Some capacitor microphones can produce enormous output voltages when placed close to a loud sound source. To eliminate any possibility of overload under these conditions, a 20dB pad switch is fitted. It's Fletcher Company theory that 'big' sounds are only possible if the recording channel keeps the response and phase of the lower frequencies flat and under control. This ensures that there are no sudden phase shifts in the low end. The proof of the theory is that the sound from the Tube Channel is characteristically full bodied and rounded in character.
Balanced operation means that the audio is carried on two wires working in opposite phase.
The compressor is a photoelectric device where the sound triggers light emitting diodes which in turn control the resistance of a photo sensitive resistor. This form of compression used to be common in the 60's and 70's but has been superseded by so called 'improved' voltage controlled amplifiers. The advantages of the older system are that distortion is virtually eliminated, noise is extremely low while overload margin is extremely good. Totally unlike a modem compressor, it can pull voices forward, help with internal mix balance, and add 'presence' to the sound as well as controlling recording volume levels. But its main and unique attribute is it's ability to produce the characteristic 60's compressed exciting sound without losing the transient sparkles that are such a feature of good digital recording.