Limiting Amplifier G51501

LIMITING AMPLIFIER TYPE G51501

Uses:

Limiting amplifiers are essentially protective devices employed to guard against modulation in excess of a pre-determined maximum value. The use of limiting amplifiers is presumed to be supplementary to the volume control exercised by a studio engineer and therefore serves to reduce or limit only the high programme peaks or programme material in which the ratio of peak to R.M.S. value is relatively high. Three common uses of the limiting amplifier are:

1. A line amplifier at transmitting stations.

2. In place of a "B" Amplifier for studio racks.

3. As a general purpose programme amplifier for recording equipment.

Description:

The A.W.A. Limiting Amplifier type G51501 is a six valve unit using single ended type valves. It is designed to limit its output signal to certain pre-determined levels and the input level, above which limiting occurs, is also adjustable. The input and output controls are calibrated in programme levels (VU).

A special feature is the arrangement of timing circuits. These provide a rapid forward action time of less than 1.0 millisecond and release times dependent on the nature of the programme material in such a manner that a peak of short duration (say 0.1 second) will cause compression in the normal way and a recovery of gain in a very short period (e.g., 10-20 db. in 1-2 seconds). Peaks of longer duration (e.g., several seconds) initiate a longer recovery period of 5 to 15 seconds (depending on duration and amplitude of peaks). This special arrangement achieved by the use of two limiting stages in cascade, is not available in most other types of limiting amplifiers; it avoids audible surging effects and "thumps" while maintaining a high average output level.

A meter and selector switch for setting up the unit and checking of plate currents is included. A compression scale on this meter, graduated from 0-30 db. (compression increases clockwise - by use of a special bridge circuit) gives indication of the amount of compression, and provision is also made for the connection of an external compression meter.

Control ranges and pre-set adjustments are adequate for all normal requirements; initial setting up is facilitated by the convenient arrangement of controls.

Power supply from an external source is required, but is not critical in respect to regulation or bias provision, and can be common to other amplifier units (with negative H.T. supply earthed). Power supply Unit type H8027 is suitable.

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