The ultimate guide to speaker cables



Last edited: March 10, 2019

This is my guide to speaker cables and wire gauge selection. The first part is a quick overview of the basic principles, rules and the important electrical parameters. For those who are familiar with wire gauge, resistance, inductance etc. the second part will be more interesting. The second part has an interactive table and a some discussion about high frequency losses due to inductance.

▶ Table with recommended cable distances



Construction

Speaker cables are made of two stranded copper wires surrounded with PVC insulation. The role of the insulation - apart from isolating the two wires from each other - is to prevent the copper from oxidation. There are speaker cables that cost 100$ per meter or more, but in reality these are just 'audio jewelry', they look cool, but have no sonic benefits (and some of them can be worse than an ordinary speaker cable). A speaker cable should have very low series resistance and series inductance - and that's all. More about speaker cable myths can be found here.

Using exotic materials (OFC, gold, silver, Teflon) and construction (matched propagation and other nonsense) have no benefits in audio cables. These are just raise the cost and give cables an extraordinary look. In humid environments cables with tinned copper and double insulation should be considered.

Although any ordinary speaker cable will do the job if the resistance is low (more on this later), speaker wire connectors have some real advantages over bare wires:

There are two basic types of amplifier and speaker terminals in home audio: binding posts and spring clips. The chart below summarizes what type of plug fits into binding posts and what type fits into spring clips.

chart, speaker terminals, binding post and spring clip, pin tip plug, banana plug, spade plug



Resistance, inductance and capacitance

Since speaker cables connect an amplifier with low output impedance (~100 mOhm) to a low impedance load (3...50 Ohm), cable resistance and inductance are more important than capacitance.

The resistance, inductance and capacitance of a cable are directly proportional to its length. So the longer the wire, the more resistance, inductance and capacitance it will have. A thicker wire will have less resistance at the same length as a smaller gauge. Doubling the effective cross-sectional area of a wire reduces its resistance by half.

The current flow through a wire results in a voltage drop according to Ohm's law (voltage = resistance * current). Therefore a speaker wire should have low resistance to minimize voltage drop. The power loss (heat) is not a dominant factor in speaker cables.

Cable geometry affects inductance and capacitance. Greater the distance between the two conductors, larger the inductance of the cable and lower its capacitance. So separating the wires over a long distance is not recommended, because it will add a lot of inductance. (For zip cord cables the typical inductance per meter values are between 600 nH/m - 700 nH/m.)



Speaker impedance

A speaker's impedance rating is merely a nominal figure. In fact, the speaker's impedance (~ AC resistance) is frequency-dependent: a speaker with 4 Ohm rating can drop down to 3.2 Ohms, and get very high - say 40 ohms or more - at various frequencies.

The minimum value of the speaker's impedance will determine the largest attenuation due to wire resistance and output resistance of the amplifier. Lower the minimum impedance, higher the attenuation with a given cable and amplifier. According to the IEC 268-5 standard the minimum impedance of a loudspeaker must not fall below 80% of the nominal impedance, so for an 8 Ohm speaker the minimum would be 6.4 Ohm, and for a 4 Ohm speaker this would be 3.2 Ohm.

Speakers with 4 Ohm nominal impedance rating are more 'sensitive' to wire resistance, inductance and to the output resistance of audio amplifiers than speakers with 8 Ohm nominal impedance.

Sometimes the label on the rear panel of a loudspeaker displays something like '4-8 ohms'. In this case, the speaker has drivers with different impedance ratings, e.g. a 4 Ohm woofer and an 8 Ohm tweeter. These type of speakers should be taken into account as 4 Ohm speakers when determining the cross-section. In inductance calculations the impedance of the tweeter (or tweeter section) is what matters.



Determination of the minimum cross-section

There is a minimum wire cross-sectional area or gauge (AWG) for a given speaker impedance, cable length and allowed loss (dB). Or put it differently: there is a maximum cable length for a given speaker impedance, wire cross-sectional area and allowed loss.

A more accurate calculation may include the output resistance of the amplifier and the inductance of the cable. For even greater precision the amplifier's output inductance can be used as an extra parameter.

The most important parameters are wire gauge, length AND the nominal impedance of the loudspeaker.

The output impedance of an amplifier (in the case of audio power amps this is the output resistance) can be calculated from the damping factor. Both the damping factor and the output impedance vary with frequency. Even for amplifiers with virtually zero output resistance on paper (damping factor is much higher than 100) the output resistance will get close to 100 mOhm at 10 kHz. So 100 mOhm is a good approximation in loss calculations.

Output inductance is between 1 uHenry and 2 uHenry. The source of this inductance is that in the vast majority of amplifiers there is a small inductor parallel with a resistor to prevent oscillation with long (and 'bad') cables. 1 uHenry is the inductance of 1.5 meter zip cord.



Table with recommended cable distances

The table below describes the recommended maximum cable distances for various speaker cable gauges (cross-sections) and speaker loads with 0.3 dB and 0.5 dB loss. The amplifier's output impedance is an adjustable parameter: it can be set to zero (ideal amp) or 100 mΩ (close to a real-world class AB amplifier).

About AWG (American Wire Gauge): the higher the gauge number, the smaller the diameter, and the thinner the wire.

Switching between units and amplifier output resistance require JavaScript, which shouldn't be a problem for modern browsers.

Up ▲


   

Maximum length in meter
Sq. mm 0.3 dB loss 0.5 dB loss
4 Ohm 8 Ohm 4 Ohm 8 Ohm
0.75 2.5 5 4 8
1.5 5 10 8 16
2.5 8 16 12 24
4 13 26 22 44
Amplifier output resistance:
   


This table was calculated for pure copper wire. Cables with copper-clad aluminium wire (CCAW) should have two AWG sizes larger (e.g. 14 AWG instead of 16 AWG) or 1.5 times larger cross section to be selected. But it's better to choose a cable with pure stranded copper conductors, because CCAW cables have poorer flexibility.

"Which loss value should I use?" If you are a perfectionist, then set the amp output resistance to 100 mΩ and select the column with 0.3 dB loss, then select the length and the appropriate cross-section. This will give a flat electrical response (the acoustical depends on the speakers and the room). If we consider room reflections and speaker response errors, then a 0.5 dB loss is still a reasonable target. Fortunately, the 100 mΩ output resistance has little effect on the frequency response of 8 Ohm speakers. 4 Ohm speakers are a different story and they require really thick wires even for very short cable runs.

Loss (transfer function) calculations:

   loss = 20 * log ( Rspeaker / ( Rcable+Ramp+Rspeaker ) )  [dB]
   Rspeaker = 0.8 * Znominal   [Ω]
   Rcable = 2 * ρ * l / A   [Ω]
   ρ = 0.017 Ω·mm2/m (resistivity of copper)



Attenuation due to inductance

It's really hard to predict the high frequency attenuation of speaker cables. The actual roll off depends on the tweeter or full range driver impedance curve, and the nominal impedance rating can be misleading.

The problem is the following: If a tweeter has an 8 Ohm nominal impedance and the impedance graph is almost flat above 10 kHz, then the high frequency attenuation response will be close to the response of a 8 Ohm resistor (this is good). If the tweeter impedance curve is inductive in nature and rising, then a '8 Ohm' tweeter will behave like a 4 Ohm resistor above 10 kHz (this is bad). Cone tweeters and some old dome tweeters have rising impedance, while most 'modern' dome tweeters have a flat impedance curve.


attenuation due to inductance at 20 kHz

Sometimes there is a little overshoot in the response due to the interaction between the reactive load presented by the crossover and the cable inductance. This may happen when a cable is longer than ten meters and the cross-sectional area is large (>2.5 mm2). The magnitude of the overshoot is very small (<0.05 dB) and negligible.



Final notes

Speaker cables are the most over-mystified components of the audio signal chain. And yet they are the simplest and cheapest ones. Changing the listening position has more dramatic effect than switching to a cable with slightly larger cross-section.

Csaba Horvath

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