MusicWiring and connecting a PA or recording system can be very complex and potentially dangerous. Here are some issues to consider.

A Little Background to Music Connectivity

The world of connectivity and wiring within backline, PA and recording setups can seem at times bizarre and bewildering. There are numerous connection types, from XLR and TRS Jack to RCA Phono and Mono Jacks, along with screened and unscreened cables. We’ll attempt to unravel these mysteries and hopefully offer a clear guide to what you need to use and when to use it!

First, it’s worth pointing out that recording connectivity is very different to PA (public address) or backline (monitoring) connectivity. PA and backline sound rigs involve feeding signals to speakers at high power, whereas the signals used in recording are rarely more powerful than a home hifi unit, and are mostly much lower than that.

As a result, we’re dealing with significant safety issues in these circumstances, as well as the usual considerations of signal quality and connector type. But let’s kick off by looking at one of the most basic parameters – signal strength.

Signal Strengths

There exists a standard protocol within Pro Audio for unamplified, sometimes referred to as preamplified, signal strengths to ensure that when ancillary products are connected they work as desired. This protocol is referred to as a ‘Line Level’ signal.

For example, when you connect a microphone to a mixer or a guitar to an amp, the first section the signal hits is a preamp of some form. The function of the preamp is to condition the low level input signal (e.g. a vocal from a mic or a guitar/bass), by bringing it up to line level strength. A classic demonstration of this is a guitar amp with a line level/effects output, which always follows near the end of the input stage but always before the power amp stage. Similarly with mixers, all channel inputs are conditioned by the input gain and fed to the output of the mixer, which can then be connected to a power amplifier.

In any high-quality preamp, there is a lot going on besides simply correcting the signal strength. This isa why the term ‘conditioning’ is used and it’s why so many sound engineers orientate towards certain preamps for particular sounds or situations.

However, Line Level comes in one of two varieties, namely Balanced or Unbalanced and two associated signal strengths -10 dBU or +4 dBU and therefore users need to be extremely careful when connecting products to ascertain the appropriate signal level to avoid any related issues, e.g. distortion or poor output levels.

Screened or Unscreened Cables

There are two general types of cable used with Pro Audio/PA/Recording applications, one screened the other unscreened.

When connecting any products before an amplification stage it is a requirement to use screened cables to avoid an unnecessary introduction of noise within the signal path. Note that any cable acts as an aerial and superimposes the abundant amount of RF noise kicking about the atmosphere onto the signal it is carrying. This is a really important, but often overlooked, issue.

One way to help minimise this is to use a shielded or ‘screened’ cable. The shielding/screen wraps completely around the core carrying the signal and shunts the noise to earth, thereby protecting the integrity of the signal it is passing. However, please note that unscreened cables are designed for very low power usage.

Unscreened cables are generally referred to as speaker cables and come designed to handle serious power. There is no need to use any shielding as the difference in level between the signal and any potential interference is absolutely huge, making any noise simply negligible. If for some reason a screened cable is used as a speaker cable there is the serious potential for trouble as the signal strength will simply burn the cable out. Best case scenario is that the cable essentially breaks (I’ve seen this in practice when a PA stopped working as a result of using incorrect cables), but if the core fuses with the screen it will short the output stage of the amplifier, which can result in rather a large repair bill.

UnBalanced Cables

A single core screened cable generally carries an unbalanced line level signal (-10dBU). The screening, which is wrapped around the core that carries the signal, provides the earth connection and protects the core from unwanted interference or noise. However, within long cable throws it is still possible for RF noise and interference to penetrate the screen and to become superimposed upon the signal.

It is relative easy to spot and unbalanced cable/connection as it simply uses a two core connector, i.e. one for the screen (earth) and the other for the signal (+’ve). Typical connectors used in unbalanced cables are ¼” Mono Jacks, for example a guitar lead, or Phono to Phono (RCA) or Phono to Jack cables.

Balanced Cables

To counter the issue touched upon above, wherein RF noise or interference may still become superimposed upon signals carried by long unbalanced cabling, balancing was introduced.

one may use balanced connections providing the products within a particular system support this protocol.

Balanced cabling uses a twin core screened cable. As in an unbalanced lead the screen offers a certain degree of protection against unwanted noise, which may be superimposed upon the signal carried by the core. However, within a balanced signal there are two component parts that carry the signal, a positive connection and a negative connection, which are precisely 180 degrees out of phase.

Upon transmission within a balanced system the signal is split into two components, one remains untouched (positive) whereas the other (negative) is phase shifted by 180 degrees. Upon receipt, the negative component receives another 180-degree phase shift and at this point is recombined with the positive signal. As a result any noise, which may have been superimposed upon both the positive and negative components of the signal within the cable, becomes completely phase cancelled leaving a ‘clean’ signal with a far higher Signal to Noise Ratio, than would otherwise be the case if using unbalanced connections.

Balanced signals also have a higher signal strength, at +4dBu, than unbalanced signals, which operate at a standard –10dBu. Typical connections are either XLR to XLR leads (i.e. microphone cables) or ¼” TRS (Tip, Ring, Sleeve) Jack to TRS Jack – not mono Jacks. Note that within XLR’s, pin one is the screen, pin 2 positive and pin 3 negative whereas on a TRS Jack the sleeve connects to the screen, the tip is positive and the ring negative. Although, again pending the products, sometimes XLR to TRS Jack leads may be used.

I would therefore recommend using balanced connections, particularly over long cable lengths, to unbalanced – providing the products used within a given system support this protocol.

What to Buy

When it comes to connectors and cables, there are three key considerations – and none of them relate to which brand you should buy!

  • Look at your set-up and make sure that you understand the best cabling configuration
  • Buy the highest quality connectors and cabling you can afford. It’s pointless having good kit and feeding it a poor signal
  • Buy robust cabling/connector combinations as, they’ll be much better value-for-money, especially in mobile PA rigs.

For more guidance on this, try talking to the PA guys at one of the knowledeable retailers. I happen to use Soundslive a lot.

Finally, and on a very serious note, screened cabling must never be used as speaker cabling, since it extremely inefficient at carrying the high power levels generally delivered by power amplifiers (most amateur bands don’t appreciate the Watts being carried by these cables). The effect of using screened cables for speakers is normally twofold, firstly reduced output due to the resistivity of the cabling and secondly cables failing as they burn out – speaker cables use much thicker wiring designed to efficiently pass high power, unlike screened cabling.

By Andy Atkins

Andy Atkins has written about music gear for some of the biggest and smallest names in the business, including Fender, Roland and Soundlive. He still gigs occasionally in a band composed almost entirely of ex marketing managers! You can find him at Google+

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