“What gauge of speaker cable do you recommend?” is a question that I get asked often. And to answer, I always have to ask, “What is the impedance of your speakers and what will be the length of the longest cable?”
This article is a brief explanation of the essential science behind selecting cables to connect passive speakers to stereo amplifiers and AV receivers. It is not applicable to the cables used with active (powered) speakers and subwoofers.
Cable resistance and power loss
The purpose of a speaker cable is to transfer the output power of an amplifier to the connected speakers, with as little loss and distortion as possible. How good a given cable is at doing this depends on three basic physical properties which can be objectively measured – the resistance, inductance and capacitance of the cable. The inductance and capacitance of most speaker cables would typically be low enough at audio frequencies as to be considered insignificant, so what we need to focus on is its resistance. Resistance is a measure of the opposition to current flow in an electrical circuit. The higher the resistance of the cable, the more the power that is lost in the cable as heat. The lower the resistance, the more the power that is transferred from the amplifier to the speaker.
What determines the resistance of a cable?
- The conductivity of the material used to construct the cable. The higher the conductivity, the lower the resistance. Silver is the best conductor of electricity (at room temperature) that we know of but is rarely used due to its high cost. Copper is the material of choice for most cables. Aluminum is also used, because it is cheaper than copper, but it is not as good a conductor as copper and is best avoided in speaker cables.
- The cross-sectional area of the cable. The thicker the cable the greater the cross-sectional area and lower the resistance.
- The length of the cable. The longer the cable the higher its resistance will be. So, keep cables as short as possible and compensate for the increased resistance of longer lengths by using thicker cable.
Speaker impedance and frequency response
As a general rule, we should aim to keep the total resistance in the connection between the amplifier and speaker as low as possible, and never more than 5% of the nominal impedance of the speaker. So, the lower the impedance of the speaker, the lower the speaker cable resistance needs to be. And what is impedance you ask? Well… you could think of it in a simplified way as ‘resistance which varies with frequency’. The impedance rating mentioned on the back of most speakers (4, 6, 8 ohms etc.) is the ‘nominal’ impedance of the speaker. The actual impedance varies with frequency, which complicates the interaction between the cable and the speaker. If the impedance of the speaker remained constant with frequency (in which case, we could have just called it resistance) the power lost in the cable would be a fixed proportion of the power that is output from the amplifier. But as the impedance of the speaker varies with frequency, the power lost in the cable would also vary with frequency. This means that using speaker cables with excessively high resistance will not only cause an increased loss of power; it will also alter the frequency response of the speaker. This makes it all the more important that we select the right speaker cables for our application.
So, does this mean that thicker the cable, the better? Yes, but up to a certain point only. Beyond that, we would just be wasting our money, as the continued decrease in resistance will make no audible difference to the sound.
If speaker cable manufacturers mentioned the resistance of their cables on an ohms-per-meter basis, it would be quite straight forward for us to select a cable of suitable gauge based on the nominal impedance of the speakers and the required length. But this information is rarely made available in an easy-to-understand format, so I am putting together a chart to help select the right speaker cable according to the impedance of your speakers and the length of the cables. Will upload it here as soon as it’s ready…
Speaker cable termination
Something we should not forget to consider is how the ends of the speaker cable will be attached to the terminals on the amplifier and speakers. While banana plugs are popular and make it easier to connect, disconnect and reconnect speakers to amplifiers, they may not necessarily provide the lowest resistance contact between the cable and the amplifier/speaker terminals. But more on that later…