Hum problems in Audio from the mains and other electrical sources

Hum problems in audio can be notoriously difficult to track down. Usuallly they result from earth loops between equipment that somehow creates a mains derived voltage oscillation that feeds into the signal path in your amplification. This is then amplified internally by your amplifier to produce hum through your speakers.

Over the years we have compiled a list of the various causes that can give rise to hum problems and their solutions. To save time just skim down the sub-headings till you get to a relevant section.

Electro-Magnetic Interference

Mains cable or transformers and amplifiers in the vicinity of low level signal cable can easily cause hum so ensure the arm cables and any phono stage interconnects and phono stage are spaced well away - you will hear hum diminish as they move so you can judge how far is safe enough.

Connection of earth leads to equipment

Never tamper with earth lead connections from the mains. These are critical to ensure that you are not exposed to lethal electrical shock and they are often essential for equipment to function correctly.

This section deals with seperate earth leads that connect to the outside of equipment. These are leads that screw onto a rear binding post on the back of your amp. They may come from a tonearm, turntable, interconnect cable (if it has an external shield) or other item of equipment.

It is usually a bad idea to add a wire to join items of equipment like a pre-amp and power amp but this is a matter of experimentation as to whether it does any good. It should also only be done where a rear binding post exists on the rear panel of any item of equipment. The reasons why some equipment carries an earth terminal and some don't is complex and certainly not something that can be explained here.

Ensure there is no break, bad termination or poor connection in earth wires.

You should also connect external earth wires to different equipment as well as experimenting not connecting it to others. For example the earth lead from a deck or arm is best connected to the phono stage binding post or if you don't have one, the pre-amp.If you only have an integrated amp there is only one choice so you can't go wrong.

Deck sub-chassis not earthed to arm

Some decks need an earth connection between their chassis and the tonearm to prevent hum. The Technics 1200 series is one such item. Although Linn decks have this option it is seldom necessary. 

Mismatch of Amp Input

If you are using a moving magnet cartridge make sure that the phono stage setting is correctly set for moving magnet input and vice versa for a moving coil cartridge.

Unshielded Cartridges

Grado cartridges are unshielded and this can cause problems with a/c turntable motors which kick out quite a strong magnetic field. The only solution is either to change the cartridge or use a DC motor.

Earth Loops

You can experiment with attaching and detaching earths - particularly if you have a pre-power combination - sometimes it is best not to link the two together by an earth wire.

Fault Within the Cartridge or the Amp

Cartridges can sometimes develop hum as can amplifiers. To eliminate this possibility you can swap channels if the problem is only on one channel and hear the result or you can substitute the cartridge or arm if possible.

Areas of Very High RF

i.e. residences near radio transmitters - Try enclosing the cable in a large diameter braid earth ed to the arm or pre-amplifier- you can try it in foil but it is limited in its screening effectiveness. Once something works, a more elegant solution can be found - If you are stuck for braid try Maplins.

Earth Lead Connection Between Arm & Deck (Technics 1200 Especially)

When we internally rewire an arm then a separate ground lead is installed which enables easy connection of the arm to earth. However for the OL1 without any modifications, the arm earth is attached to the return signal of the left hand channel i.e the outside metal of the black plug (at the other end - the wire with the blue cartridge tag on the end). Try touching the earth wire from the deck to the outside metal part of the black phono plug with everything playing. If this eliminates hum then you can splice into the arm cable for permanent attachment.

Tonearm Wiring

We test all our arms and rewiring jobs for hum before they are dispatched, however faults can very occasionally develop in transit, fitting or subsequent use. From time to time there is an internal wiring fault that may develop which causes hum - this is something that is often possible for you to rectify - However before trying to fix or return the arm to us, it is worth listing the other causes of hum - this list is not exhaustive.

If the following list does not identify or solve the problem, please refer to the section much lower down page on "One channel down and no sound coming out of one speaker or hum problems" - section (e).

Unshielded Cartridges

Grado and Music Maker cartridges can hum when used on turntables which have an a/c synchronous motor not positioned on the far side of the platter. You can either change the cartridge of fit one of our DC motor kits which emit a very low electromagnetic field.

Tonearm interconnect issues

Occasionally there can be an impedance mismatch of tonearm cable litz wire capacitance with the pre-amp input - if you have another pre amp or can get hold of one it is worth checking this.

 

 

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