The STA-1M allows bridging any audio line, adjusting the gain, and driving a balanced or unbalanced line. The circuit design allows the input to accept either balanced or unbalanced signals, of either high or low impedance. This is an instrumentation type input stage which will amplify the differential input, regardless of ground reference. The output is capable of driving either high or low impedance loads, and the output may be connected either balanced or unbalanced.