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Noise - Noises represent problems in PA system

Causes of noises associated with the PA system, especially with the amplifier, may be divided into the following 4 sources:
1) Input devices and lines
2) External radio wave
3) Output line
4) PA system itself

causes of noise

Possible causes and countermeasures against noises from input device and line

  • Possible cause
    Input devices and related apparatus may frequently generate whirring sounds called hum noises. They are, long signal cable, closely coupled power cable (AC mains) and signal cable, mismatched connections, poor insulation of aging cables, etc.
  • Countermeasure
    Use of balanced cable is recommended because it is less susceptible to external noises than unbalanced one. For example, balanced cable connected to 600 Ω microphone can be extended up to 80 m without picking up noise while a 20 m or longer unbalanced microphone cable will introduce noise.
  • How to use a balanced cable
    The figure below shows necessary connections of balanced cable.
    balanced cable
  • Problem with CD player
    transformer IT-450
    In case where the CD player is far away from the amplifier, prolonged cables tend to pick up external noises easily. The output voltage level of the CD player is higher than that of a microphone, and is still kept at a level enough to drive the amplifier after significantly attenuated in the cable. However, the amplifier will distort the signal and increase noise as the gain control is turned.
    This is because the output impedance of the CD player is high (typically approx. 10 kΩ) and the output signal is connected through unbalanced pin jack. When the cable from the output of the CD player is long, it will easily pick up noise interference. To minimize this type of noise problem, connect the output to a preamplifier and connect its balanced 600 Ω output terminal to the amplifier.
    problem with CD player

Noises caused by external radio wave

A geographic location in which signals from radio stations are poorly received is called dead spot.In contrast, there is a location where certain signals are intensified. Output powers of some unlicensed CB transmitters are higher than those of miniature FM stations. It may be difficult to believe but true that connection cable and circuit on the equipment will receive radio signals. In intense electric field, radio signal picked up by the speaker cable sounds the amplifier even if the power to it is not turned on. To prevent these radio interferences, sensitive area should be provided with electromagnetic shield or grounding but it is practically difficult.

Noises on output line

noise on output line

Sound system may chee at high pitch or swing meter pointer while no sound is input. These phenomena indicate oscillation of amplification loop caused by closely coupled speaker lines. When high amplitude oscillation continues, the amplifier will be damaged or the fuse will blow. Possible causes of the previously described problem: fuse blows as the emergency line is pressed are as follows.
1) Amplifier is overloaded, i.e. it is connected to many number of speakers.
2) Low-impedance and high-impedance speakers are used in one speaker line.
3) Speaker line is not completely isolated (insulated) acoustically and electrically.

Noises generated inside the equipment

Cause of rasping sound heard as the volume control is turned, noise heard as the switch is handled, and other sounds or noises generated inside the equipment will be easily located and removed. Loose connection of rusty plug and socket also cause noise. Especially, phone plug should be frequently checked.PA system for professional-use, once installed, may be operated without maintenance for 5-10 years. The PA system processes low level electric signals. Treat these noises as "sound of crying" of the PA system and locate and remove them through periodic inspection.