Advice on setting up hum free safe PA system
This text is based on my personal experiences with PA systems, RaneNote 110 and the advice given in Concert Sound and Lighting Systems book written by John Vasey (second edition, published 1993 by Focal Press). Following those simple rules I give you will avoid almost all causes of humming problems and you will save many hours for more creative work in your system setup.
Use single power feed everywhere you can
If your system power requirements allow take power only from one electrical power outlet. This will avoid the ground potential difference problem which can exist if power is taken from multiple power outlets. If the distance and electrical code permits is is a good idea to run a power extension cords from your single power feed points to all subsystems connected to your audio system instead of using the local power sources.
Wire all audio equipments which you connect to mixer to same phase
Use dedicated power feed for your audio system. Wire all sound sources, musical instruments, effects and your mixing desk to same grounded dedicated audio power feed. Do not connect any other equipments than audio equipments to this power feed. This saves you from many humming problems. It is a good idea to mark clearly which extension cords are part of your audio power feed so that you don't accidentally connect any other equipments to them (it is good idea to tape over all power outlets from this audio feed which are not used by your audio system so nobody will accidentally plug any other equipments to your audio power).
Especially avoid plugging fluorescent lights, lighting dimmers, computers and video equioments (like video projectors and TVs) to your audio power feed.
Use balanced connections everywhere you can
Balanced connections are much less sensitive to to pick up interferences and humming. If you still get himming with basic balanced wiring, there are much more to do to avoid it than with unbalaced connections.
Most professional audio devices are connected via balanced cables to minimize pickup of stray electrical noise. Consumer audio devices use unbalanced cables and are very prone to picking up noise, especially at low signal levels from devices such as microphones. Balanced circuits have an inherent ability to only pass audio signals and reject unwanted noise.
Balanced refers to the fact that there are two symmetrical signal lines and one ground, while unbalanced uses just one signal line in reference to ground. Normally, XLR connectors are used in most balanced devices while unbalanced consumer gear normally use mini-plug or RCA connectors.
Do not ground the XLR connector shells
The recommended practice in professional audio community is not to ground the shells of XLR connectors. The reason is XLR shell can come easily in contact with metal railings or somethign else which is grounded to somewhere else than your audio system. If your shells are not grounded nothing harmful will happen. But if you have for some reason grounded the XLR connector shell to audio ground you will get hummign to your system because all kind of ground paths/loops and sorting yout those can drive you crazy. It is best to leave the sells not connected so you don't add a new, intermittent problems, to your audio systems.
Transformer isolate all unbalanced connections
If you have a mixed balanced and unbalanced system, do yourself a favor and use isolation transformers in every connection between balanced and unbalanced system. When unbalanced lines cannot be transformer isolated, use special cable assemblies to solve the humming problems.
Use unbalanced cables for short distances
Any unbalanced cable must be kept under few meters in length. Cable lengths longer than this will amplify all the nasty side effects of unbalanced circuitry's ground loops.
Keep your audio cables away from power wires and transformers
Power wires generate magnetic field around them and this can cause humming to the audio cables. Especially avoid placing your audio cables near you main power feeding cables and cables which are connected to lighting system (the dimmers cause very fast current changes to cables which induce easily humming to audio cables).
Keep the audio cables at least half meter away from power cables. If the audio cables must cross power cable then they must cross them at axactly 90 degree angle to keep the hum coupling minimum.
Isolate ground in all wires which go to equipment connected to separate power feed
If you must connect equipments which are powered from other power source than your mixing desk then you must make sure that they don't cause ground loop problem. Safest choice is to use audio isolation transformer in every connection you need to equipments connected outside your dedicated audio power feed. For balanced connections also ground lift in audio wire is an acceptable way to isolate audio grounding between different system
If your power amplifiers are connected to different power feed than your mixing desk then use balanced connection between your mixing desk and the power amplifier and use ground lift to isolate audio ground in the cable (the ground lift should be done in the amplifier end). if you can't use balanced connection and ground lift then use isolation transformer in that audio line.
If your system has also video and/or computer equipments then give them a separate power feed. Use audio isolation transformers in all audio connections between your computer/video system and your mixing desk (computers and video equipments typically have unbalanced audio connections so isolation transformer is the only proper way to get rid of humming problem).
Keep some spare audio isolation transformers always available
Audio isolation transformers are very powerful tools for solving unexpected humming problems when you connect some extra equipments to your system. When you have some extran isolation transformers always available the problems can soleved easily ad effectively whenever they pop up.
Keep transformers and power distribution panels away form your audio equipments
Audio equipments and wiring can very easily pick up humming from magnetic fields. So avoid putting any sources of such magnetic interefrence near your system. This applies to power transformers, video monitors, computer monitors, electric motors, fluorescent lights and any mains wirign which carry large currents.
Do not keep your cables coiled
Coiled cables for a coil which will easily indice magnetic field around them and pick up magnetic fields. If your power cables are tightly coiled they can cause large magnetic fields and heat too much up if loaded very much. Coiled audio cables can easily pick up interferences. If you have spare cables you need to put somewhere do not coil them just to a single coil, but instead put them on 8 shape to ground (this shape greatly reduces the magnetic interference problems).
Star Grounding: One Path to Ground
Star grounding is the name Audio and Electrical engineers use to describe a wiring system where all the electronic SIGNAL grounds (cable braiding, jack sleeves or pin 1 on an XLR ) either
- Flow through in a serial manner without coming into contact with the electrical or chassis ground on the way, or
- Join the mains power supply ground (the really really big thick green wire) at the one place and one place only.
With star grounding systems the centre that is usually chosen is either the Main Mixing Console or the multicore snake or patch bay that is usually plugged or hardwired into it. The reason for this choice is that most Audio Systems only have one main mixing console and the vast majority of devices are connected to it.
Conformity with this grounding system will usually ensure that your system no matter how large will remain hum free.
Avoid connections to other grounds than your system ground
There are other things than the power connections which can me an unwanted grounding connection to your audio system. Audio connectors which have metal shell which is in contact with the cable shield can cause unwanted grounding if they touch any grounded metal in the building or some other grounded equipment. Same applies also to microphones and snake cable connection boxes. So if there is change thet your exposed audio wiring ground can touch any building ground then put some insulation between your wiring and that ground. Avoid putting equipment racks over any metallic parts in the floor because it is possible that they are grounded for some reason. If you have to place equipments on such place then put some kind of carpet between the metal parts in the floor and your equipment rack to provide necessary isolation.
Use correct gain structure in your system
By using the correct signal levels throughout the system will minimize the humming, noise and distortion. Gain controls in PA power amplifier be typically set between half-way and fully up. Amplifier gain controls set at a lower position require input signals to be set to a higher level to obtain suitable power levels. Particularly with unbalanced input lines, the hotter your signal is at the input of an amplifier, the more noise propogation you will have into your amplifiers. If you have set the amplifier gain set too low your system may become such that you will reach the maximum gain travel of a fader, at your source device, before obtaining expected power within your amplifiers.
Tomi Engdahl <Tomi.Engdahl@iki.fi>