Service manual

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3. Mono Input Pod.
The input pod has separate mic and line inputs, hi and lo pass filters, a direct output and a 20 led
meter.
The mic input signal passes through T filters L1 and L2, via a switchable 20dB pad and AC
coupling capacitors C2 and C3 on to the base of low noise transistors Q4 and Q6. Further filter-
ing is provided by capacitors C5, C6 and C7 and pre-set VR1 sets the pad CMR. 48 volt phantom
power can be supplied to the input via current limiting resistors R1, R3 and switch SW1. Transis-
tors Q4 and Q6 provide amplification of the signal and their collectors form an output to IC1
which functions as a comparitor. This gives a buffered output to the following circuits and ensures
the amplification transistors are operating at constant current by providing feedback via transis-
tors Q3 and Q5. Transistors Q3 and Q5 form a voltage to current converter and phase splitter in
conjunction with the constant current source formed by Q1 and Q2. By providing this balanced
current feedback to the amplifier transistors Q4 and Q6 the common mode load resistance seen at
their emitters is infinity and the common mode gain of the overall circuit is zero. The differential
load is set by the potentiometer VR3 which adjusts the gain of the circuit by 55dB; effectively
shorting out the negative current feedback to greater or lesser extents. High frequency stability
and constant bandwidth at all gains is assured by capacitors C8, C9, C10 and C12.
The mic amplifier output feeds the frequency adjustable hi and lo pass filters which are second
order active networks formed around IC2.
The line input signal passes through T filters L3 and L4 and on to the differential amplifier formed
by IC3. Pre-set VR6 provides CMR adjustment and capacitors C23, C24, C25 and C28 provide
additional high frequency filtering; C29 and C36 provide AC coupling. The signal passes on to
amplifier IC4 which provides 40dB of gain adjustment in conjunction with potentiometer VR7.
FETs Q8, Q9 and the second half of amplifier IC4 select the mic or line input to pass on to the
phase change over amplifier formed by IC7 and FET Q10. Control of the mic/line and phase
change is achieved by invertor IC9 via transistor Q11 and CMOS switch IC8. IC9 is configured as
a pair of bi-stable flip flops which are triggered by switches SW3, SW4, capacitors C65 and C67
and is connected to the automation system via R86 and R83.
CMOS switch IC8 also selects the signal feed to the 20 led meter in conjunction with amplifier
IC6. This automatically selects mic or line amplifier to feed the meter if the meter pre change over
line is logic high and monitors the input module signal level via the meter post feed if it is logic is
low.
The meter circuit is formed by the peak responding full wave rectifier and buffer IC10 and meter
drive IC11 and IC12. 0dB adjustment is via pre-set VR9 and offset adjustment is via pre-set
VR10.
The direct output signal from the input module is buffered by amplifier IC6 and fed to the differ-
ential output amplifier formed by IC5. Output symmetry is adjusted by pre-set VR8 and high
frequency stability into any load is assured by filter capacitors C46, C47, C48 and C49. Further
filtering is achieved by T filters L5 and L6 and AC coupling is via C44 and C45.