The 2-Wire to 4-Wire Hybrid Interface Module, 41690, provides bi-directional access between the 2-wire line and the 4-wire network. The levels of the transmit and receive paths are each adjustable over a 23 dB range. The 41690 module is always ON, passing signals between the 2-wire line and the 4-wire circuits. The module is provided with one or two 2-wire to 4-wire interface circuits.

The advantages of this circuit board over the one you are using is that it can fit in the Raven Bridge Shelf .  And, since the Hybrid is also active, the levels of the transmit and receive can be adjusted.

Like the notch filter , we can install up to four hybrids on a single board and it is an active hybrid, not passive, so no losses.  They, too, can be installed in the
same shelves as our bridges.

THEORY OF OPERATION
4-WIRE PORTS

Operational amplifier U18A and its associated components provide a balanced 600Ω receive input on pins P and R. The receive input level is adjusted by potentiometer R63.

Operational amplifier U-18B and its associated components provide a balanced 600Ω transmit output on pins U and V. The transmit output level is adjusted by potentiometer R35.

HYBRID BALANCE
Transformer T2 and its associated components form the hybrid balance circuitry. Potentiometer R24, and capacitors C42 through C45 via rocker switch SW2, form an adjustable circuit which matches the 2-Wire impedance reflected across pins 8 and 9 of T2. The Line Build Out (pins Y and Z) is also available to aid in hybrid balance adjustment.

POWER and BIAS VOLTAGES
U25 U23 and its associated circuits provide the bias voltage and analog ground for the analog circuitry.