Analogue Cabling: Signal Types

by Pablo Bellinghausen

In this article we will be looking at the different types of analogue signal, as well as the right cables to use for each of them. We’ve kept it simple yet thorough so it can work as a quick guide. If you’re a bit hazy on your electrical terms, balanced audio is explained in our previous instalment, and a small glossary for things like impedance and capacitance can be found at the end of the article.

Powered vs. non-powered signals

One can divide analogue leads between non-powered signals, that only carry audio information, and powered ones, that carry power alongside it.

Non-powered signals can be line, mic, phono or instrument level, and use a technique called voltage bridging, which minimises current in the circuit for best voltage transmission. Without getting technical, this means that the resistance and inductance of a cable don’t have an effect on the sound of non-powered signals. Depending on the inherent impedance of the source however, cable capacitance may attenuate treble, and poor shielding may let interference seep into the audio as noise or buzzing.

Powered audio signals, which can be either speaker or headphone level, are optimised for power transfer, and have different electric characteristics and requirements – we’ll see that later on.

Do note that in most situations, these signal types are not intercompatible, even though they often use the same connectors. If you’re not sure what you’re doing or require guidance when hooking up your equipment, it’s always best to give a free call to your favourite pro audio retailer, whose team should be very happy to help. Better safe than sorry!

Allen-Heath-Zed10 Allen & Heath ZED10 channel inputs – they are not intercompatible! The TS Jack “hi Z” inputs contain a DI, and the XLR mic ins have a preamp; the TRS Jack and phono line ins bypass both.

Line level

This is the best type of signal to send sound from one device to another, and therefore the industry standard for interconnections. Most devices will use line-level inputs and outputs whenever possible. A line-level source has:

Moderate voltage: an average of 0.2–2 V means that interference can be on unbalanced signals, which shouldn’t be more than a few metres. Balanced connections remove most interference noise and can be much longer.
Low impedance: a nominal impedance of 100–600 Ω in a resistive output means it can be sent through long cable runs without losing treble. Balanced connections can be hundreds of metres long if the cable has good shielding and low capacitance.

Line level leads most commonly use Phono, Jack or XLR connectors. Phono is always unbalanced, XLR is balanced (unless it’s linking to an unbalanced signal, like on a phono-to-XLR lead), and jack can be either balanced (TRS) or unbalanced (TS). 2-Line

Microphone level

This is the type of signal that usually comes out of professional microphones. A professional mic’s output has:

• Very low voltage: an average voltage of 2–200 mV means the signal is much smaller than that of line level, and therefore that interference noise is a far bigger issue. The connection should always be balanced to remove interference; good shielding is still however a good idea.
• Low impedance: a nominal impedance between 50 Ω for condensers and 300 Ω for dynamic mics means that, as with balanced line cable, they can be sent through cable runs of dozens of metres. Dynamic mics are inductive, which means in very long runs (over 200 m) cable capacitance might affect the sound slightly, but condenser mic signals can travel even longer distances without obvious degradation.

The only major difference between mic and balanced line signals is the voltage, and therefore well-shielded balanced line cable is effectively the same as mic cable. Pro mic level leads normally only use XLR connectors. TRS jacks are not recommended because they short the circuit when plugged in or out, which causes phantom power to create huge spikes that can damage equipment.

Do note that consumer mics and the devices they plug into will often have a TS jack or mini jack connector; this signal will be unbalanced, moderate-voltage (sometimes almost reaching line level) but high-impedance (dozens of thousands of ohms), very similar to the instrument level described below – with all of the associated drawbacks and reduced sound quality. They require a DI to be plugged into a standard XLR mic input, and phantom power will not work. High-impedance mic signals, just like guitar ones, should be as well-shielded and as short as possible. For more information on this, you can read our post on connecting mics to a computer. 3-Mic  

Instrument level

This is a somewhat vague connection standard that is most commonly used for electric guitars, basses and keyboards. The output of a guitar using passive magnetic pickups has:

• Very high impedance: although the resistance spec of a guitar pickup is usually under 10 kΩ, the total impedance (including capacitance and inductance) is much higher, and wildly varies with frequency between 50–800 kΩ. The cable will be quite prone to interference noise, which means good, strong shielding is essential. Also, the cable’s capacitance will form a tuned circuit with the pickup coil, creating midrange bumps and audibly reducing the treble, with measurable losses of several dB on even a few metres of cable. Sometimes this capacitance rounds off the signal in a pleasant way, so experimentation is recommended; a lead with higher capacitance will have a warmer sound that might work for classic rock distortion, whereas a very low capacitance will best carry the treble “sparkle” of an electroacoustic guitar. Since capacitance is proportional to length, cables longer than 10 m are not recommended.
Moderate voltage: an average voltage of 0.5–10 V means the output can vary from well below to even higher than line level; unfortunately, because of the impedance, interference noise will comparatively high even with “hot” pickups.

The connector is pretty much always an unbalanced TS jack, but can be a mini jack when space is an issue. Guitar leads can be used for line signals but the other way around it is not recommended, due to how much more fragile a guitar signal is. Passive electric guitars should never be plugged into line inputs as the impedance mismatch will distort and mangle the sound.

Do note that the output of guitars with active pickups and keyboards usually has a much lower impedance (very often almost reaching line level specs) and in that case, the cable is not as critical to sound quality. It is still however still unbalanced, so either short cable runs or balancing boxes should be used.

1-Instrument

 

Phono level

This is the signal that comes out of old-fashioned turntables. It is always unbalanced, and has:

• Low voltage: an output voltage of 1–10 mV means the cable will be quite susceptible to interference.
• Moderate impedance: cartridge output impedance varies a lot, from 500–5000 Ω depending on the design.

Although the signal is in theory very lossy, turntables play mastered material, which has a smaller dynamic range than raw recordings, and have high self-noise, which buries low-level cable interference. Since most often there is no reason why the cable should be more than a few inches long, any decent unbalanced cable will do fine without audible degradation. In the very unlikely case the lead needs to be longer, sturdier instrument cable can be used. As two cables are required for stereo, many phono leads are dual, with two cables glued together.

Phono level always uses Phono (RCA) plugs, which can be a source of confusion, since these are also often used for line level signals.

Due to the physical constraints of turntables, vinyl is recorded with boosted treble, using an equalisation called the RIAA curve. Plugging a phono output directly into a line input will result in a very quiet and tinny sound; to fix this problem a phono preamp is required. This stage will boost the signal up to line level and apply the opposite equalisation curve to attenuate the treble and straighten the frequency response. RIAA-Curve 4-Phono  –

Speaker level

This is the type of signal that goes from a power amplifier into a passive speaker. It carries not only the audio but the necessary power to drive the speakers, and is therefore quite different. It has:

• Very low impedance: a nominal impedance can vary from 4–16 Ω, but often falls well below 1 Ω depending on frequency. This impedance is so low that capacitance is not an issue, but in longer runs, cable resistance becomes a major factor; high cable resistance will cause some of the signal to be lost as heat, and will somewhat change the sound of the power amp. This is because the extra impedance lowers the amp’s damping, which is its ability to control the movements of the speaker. This translates into boomy, ringing bass, and in very bad cases it can even cause mechanical damage to the speaker cone.

• High voltage: an average voltage of 5–100 V in PA systems (added to the low impedance) means that, for all practical purposes, interference will be negligible compared to the strength of the signal, and therefore cable shielding isn’t required, unless it’s running alongside AC power lines for several metres. Shielding might be required to keep the signal out of other cables however, for example if the speaker cable is running alongside microphone signals.

 High current: the average current will depend greatly on the power of the speakers and the playback level, but it will usually range from 0.5–10 A. Thin cables with a relatively high resistance (like most audio interconnects) can actually melt down at high levels, which may irreparably damage certain amps. Inductance, which is pretty much non-existent in interconnects, can attenuate the signal slightly in very long runs, although resistance will play a much bigger part in any change in sound.

The best way to increase power handling and decrease resistance is to use shorter and thicker cable and, up to a point, the thicker the cable, the better. A cross-section of 4 mm² is more than enough for most PA applications, but often can be increased to 6 mm² for long runs of large subwoofers, and down to 1.5 mm² for short runs of bookshelf speakers.

Small speakers often use binding post connectors, which take either bare wire or banana plugs; guitar or budget mid-size amps and speakers will often use TS jacks, but quality PA systems nowadays all use the industry-standard Speakon connector (stylised as “SpeakON”), developed by high-end manufacturer Neutrik to address safety concerns when handling very powerful signals.

Do note that audiophiles will often claim huge increases in the sound quality of different cable types, often of exotic configurations, and to a certain extent they are right; there are indeed measurable differences between different models. However, unless the cable is incorrectly manufactured, these differences are very small (tiny differences in phase at the lowest frequencies, or differences of under 1 dB in the treble), and almost always favour the thicker gauge. 5-Speaker  –  

Headphone level

This is the type of signal that comes out of a headphone amplifier, which is essentially a miniature power amp. It is dual unbalanced, usually sent through the same connector. The electrical characteristics are however very dependent on the headphones that are plugged in, which can be a bit all over the place. The signal can therefore have:

• Low impedance: the nominal impedance can vary from as little as 16 Ω for in-ear monitors, to about 600 Ω for large over-the-ear models.
• Low to moderate voltage: the average voltage can vary from as little as 2 mV for in-ear monitors, to over 10 V for large over-the-ear models; for this reason, extra shielding might be preferable for the former.
• Low to moderate current: the current can vary from 2 mA for in-ear monitors, to over 200 mA for large over-the-ear models; for this reason, slightly thicker cable may be preferable for the latter.

Matching headphone amplifiers to different models is a big subject in and of itself, but the cable is relatively straightforward. In general, due to the low impedance and the relatively low current, balanced mic or line cable can be used, wired as dual unbalanced. If long runs and ultimate sound quality are required for large headphones, thicker cable and lower capacitance are technically better, and for small in-ears, extra shielding might be required because of the low voltage. However, whenever the signal needs to travel long distances, it really is best to send line-level signals to a headphone amp as close as possible to the listener.
Since headphone signals are unbalanced, long lengths of multi-core cable can add a bit of crosstalk from other channels.

The connectors are most of the time either TRS jack or mini jacks.

Do note that although they can often work well with earbuds, line outputs do not have the circuitry required to provide enough current to large headphones and should not be used as headphone outputs. Some audiophile headphone systems used balanced, bi-amped connections through XLR; headphones need to be adapted to use these connections, and whether this improves performance in short runs is debatable.    6---Headphone  –  

Shielding

The ability of a cable to reject electromagnetic interference of all kinds (from mains hum to radio frequencies from mobiles and taxis) has to do with the quality of its screening. There are four types of screening material:

Carbonised plastic consists of a tube of carbonised plastic which surrounds the signal wires. This is only seen in the oldest and cheapest leads, since the rejection is quite poor. The only advantage is cost, plastic being cheaper than copper. Carbonised plastic is used in guitar cable, not as a screening agent but to discharge static generated when flexing the cable, which can create scrunching noises.

Aluminium foil is effective, but constant flexing will inevitably cause the aluminium to break. Therefore such screening is confined to fixed installations where the cable remains static. It is generally used in mobile multicores where the cable diameter is big enough to make flexing less of an issue. Such cables also contain an uninsulated drain wire which lays alongside the signal wires. Being bare the drain wire maintains electrical connection between the broken halves of the aluminium screening foil. Being so thin, the foil hardly adds to the cable diameter so keeping it compact.

Spirally wound copper screening consists of many thin strands of copper wire spirally wound round the signal wire, which is highly effective in keeping hum and RF interference at bay. Cheaper cable may contain fewer strands and be less effective. When flexed, the strands can open up leaving gaps allowing interference to penetrate. Cost is reasonable as manufacturing time is minimal.

Braided copper wire screening consists of thin strands of copper wire woven round the signal conductor. Technically it is the best option, since it is less likely to allow interference to penetrate and the woven strands cannot easily open. The disadvantages are cost and a lack of flexibility, particularly when used for microphones used by stage performers.

Some cables become very brittle at low temperatures (when for instance a cable is left in a vehicle overnight during winter). The better the cable, the less likely this will be a problem.

Connectors

Most people will identify a lead by its connectors; however, as we’ve seen above, this can cause problems ranging from degraded sound to broken equipment; it’s best to determine the type of cable, and then just use the connectors that the products require at each end. There is little to be said about the connector types themselves, since more often than not we are constrained by whatever the product designers have chosen; here are however a couple of useful tips:

• Most leads fail at the connectors; it pays to get the best possible quality whenever they are in constant use. Poor connectors will also add far more resistance to a lead than dozens of metres of cable.
• Removable connectors tend to be more reliable than moulded ones, and unlike them, they can often be repaired.
• XLR is the sturdiest type for interconnects and should be used whenever possible, as it is locking, mechanically sturdy, and easily daisy-chainable; jack plugs can bend the sockets from the inside if pulled at an angle, and TRS versions short the circuit when plugged in and out, causing large pops; Phono/RCA connectors are the least reliable, as they are mechanically fragile and tend to make a poor electrical connection.
• Speakon is the best and safest speaker connector type and should be used whenever possible, as it is locking, moisture resistant, and easy to daisy-chain with couplers.
• Americans use the word “jack” to refer specifically to any chassis socket, not to the connector type, which they call “phone”.

We’ll speak more in depth about specific mistakes to avoid when linking equipment in our next and final instalment of the cabling series.

Mini Glossary

Analogue audio is an alternating electrical signal, which means it’s a flow of electrons moving back and forth in a circuit, just like AC power. Here are its most important characteristics related to cable:

Voltage: it’s the electric force that pushes the electrons back and forth. Voltage carries the audio information, and is what one usually sees in a waveform. When electromagnetic interference gets inside the cable, it is added to the voltage in the form of noise and buzz. It’s measured in volts (V). #

Current: it’s the quantity of electrons being pushed around in a given amount of time. It also carries the audio information, and gives the signal the power to drive speakers, but it is all too easily turned into heat, so it’s always kept at a minimum besides in amp-to-speaker connections. It’s measured in amperes (A).

Impedance: it’s the combination of everything that obstructs the electron flow. A component with high impedance will act a bit like a nozzle, lowering the current that can go through the circuit. Inputs, outputs and cables all have their own impedance ratings. Impedance is made up of resistance, capacitance and inductance, which can either be important or not depending on the type of signal. Resistance and impedance are measured in ohms (Ω).

Resistance: it’s the opposition of a material to let electrons through it. It converts current into heat, and causes signal loss at all frequencies equally. Thicker cable will lower resistance.
Capacitance: it’s the opposition to the direction change of voltage across a component. This happens because the component will keep some electrical charge for a little while, which resists voltage change. In circuits with a high impedance source, capacitance lowers the ability of the signal to change quickly, which causes a loss of high frequencies. It’s measured in farads (F).
Inductanceit’s the opposition to the direction change of current through a component. This happens because current through the component will create an electromagnetic field that resists current change. In circuits with a high current, inductance lowers the ability of the signal to change quickly, which causes a loss of high frequencies, just like capacitance in high-impedance circuits. It’s measured in henries (H).

Power: it’s the total amount of energy travelling through a circuit. It isn’t a separate electrical property, but rather the combination of voltage, current and impedance of the signal: the more current and voltage, and the lower the impedance, the higher the power. It’s measured in watts (W).

Voltage bridging: this is a connection design in which the two audio components of different impedances are linked together, with the source having an impedance over 10 times higher than the input. This minimises current (and therefore heat and inductance), whilst optimising voltage, which is what actually carries the audio information. It also renders cable resistance a negligible part of the circuit impedance.