Low-frequency transformers are used to transmit signal voltage and signal power, and can also achieve impedance matching between circuits, and have an isolation effect on DIRECT current. It is divided into interstage coupling transformer, input transformer and output transformer, which are similar in shape to power transformer. 1. Inter-pole coupling transformer: An inter-stage coupling transformer is used between two audio amplifier circuits as a coupling element to transmit the output signal of the former amplifier circuit to the latter stage and perform appropriate impedance conversion. 2. Input transformer: In the early semiconductor radios, the transformer used between the audio push stage and the power amplifier stage is the input transformer, which plays the role of signal coupling and transmission, also known as the push transformer. Input transformers have single - end input type and push-pull input type. If the drive circuit is a single-end circuit, the input transformer is also a single-end input transformer; If the push circuit is a push pull circuit, the input transformer is also a push pull input transformer. 3. Output transformer: The output transformer is connected with the output circuit of the power amplifier and the loudspeaker, which mainly plays the role of signal transmission and impedance matching. Output transformers are also divided into single-end output transformers and push-pull output transformers.