A simple transistor circuit and lamp are connected at the power amplifier output. A photocell coupled to the lamp is connected at the power amplifier input. By switching the photocell in series or in shunt with a voltage divider, the device will expand or compress the audio signals. The high input impedance of the power transistor does not affect speaker performance when connected to the amplifier output terminals. DC balance controls allow adjustment so the lamps just extinguish when there's no input signal.
Construction is described for use with a stereo power amplifier.
The photo shows the front of the expander-compressor. The DC balance lights are at the top. The power switch is at he left. A power indicator light is at the bottom center. It works just as good today as it did back in 1964.