor. The technical description for this is the Transfer Impedance of the cable and connectors, which must be in the low milli-ohm range. Unfortunately I have not seen this specification used by the audio industry. 

An excellent description of the theory and treatment of hum and buzz problems in equipment setups with mixed two and three prong AC plugs is given in AN-004 by Jensen Transformers, Inc. I have not found balanced interconnections to be necessary for the high level circuits past the preamplifier. But sometimes it requires to experiment with AC outlets in different locations to reduce to insignificant level the buzz that one may hear with the ear close to the speaker cone. So, when choosing a coaxial audio interconnect look for good mechanical construction, direct contact between shield and connector, and well plated contact surfaces.  

I find what is needed at Radio Shack or Amazon. I solder speaker cables to terminal strips on the speaker end and use dual in-line banana plugs on the amplifier end.

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