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7. Level issues

7.2 Head amps

As networked audio systems allow the placement of analogue inputs at remote locations - with the connection between the remote location and the control location (a mixing console) using an audio network protocol - the head amps that align the analogue microphone input levels with the remote input’s A/D converter is also located at a remote location. To control the head amp’s gain, high pass filter and phantom power, a remote head amp protocol must be used to control these functions from the control location. The head amp’s gain is most commonly a combination of relays and/or analogue switches that provide a variable gain in the analogue domain. A digital make-up can be added to increase the gain step accuracy and to provide constant gain to the network (see chapter 6.3).

The noise floor and distortion of most high quality head amp circuits are normally very low. Designs from manufacturers with a natural sound philosophy provide a ‘flat’ transfer function from analogue input to A/D converter input - with as less as possible colouration, allowing DSP processes to apply sound colouration to the full control of the system designer and sound engineer. Some manufacturers apply a deliberate EQ curve in the analogue circuit to create a default Response that suits dedicated applications - eg. ‘British sound’ or ‘warm sound’.

To support a flexible use of networked audio systems, it is of vital importance that the gain step accuracy is high - so that when a preset is recalled on the control user interface (mixing console), the recalled gain is as accurate as possible. The human auditory system is capable of detecting level differences of less than one dB. Also, to provide a high consistency when loading presets in a different console of the same type, or when reconnecting a sound source to another input in the system, the gain consistency between channels and between input head amps in different stage boxes must be very high. Still, because the total gain between a system’s analogue input and an analogue output involves many electronic circuits, the accumulated signal chain gain consistency (or gain error) can be audible. This means that - even with very consistent head amps (and power amps) - every time a system is set up and connected, the head amp gains (and power amplifier gains) might have to be adjusted to comply to the design specifications.

>>7.3 Gain compensation

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