FORMULA SOUND PM-80R [5/8] You must not disconnect the mains earth wire from the mains plug of any

Gain

In an audio mixer different signal levels from various items of equipment need to be amplified or

attenuated to a common level so that they can be added or mixed together. These signal levels may

differ considerably depending on the equipment. The signal from a microphone for example may be

1000 times smaller than that from a CD player. For a mixer to be as flexible as possible and to accept

signals from a variety of equipment it is necessary for the first or early stage in the input circuit to be a

variable gain amplifier. The gain of this amplifier is set by the gain control.

adjustment on the front gain control.

When amplifier gain is introduced in a circuit noise is also introduced (this is a fact, you cannot have one

without the other). The PM-80R has been designed to keep this noise as low as possible by using the

latest technology. For the best performance set the gain control as low as possible to achieve the

desired output level. Gain introduced into the system and then held on the channel fader is a waste of

performance. Too much gain could result in overloading the first stage causing distortion and clipping.

The level can be checked by selecting MON and observing the output meter. Normal operating level is

with the meter just illuminating into the red section.

possible. The interconnection of audio systems can be fairly complex depending on the type and size of

system and obviously well outside the scope of this handbook. We have included a few basic points for

information for anyone who is new to audio systems.

Good wiring practice should be observed when connecting any audio equipment. Good quality

connectors and screened cable should be used for all audio connections.

Twin screened cable should be used for all balanced lines particularly microphone connections.

Always ensure cable clamps in connectors are fully tightened and gripping the outer sheath. Good strain

relief and mechanically sound connections will increase reliability at virtually no extra cost.

GROUND LOOPS

connected to mains earth this will form a loop (hence ground loop). Current will flow in this loop and

appear in the form of hum (50Hz mains frequency) added to the audio signal. The problem is

aggravated if the equipment is located a distance apart as the loop is larger. It is possible to have

several ground loops within a system. The solution is to connect the system to mains ground only

once. This is usually done at the mixer. You will need to investigate the various items of equipment you

are using and isolate their signal common from mains earth. Many manufacturers fit a ground lift switch

for this purpose. On some equipment this is in the form of a removable link. Unfortunately with some

equipment you have to get inside to identify where the connection is and remove it.