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for this discussion, I will consider "Amplifiers"

as covering all Amplifiers including Mixers

Audio amplifiers take a small signal and amplify it to a larger signal. This diagram shows a simplified example of the amplifier stages. (An actual circuit includes bias, coupling, and feedback elements. Amplifiers and mixers have many of these circuits.)

When the audio signal applied to an amplifier is too loud. It can exceed the
capabilities of the circuit and distortion of the signal is introduced.

Notice the flat top and bottom of the amplified signal. The top flattens when the gain of the amplifier reaches the upper limits (B+). The bottom flattens when the amplifier approaches the lower limits (Ground). This is referred to as the clipping of the signal. Depending on the circuit, it may not be symmetrical (ie may clip at one extreme before the other). In vacuum tube circuits the clipping will normally start earlier and thus be more gradual..

A clipped audio signal will introduce extra harmonic content into the signal
similar to the harmonics of a square wave.

another component of the audio signal is "Noise"

All Amplifiers generate noise internally and all inputs will bring noise into the unit. Normally amplifiers are designed to keep the internal noise down 60 db (1/1000) from the normal operating level. Hopefully the incoming noise can also be kept to a similar level.

(Since we are dealing with voltage levels ... all db measurments in this article will be in voltage ratios)

Poor shielding, grounding and ground loops often contribute to excessive incoming Noise

This is an example of 60 cycle hum. This would normally be introduced from external ciruits unless there is a fault with the amplifier.

Operating Levels

Every amplifier has a normalized operating level. When setting the amplifier up, this is the level that should be used. For rotary knobs, the controls should be set at about 1:00 (one oclock). For the slider controls, the inputs should be set at "0". This will be approximately 10 db below clipping, and 60 db above the noise threshold (on a properly designed amplifier).

When the Amplifier has input trim pots (as in a mixer), adjust the main Input Control and the Master Control to the normallized setting (described previously). Apply the input signal you are trying to normalize, and adjust the Trim Control to get "0" VU out of the Mixer. When using a powered amplifier (normally without trim controls), adjust the input as suggested previously, and adjust the output for the desired volume. This will adjust the amplifier to work in it's best range (without clipping and 60 db above the noise level).

This should apply to each amplifier and/or processor throughout the system (ie each item should be normallized to work in it's design range). Usually this would be setup with a tone into one of the inputs.

Sources of Noise

• Radio Frequency (RF)
       This type of interference can enter the microphone inputs ........ Check grounds and shields
  
  
  

RF noise can often be reduced with bypass capacitors on each mic line to ground.

Ferrite beads on the microphone lines can also reduce RF. The ferrite beads act as chokes at the radio frequencies.

Transformers can eliminate RF noise and 60 hertz hum

• Electrical Transients
       This type of interference can come into the microphone inputs or thru the power line connection.
  
  • Switching heavy loads on and off
  • Motors starting and stopping
    
    
    
  
  These transients will depend on the exact instance that the switch is made.
  The worst case is at the peak of the 60 Hert Power Cycle.
  For this reason, the interference may not always be there.
  
  
  
  
• Electrical Noise due to continuously switching in the middle of the 60 Hertz Power Cycle
       This type of interference can also enter the microphone inputs or come thru the power line connection.
  
  

This is usually caused by Lighting and Fan Dimmers. This is normal with all of the professional dimmers. However, a choke is usually part of each dimmer circuit. This reduces the sharpness of the switch and hense the noise.

You have seen what happens when the signal level is too high for the system (clipping).
What happens if we keep it too low?
   Consider the following

The mixer's output, and amplifier's input are at their designed level of " 0 dbm ".
The noise (-60 dbm) will mix at 60 db down
(can't hear it).

The mixer's output, and amplifier's input are set to " -20 dbm ".
The noise (-60 dbm) will now mix at 40 db down (noticeable noise in the background).

Reducing Noise pickup

With balanced audio cable, the signal is applied between the two conductors. This means that the signals on the two conductors are opposite phase to each other. The external noise will be induced on each wire in the same phase . The amplifier will amplify the difference between the two conductors. Therefore this will amplify the signal and will cancel the noise .

Induced noise (low power) has more trouble entering a low impedance input (eg 600 ohm) , than it has entering a high impedance input (eg 10,000 ohms) , thus amplifier A would have less noise. This is why the professional equipment normally uses low impedance line and microphone inputs.

13-Feb-07

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