Other than ground loop sources for humming

Not all humming and noise is causd by ground loops. There are lots of other interference sources which can cause humming and other kinds of noise. An earth loop will typically inject a 50Hz ot 60Hz hum into the signal cables. There can be also exist other harmonic frequencies in ground loop soiation, but practically always there is that 50 Hz or 60 Hz strongly around.

if you have a 100Hz or 120Hz hum (which often has a hard "edge" to the sound) without that lower frequency 50Hz or 60Hz aroud, you should definately check also other sources of interference. Possible causes for this kind of only higher frequency interference is something wrong in the wiring of the power supply or some noise source in the electrical power system which causes that harmonic noise (light dimmer for example).

I am getting a lot of hum and hiss from my system. Is there a quick way to tell if the amplifier is making the noise, or is it a problem with my system?

The fastest way to tell if the amplifier is the cause of the noise, is to disconnect the input cables from the amplifier. If the noise is still there, it may be amplifier; if it's gone, it's a source device inducing noise into the amplifier. If after removing the input connectors from the amplifier you find the noise still present, it will then be necessary to determine if the noise is coming from the AC line. This further isolation may be helpful. Try relocating the amplifier using a different AC service, if the same level of noise is present, the amplifier is likely to be the cause. If the noise is lower, the AC service may be the cause.

Magnetic fields from transformers can cause humming

Mains transformers typically cause an AC magnetic field around them. This magnetic field can be copled to wiring, electronics or signal transformers and cause humming. Typically audio electronics are designed so that the magnetic felds caused by the mains transformer are not coupled to the sensitive elctronics in the equipment, but the magnetic fields from other equipments can be coupled to the electronics.

If you have many equipments which have transformers inside them stacked together then you have a risk that the magnetic field form other equipment's transformers can cause hummign to the other equipments. The normal aluminium case does not provide much protection against low frequency magnetic fields and only steel case can shield agains magnetic fields. Some power transformers in powerful equipments (like amplifier) are leaking a pretty heavy magnetic field. This has something to do with it being cheaply constructed without good core material and something to do with the design configuration. Leaving a couple blank spaces between the equipment with that leaky power transformer and anything else might help. You can also try moving things around so the units that are least suceptable to hum pickup are closest to humming source.

Many equipments nowadays use small wallwarts which you plut go wall socket. Those transformers are usually chapely built (like those cheapest universal adaptors) and can cause lots of magnetic fields around them. It is not advisable to put any sensitive signal wires or equipments near those wall transformers because they can cause serious humming problems. Look around and leave adequate space between the gear with transformers inside of them.

Magnetic fields induced from other wires

Hign current carrying mains wiring can cause quite strong magnetic fields around the cable. If you run an audio cable near high current mains wiring it can easily pick up humming. It is a good idea to keep at least 0.5 meter distance between high current mains wiring and your audio cabling. If the audio cable must cross a mains cable then make this crossing at exactly 90 degree angle so the mains cable magnetic field will not be coupled to audio cable.

The interference coupled from mains wiring can become especially noticable if there are some problematic loads like fluorescent lighting, light dimmers or lots of computers connected to the mains wiring.

Magnetic fields from distribution panels

Main poer distribution panels have high current carrying wires, relays and contactors in them and they can cause quite storng magnetif fields around then. It is not advisable to put any computer monitors or audio equipments near them. Few meters clearance from mains distribution panel is usually enough to stop any magnetic interference.

Magnetic fields from electrical motors

Electrical motors use strong magnetic fiels inside them, so typically those motors also cause a noticable magnetic field arount the motor. If your equipments are near some powerful electric motors you might have quite strong magnetic fields near your equipments. Remeber also that fans used in air conditioning and in many equipments can generate problematic magnetic fields so it is a good idea to keep yout equipment away from those.

Radio interference interference

Radio interference can cause serious noise problems to the audio system which is not properly shielded. The most typical noises are all kinds of interference noises and the transimmission is heard in the audio system. Some strong radio transmitters and industrial systems which use radio frequency signals can sometimes be modulated with mains frequency humming (for example because of poor power supply). If this kind of modulated radio frequency signal enters to your equipments it can cause mains hummign like interference which is very hard to get rid of.

Main voltage quality problems

If mains voltage is not nice sine wave then it contains harmonics which can usually more easily enter to the circuit than 50 Hz mains. Typical sources for this kind of power quality problems are high frequency interference caused by dimmers. Computer power supplies and fluorescent lighting can cause harmonics to the power and thus interference to sensitive equipments. Some cheapest UPS equipments put out vety low quality main voltage and if you connect sensitive audio equipments to such UPS you can get all kinds of noise problems.

Mains quality problems are usually solved by using mains filters and isolation transformer. Many professional studios used high quality on-line UPS equipments to power the whole studio. This UPS will filter out any problems from the incoming power and also useful protection against short mains power breaks.

Sources for hissing noise

Sometimes the phumming in the audio system is nor the main problem, but he hissing like noise. Here are few noise sources and how to avoid that noise.

First and foremost there's what is called "thermal noise". It is generated by all active and passive electronicd components (ncluding cold solder joints) and is simply a result os all those un-cooperative electrons smashign each other. The only way to avoid the hermal noise in components is to make them very cold, but this is not an option in audio electronics. The gain structure of your system becomes on important to get madimum signal/noise ratio.

Radio frequency interference (RFI) is a big source of all kinds of noises. Many RFI interferences are easily un-detected if you can't hear the actual "radio signal". Radio interference can easily take the form of hash that sound like white noise. Proper grounding and shielding practices are your only real defence against RFI.

Digital noise usually takes a form of high-pitched whine (usually round 15 kHz or so), but at lowe levels can be mistaken for hiss. Digital keyboards, lighting, computers, effect processors and just about everything which uses fast clock pulses can cause this problem. This includes the switched power supplies in many modern audio equipments and some amplifiers. Some poortly designed equipments can radiate excessive levels of digital noise, often via mains AC cord. Carefully routing the AC power cords and audio cables away from each other can help to avoid this problem. Digital noise can also be in forma of radio frequency interference, so do not place a wireless microphone receiver near difital effect units and computers.

An electronics component which is failing can be a source of noise. You cqan identify this hiss because it is not consistant. It will vary fairly rapidly, depending on run time and temperature. The noise can also be "popping" noises, similar to popcorn. To get rid of the problem locate the bad part/equipment and get it replaced or repaired.

One problem is the poorly designed equipments. Some companies use active components with poort noise performance. You can get rid of this kind of problem by replacing the nasty components or the whole equipment. If you must use the existing equipments as it is then careful gain structure adjustment can usually keep the noise at acceptable levels.


Tomi Engdahl <Tomi.Engdahl@iki.fi>