A separately packaged film sound editor mixer is disclosed which includes a plurality of low level audio signal input terminals adapted for connection to outputs from playback heads of a synchronizer. Magnetic tape preamplifiers are included for preamplification of the low level signals. The preamplified signals are connected through slide attenuators and muting switches to inputs of a first summing amplifier. The output from the first summing amplifier is connected to inputs of left and right channel summing amplifiers through a tone control circuit and a master attenuator of the slide type. A selector switch is included in the input for one of said magnetic tape preamplifiers whereby, in one switch position, an output from the synchronizer is supplied to the preamplifier, as mentioned above. In the other switch position, the preamplifier input is connected to another input terminal to the mixer through a variable attenuator, which input terminal is adapted for connection to an audio signal source from an upright editing machine, or the like, for mixing with signals from other playback heads of the synchronizer. Also, left and right channel input terminals are included with are connected through variable attenuators to inputs of said left and right channel summing amplifiers. A stereophonic signal source, such as the output from a stereophonic tape deck, may be supplied to the left and right channel input terminals for mixing with monaural signals from the first summing amplifier. The power supply for the mixer includes a ground lift switch for removal of the power ground prong of the three-prong power plug from the chassis of the mixer to prevent ground loop currents.