GENERAL
This is a 1U subrack that contains two separate loop amplifiers fed from a common source.
Each amplifier is capable of driving a maximum output current of 16amps peak to peak. Also
each amplifier is capable of swinging up to 42volts peak to peak. Both limits apply so the
actual maximum loop current is a function of the loop impedance. Drawing
number A2-14525 shows the front and rear view of the unit (adobe .pdf file).
COMMON INPUT STAGES
The input is electronically balanced and feeds a switched gain input stage. This lines up,
in 3dB steps from –9dBu with the control fully clockwise to +6dBu with the control
fully anticlockwise. This input stage is followed by a 12dB per octave band pass filter
100Hz to 5kHz. After the filter there is a compressor with four LEDs showing the degree of
compression.
PHASE SHIFTER
To drive complex loops two amplifiers in quadrature are sometimes required. A +45 degree
and –45 degree phase shift pair are therefore provided and can be switched on the
front panel.
POWER OUTPUT STAGES
The power output stages have a high defined output impedance of 1kOhm. Therefore the loop
current does not depend on the loop inductance. Each amplifier has a switch to control the
peak loop current. It is practical parallel the two amplifiers if a larger current swing
is required.
POWER SUPPLIES
Each amplifier has it’s own independent power supply, but the input stages are
supplied by either power supply.
CIRCUIT DETAILS
Drawings A3-14524 shows a block diagram of this unit (adobe .pdf
file).