VOGAD is an acronym for Voice-Operated Gain-Adjusting Device. A
device that has a substantially constant output amplitude over a
wide range of input amplitudes i.e. an AGC circuit working at
audio frequencies. This simple circuit was developed following
the demise of the SL6270 IC. The circuit consists of an
amplifier driving a variable attenuator made up of a series
resistor and shunt bipolar transistor acting as a variable
resistor. After the attenuator is a further amplifier stage
followed by an emitter follower providing a low output impedance
for the detector and following stage(s). The output is rectified
by a voltage doubler and applied to another emitter follower
which drives the attenuator. The higher the input level, the
higher the attenuation. The circuit has a working input range of
about 30dB. The above circuit is designed for use with low
impedance magnetic microphones with output levels in the low
millivolts at normal speech loudness. The gain of the first
stage is at least 30dB so make sure th