Dealing with balanced and unbalanced audio connections
Introduction
In this document I hope to explain where the funny buzzing noises in your sound gear come from.
Then I will suggest ways of improving the wiring to get rid of them. As a youngster I was always
plagued by humming noises when hooking tape decks, radios, guitars, etc. together. Now I know
how to get rid of them I would like to share that knowledge with you...
First check your leads!
Broken leads are a very common cause of interference.
- Open XLR and jack plugs, and check that all
wires are firmly soldered in place.
- Wiggle and stretch the cable while feeding a signal through it, to
check for breaks within the cable.
- Plug and unplug connectors several times to see if you have a
dirty contact (it's a good idea to do this to every connector in your system once a year)
- Throw cheap skinny leads with moulded plugs in the bin where they belong.
If this
doesn't produce an improvement, read on...
Theory
Unbalanced lines
These are the type you most often see in domestic and semi-pro equipment. An example
would be the lead connecting your CD player to your hi-fi amp. The cable consists of one
conductor carrying the audio signal, enclosed in a conducting screen. The screen is connected
to earth, and is used both as an interference shield and a reference for the audio signal.
Over short lengths these cables can be quite satisfactory in keeping out interference.
However they are prone to the dreaded 'earth loop' whatever length of cable you use. In
my opinion earth loops are responsible for 90% of dastardly buzzing noises you might
encounter in your equipment.
What is an earth loop?
It is a loop of wire formed when you join two items of earthed equipment with another earth
wire. The classic example is when you connect the unbalanced output of your keyboard (which
is connected to earth via its mains lead) to the unbalanced input of your mixer (which is also
earthed via its mains lead) The screen of the audio cable acts like another earth wire, joining
the keyboard's and mixer's earth points together.
The earth loop then goes; from your keyboard output, along the screen of the audio cable, to
the mixer input, via the innards of the mixer to the mixer's mains lead, down the mains lead to
the wall socket, along the mains earth wire to the socket where your keyboard is plugged, up the
keyboard's mains lead, via the innards of the keyboard back to the keyboard output!
The huge loop of wire acts like an antenna picking up interference from mains wiring.
This causes large currents to flow in the loop. The current flowing in the screen of
your audio cable is picked up as an audio signal. It sounds like BBBZZZZZZZZZZZ....
Balanced lines
Balanced lines are different and a lot better. They use two signal conductors; one carries the
audio signal and the other carries an inverted copy of it. At the receiving end, a special
circuit takes the difference between the normal and inverted signals. Any interference which
gets into the cable is cancelled out because it affects both signals equally.
Balanced cables have a screen too, but it plays no part in carrying the signal. If the screen
forms part of an earth loop, there will still be no interference.
Practice
Balanced to Balanced
If you only ever connect balanced equipment to other balanced equipment, you should never get any
horrible buzzes. But we don't all work at Abbey Road or the BBC ;) so we need to deal with other
situations.
Unbalanced to unbalanced
The most difficult is when both pieces of equipment are unbalanced. It is vital to break the earth
loop. Since you can't break the screen of the audio cable, you must 'lift' the ground on one of the
appliances. There will often be a switch provided for this purpose on the back panel.
If not, it is possible to lift the ground by disconnecting the earth wire in the mains plug, but
I don't recomnend it; it is dangerous.
Since earth loops are such a problem in unbalanced gear, you will often find that it is made to be
'double insulated', i.e. there is no earth wire in the mains lead at all. Therefore, you will have
no trouble with earth loops. However, if you bolt the gear into your rack, it can make contact with
other earthed equipment, which can give you trouble. There are special plastic-coated bolts you can
buy to keep it insulated.
If you are especially keen and handy with a soldering iron, you can upgrade unbalanced equipment to
balanced inputs and outputs. High-end computer sound cards like the Terratec EWS64L/XL benefit
especially from this, as it helps reject interference from the computer.
When you connect balanced equipment to unbalanced equipment, the fun really starts. By some clever
wiring, you can break earth loops and still have both appliances solidly earthed.
Unbalanced output to balanced input
Use balanced audio cable. Connect the balanced input end as usual. At the unbalanced end, do not
connect the screen; the 'hot' signal wire goes to the signal output, and the 'cold' wire goes to
ground.
Balanced output to unbalanced input
The connection is the same as above. It only works properly if you have 'transformer' or 'servo'
balanced outputs. The equipment manual should tell you. If not, test like this;
Testing for transformer or servo balanced outputs
With a signal going to the output, connect an earphone or small speaker between the 'hot' pin
and ground. You should hear nothing, or very little. Now short the 'cold' pin to ground; you
should hear the signal.
If you have the other crappy kind of balanced output, you will hear the same signal whether the
cold pin is open or shorted to ground. In this case, proceed as for connecting unbalanced to
unbalanced.
Make your own leads
Now it's time to have a stab at making your own leads. I have made up a table of wiring for the
more popular pro-audio leads. I'll add the likes of stereo mini-jack and DIN connectors later,
if I can be bothered...
Know your leads
e-mail me: "steve at scopeboy dot com"
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