Audio Connections: The 7 Deadly Sins

by Pablo Bellinghausen

Basic wiring in the consumer world  is usually as easy as using any lead with the right plug at either end, but doing this with professional equipment can often go wrong, sometimes dangerously so. As we’ve seen in our two previous instalments, there are quite a few things to consider if one wants to understand how professional analogue connections work, but a beginner can often muddle through the basics – as long as he doesn’t make any of the really big mistakes!

Here is a short list of wiring deadly sins that no musician or engineer should ever do:

1. Mistreating cables

Cables are long and thin, and no matter how much money you pour into them, the laws of physics just cannot be broken. Bending them at sharp angles, running over them with heavy equipment, pulling them out from sockets violently, knotting the the ends for easier storage, and leaving them in damp and mouldy places will eventually degrade the cable’s performance.

The correct way to store audio leads can be seen in the below video:

If you follow this method (called the “over under” technique), you should be able to grab one end of the lead and chuck the bundle away from you, and have the cable uncoil perfectly in front of you. Any other technique will start twisting the internal conductors over time (particularly the shielding), in which gaps can form, rendering the lead vulnerable to interference noise. In worst-case scenarios the conductors will eventually break (usually, but not always, at the connectors), and the cable will either start crackling (which causes damaging pops if phantom power is applied) or stop working altogether.

Knotting the end of the lead is a pretty bad idea; the only correct way to secure the bundle is through the use of cable ties. The velcro ones are usually the most convenient, particularly those that latch onto the cable so you don’t lose them.

Cheaper leads won’t necessarily sound worse (see the end of our previous instalment for more details) but they will definitely be far more fragile, and if they aren’t used and stored properly they will start crackling or failing after a few uses.

2. Mixing mic and line level signals

Since XLR connectors are bigger and slightly more expensive than jacks, they are relatively rare in budget equipment. People who are unfamiliar with them will often assume they are all intercompatible and will plug a microphone into a line-level XLR input. Nothing bad will happen, but since mic level signals are over a hundred times smaller than line level ones, the sound will be so quiet it will most likely be unusable.

On the other hand, plugging line-level signals into mic-level inputs will send a very loud signal into circuitry that isn’t designed to handle it. At best, you will have to keep the volume very low and end up with a bit of distortion; at worst, it can send incredibly loud and clipped audio into speakers, headphones or your ears, risking damage to them.

It’s good practice to put the level down whenever plugging anything in or out, and raising the gain slowly just in case the level has been set incorrectly.

Mic-vs-Line

3. Plugging powered outputs into line, instrument, or mic level inputs

Since jack plugs are used for all different types of connection, and adapters to and from Speakon are sold in most pro audio shops, it is unfortunately very easy to plug the output of a power amplifier or powered mixer into devices other than passive speakers. The most common scenario is when inexperienced installers wish to replace a passive speaker and unknowingly purchase a powered (or “active”) model; these have a built-in amplifier already and only require a line-level signal to work.

This is a lot worse than just mic-to-line mismatches; plugging the output of a powerful amp into the line input of an active speaker (or, in an absolute worst-case scenario, a mic input!) will send a signal thousands of times more powerful than what the input is designed to take, causing very high distortion at best, and burning equipment and/or hearing damage at worst.

XLR-Speakon

Speakon to XLR male adapter: a mic preamp’s worst nightmare!

4. Using instrument or line leads for speaker signals

This is sadly a common sight in rehearsal studios around the world, and is a major cause of poor guitar tone, and ultimately, of damaged amps.

The conductors used for guitar cable (particularly the shield, or ground) are very thin and are poorly suited for running several hundreds of watts through them. The sound will be degraded, and more importantly, the conductors will overheat and slowly melt, along with any internal insulation. If the amp is used at high volumes, the cable will eventually fail completely, which is deadly to valve amps; if the insulation melts enough that the conductors touch each other, this will create a short circuit, which can blow up any amp.

Speaker-vs-Line

5. Mixing phantom power and bias voltage

Phantom power is not the only type of voltage that is used to power condenser microphones; there are two other types, which are not intercompatible:

T-power, or A–B power, is a voltage of 12 V that is sent through the hot and cold conductors of an XLR. It used to be very common in the broadcast industry before they figured out how to provide 48V with low-powered batteries. There are still plenty of T-power products in the market, and care needs to be taken when using them; mics using T-power require an in-line voltage converter to be used with standard preamps, and T-power inputs can damage dynamic mics straight away.

Plug-in power (often called bias voltage) is a standard that is used in most mini jack mic inputs available in wireless transmitters, computers, portable recorders and cameras; it sends 3 or 5 V between the hot and the ground conductors. This kind of power will not damage any mics, but will not power phantom powered models and might distort the audio from dynamic ones; models designed for plug-in power will however be damaged if phantom power is applied.

Most pro headworn or lavalier / lapel mics use bias voltage with TRS mini jack, mini XLR, MicroDot, or Micon connectors; they require an adapter with a voltage transformer to be used with standard XLR preamps. Do note that some adapters that have the correct connectors might not have the correct transformer and will therefore cause irreparable damage to your mic.

Minijack-XLR-AdaptSennheiser MZA 900P and Røde VXLR mini jack to XLR adapters;
Only the former has a transformer for use with lapel or headworn mics!

 

6. Sending high impedance instrument signals through long runs of multi-core cable

Sending the output of a passive guitar through a multi-core snake into a mixer dozens of metres away won’t break anything, but it will sound dreadful; since it’s unbalanced, the other signals will bleed through the shielding, since it’s high-impedance the sound will be horribly dull, and overall noise and buzz will make the final result an unpleasant mess.

This is the reason why DI boxes exist; they lower the impedance of the signal, balance it, and lower the voltage so that it can be safely sent to the microphone inputs of a mixing desk.

DI-Box-Explained

DI boxes are required to send a guitar’s “direct” signal through
long lengths of multicore cable without degrading the sound

 

7. Mixing stereo unbalanced and mono balanced connections

Mixing balanced and unbalanced mono inputs and outputs is nowadays rarely a problem, since the “cold” signal carried by the extra conductor is just dropped, at which point the connection just becomes unbalanced – many people have done this without even realising it. However, mixing mono balanced and stereo unbalanced connections (particularly on jacks, since both use the same TRS, or tip-ring-sleeve connector) will cause strange things to happen to the sound:

• if a mono balanced output is plugged into a stereo unbalanced input, then the left and right signals will be out of phase, which gives an odd, watery sound; the audio can however be recovered by only using the left or right signal. This happens most often when plugging in an XLR professional mic into a camera’s mic input with the wrong lead. Playing a file like this on a device with only one speaker (which sums left and right signals) will result in no sound.

• if a stereo unbalanced output is plugged into a mono balanced input, then the left and right will cancel themselves out, leaving only the “side” part of the stereo signal; at best it will sound very strange, with decreased vocals and bass, and at worst it will completely cancel out the sound if the original sound is mono. This happens most often when plugging in a headphone output into the XLR or TRS jack input of a mixer.

The correct thing to do if you are having this problem is to buy the correct adapter lead, which will route the signals the right way:
Adaptor-Lead

Recommended products

Durable rubber and plastic cable protection systems for anything from studios to festivals:
Adam Hall Cable Protectors 

High-quality, durable cable ties that actually stay on the cable:
Klotz Velcro Cable Ties (5-Colour Pack)

Useful adapter for plugging line-level signals into mic preamps:
Shure A15AS In-line Switchable XLR Attenuator

DI box for plugging (perish the thought!) speaker-level signals into line or mic preamp inputs:
ART Z-Direct Passive DI Box

Large range of high-quality adapters for plugging plug-in powered lavalier and headworn mics into phantom-power XLR inputs:
Ambient Recording Eumel Electret XLR Converters

The standard in DI boxes found in studios around the world:
Radial JDI Passive DI Box

Best leads to plug portable recorders and computers into mixers and other hardware equipment:
Klotz Stereo Mini Jack – 2× Mono Splitters

If you need guidance of any kind when hooking up your equipment, it’s always best to give a call to the sales team of your favourite pro audio retailer, who will be very happy to help you out. Better safe than sorry!