Audio Terms
Gain
This is the factor by which an input signal is amplified (increased) or attenuated (reduced) when passing through an amplifier. Gain is generally measured in decibels (dB) which is a logarithmic ratio of input to output. Unity or zero gain means that the signal comes out at the same level that it went in.
With all this gain available, it may seem tempting to turn the amp up to full to get the loudest sound possible. And why is it that smaller amps have roughly the same gain even though they are less powerful? The answer is that an amp can only amplify up to a certain point, which depends on its power rating. Past this point, the output just distorts in what is called clipping. The peaks of the signal are ‘clipped’ as they exceed the limit.
max+ |
max- #1. Peaks within amplifier limit |
max+ |
max- #2.Too much gain, distorted sound
Noise
Noise is typified by a hissing sound. It is present in all analogue electronics due to random thermal vibration of atoms within the components. However, good circuit design and proper operation reduces this to a minimum.
Gain structure, i.e. how gain is set throughout the signal path, is very important in minimizing noise. Most audio signals go through several gain stages such as input trim, fader gain, main fader gain, EQ and compressor gain, and power amplifier gain. If the signal is boosted early on to a high level, any noise subsequently picked up will be relatively small in comparison to the audio signal (i.e. higher S/N, or sound to noise ratio).
Hum and Buzz
Hum typically is a lower pitched sound of around 50 or 60Hz, depending on the AC supply. If the hum or buzz is very loud, this is generally caused by bad connections - check cables and plugs for faults.
Lifting the Ground
In other cases, a ground conflict between the chassis and the circuit ground can induce hum. This happens when a magnetic field induces a small voltage in the casing of the amp, making it slightly different in potential from the circuit ground. This hum sounds more subtle than it would be for a bad connection and is generally ignored. If your environment requires the highest quality sound, it may be eliminated by ‘lifting the ground’. This is accomplished by moving the back panel switch from the ‘ground on’ position to the ‘floating’ position. The electronic ground is then dependent on other connected components. For the best safety protection and protection of other components, it is strongly advised to keep the switch in the ‘ground on’ position.
Balanced Lines
Power transformers and other devices which cause magnetic fields can induce hum in audio cables. This can be best combated by using balanced lines where inputs and outputs permit. Balanced lines use two conductors plus a shield (ground) to send a positive and negative version of the audio signal down the line. These are in close proximity and therefore pick up the same hum or interference. At the input, the inverted
Since all the inputs of the Numark amps are balanced, accepting signals from a balanced output is no problem. Should the output be unbalanced, the cable should be wired as follows to ensure the best possible performance. This applies to XLR, 1/4” and barrier strip inputs alike:
If the gain is low early on, the noise will be relatively loud compared to the signal. When amplified at the power amp more gain must be used to make the signal reach required levels. This gain also acts on the noise, hence a noisier final sound. Of course, care must be taken not to set the gain too high, this will cause clipping distortion (see first section on ‘Gain’).
+ | unbalanced output | |
| + | |
- | GND | |
GND |
|
|
Where | is a variable resistor, adjustable between 0 and 600 ohms. | |
©1997 Numark Industries | 10 | http://www.numark.com |
