Configuration & Controls
10
Sonifex HY-02 User Handbook
Input Circuits and Limiter
The line input connection at the 3 pin XLR connector is coupled directly to the line bridging input transformer TR2. The
input impedance is greater than 10K ohms balanced floating with a high common mode rejection ratio. The input
transformer is coupled via a resistive attenuator R21, R22 and R24 to the limiter amplifier Q3 bcd and Q12. The limiter
output delivers full bandwidth signals to the bandpass filter formed by Q4c, the output of which drives the telephone line
transformer through the feed resistor R41. The dummy line balancing transformer TR4 is also driven from this point
through feed resistor R42. The limiter incorporates a signal mute operated by Q3a during handset operation.
Microphone Input Circuits
The HY02 is supplied with the microphone transformer TR1 disconnected and a ground link in PL5. If the microphone
input is used it will be necessary to remove the link from PL5 and plug the transformer TR1 in. Microphone gain is set by
the MIC GAIN preset control RV1 in the feedback path of the microphone amplifier circuit Q2 which is coupled to the
limiter input stage. The microphone input level control gives a gain range of 30 dB with a maximum available gain of 74
dB and an input impedance of 200 ohms balanced floating.
Telephone Line, Barrier and Transformer
The telephone line port is a two wire connection into the apparatus through a line cord and jack BT 6/502, with a surge
arrester BT 14A from the A and B wires to earth. The line connection is switched by a divert switch and relay between the
hybrid unit and the handset connected through a BT 605A connector.
The line is held, on the equipment side, by an electronic line holding circuit Q20 and Q21 the A leg fed through a 2.2 uF
250V d.c. blocking capacitor to the line transformer ETAL P1200. A pair of zener diodes arranged in a back to back
configuration across the primary of the line transformer, are arranged to act as overvoltage protection. The line is
balanced/floating with a high common mode rejection ratio. The line transformer is designed to meet BS 6305.
Figure 2-2 : Telephone Barrier Circuitry
Output Circuits
The output from the differential amplifier Q6a which acts as a high gain nulling amplifier is fed to the bandpass stage
formed by Q4a and Q4d. The filtered output is coupled to a mute stage and noise gate formed by Q5 abc, Q15a and Q23.
The threshold of the noise gate is set by preset control RV4. The range of the noise gate threshold control is 30 dB about
a minimum threshold level of -40 dB ref. O dB output. The output amplifier formed by Q5d, Q13, Q14 drives the 600
ohm output transformer coupled by C21. The output level may be adjusted by the preset OUTPUT LEVEL control RV3
with a gain range of 12 dB. This control effectively sets the telephone return signal level and gives adjustment for low
level incoming signals. The output impedance is less than 50 ohms balanced floating.
Line Balancing Electronics
The line is balanced and offers rejection of the input signal at the output when an input signal is present above a set
threshold level. This line balance technique is achieved electronically during input signals and stored in a sample and
hold circuit. Balancing is achieved by utilising a dummy telephone line transformer arranged in a bridge configuration.
The analogue line consists of a network of electronically variable resistance and capacitance arranged in a feedback
circuit, automatically adjusting the analogue line components to provide resistive balance and reactive balance.
The telephone line transformer and the analogue line transformer are arranged in a bridge the output of which drives a
differential amplifier Q6a. The ‘unbalance’ signal of this bridge is amplified and processed as below to give d.c. voltages
which drive the resistive and reactive analogue line elements.
Amplified ‘unbalanced’ signals are switched by the gate Q15b, driven by an amplified in-phase component of the input
signal at Q7c. Q7a and Q7b together with Q7d set the threshold level of in-phase gating.
Telephone Line Disconnection Barrier,
Call Holding and Isolation Transformer Circuit.
Telephone
Line
BT 6/502
Surge
Arrestors
BT 14A
Relay Type
Series 21
BT 605A
Handset
BC546
82k
6.2V
1.2W
Zener
150R
82k
22uF
35V
51V
Zener
1N4004
Electronic Line Holding
2.2uF
250V dc
0.068uF
250V dc
680R
6.2V
1.2W
Zener
Transformer
Type Etal
P1200
Equipment
