Before I give advice on how to remove HUM! It is important to understand what a HUM is!
A hum can be many things, but the most common is an earth hum. The earth hum is basic a 50hz buzz, that is caused by either faulty cabling of equipment. You can get earth hum: –
- if your earth reference voltage is different from one end of the signal path than the other
- if you run an audio cable through a load of coiled mains cable
- if you use lighting dimmers on the same supply as your sound system supply, although this type of noise is thyristor hum not earth hum
One of the most common types of earth hum is caused by the reference voltage being different at each end of the signal path. The best way of fixing this is by taking all your mains from one central place, which means avoiding ring mains. This should eliminate any earth loops you may have.
It may be as simple as the faulty main extension has not got an earth or the cable you are using is faulty.
However some times earth hum can be very difficult to fault find, especially in a very large PA system or music studio. So one of the best things you can do, is narrow down were the hum is coming from by turning off all equipment apart from your mixing desk and then turn on all other equipment one by one.
Once you have found the source of the hum, the first thing you must do is check that is has an earth! (NEVER REMOVE THE EARTH!!) The only pieces of equipment that should not have an earth are class2 pieces of equipment, which either have external power supplies or our totally plastic.
If the earth is ok, unplug the input to the piece of equipment and see if the output is still sending earth hum, if the earth hum stops then you have an earth loop, see below.
If the earth hum continues even though you have unplugged the input to the piece of equipment, then if it has a stereo output, unplug one of the outputs (unplug the right output, will usually send both the left and the right signal down the left (MONO))
If the earth hum stops, then you have an earth loop, see below.
If the earth hum continues, then remove the earth from that one audio output, the easiest way of doing this is by putting the device through a DI box with an earth lift switch!
If the earth hum continues then remove the piece of equipment from it’s surroundings, making sure it is not touching any pieces of metal, this will usually stop the hum.
If you have an earth loop, it simply means that the either you have plugged your mixing desk and your other piece of equipment into two different mains supplies, which basically allows a ground current to form and circulate between the two pieces of equipment. Causing a earth hum!
You must take all your mains from one central place, this even means your mixing desk at the back of the hall.
The best way of doing this for most systems is to only allow all your mains to be taken from one double 13amp socket, run your amps and mixing desk from one of the sockets (using a 50 meter mains lead to run power to your desk, and then give the band all there power from the other socket.
This will work for music and video studios as well, only having one main source for all your mains requirements will resolve most of your earth hum problems.
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