Abstract: The multi-filar moving coil loudspeaker includes a magnetic field structure which provides a unidirectional magnetic flux across an air gap, an acoustic diaphragm, and a coil set positioned in the air gap and connected to the diaphragm. The coil set has a plurality of insulated coils wherein the coils have approximately the same resistance and inductance and cut substantially the same flux lines in the air gap. The coil set has two or more wires which are twisted around one another and then wound about a coil form. The coils in the coil set may be connected in series or parallel aiding, or in series bucking pairs. In a loudspeaker circuit, the coils are connected into two groups, each having one or more coils. The first group is connected to input terminals, and the second group is connected to a feedback amplifier having an output connected to the first group of coils.

Abstract: The loudspeaker telephone apparatus consists of a hybrid circuit which interconnects a telephone line and its line balance impedance with receive and transmit amplifiers. These amplifiers are connected to a bridge circuit, the arms of which are formed from two pairs of similar coil groups located in two multi-filar moving coil loudspeakers. The coil groups in one loudspeaker are connected in series aiding while the coil groups in the other loudspeaker are connected in series bucking. In one arrangement, the bridge circuit may be connected directly to the amplifier, the receive amplifier being connected across the pair of coils in each loudspeaker and the transmit amplifier being connected across the junction between the pair of coils in each loudspeaker. In another arrangement, a second hybrid circuit is used to connect the bridge to the amplifiers which are connected to the hybrid by its amplifier terminals.