Abstract: This autobalance hybrid circuit is used to connect a 2-wire duplex telephone line to a 4-wire line which has separate receive and transmit pairs. The impedance of a balancing network automatically is adjusted to equal the impedance of the 2-wire line, thereby maximizing echo return loss. "Howling" and "singing" are eliminated, regardless of changes in the 2-wire line impedance. Balancing is achieved by comparing the absolute values of the receive line audio voltage components appearing across two coupling transformers, one associated with the 2-wire line and the other with the balancing network. A control signal, indicative of the difference between these components, alters the balancing network impedance until equal voltages are achieved. In this condition, the receive signal cancels out as a common mode in the output circuit, so that none of the receive line audio is returned to the transmit pair.