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182 THREE-DIMENSIONAL MOTION PICTURES lifted and built up to a high level by a pre-amplifier, voltage amplifier, and a power amplifier. This high- level signal is then fed to each of the three main chan nel amplifier racks. A 250-cycle hand-pass filter on Rack 1 accepts the 250-cycle portion of the complex control tone and rejects the other two tones. A 630-cycle band pass filter on Rack 2 and a 1600-cycle band-pass filter on Rack 3 accept 630 and 1600 cycles, respectively, re jecting the undesired components. In this way, the com plex control tone is separated into Its Individual parts and one tone goes to each rack. After coming through the band-pass filter each con trol tone Is fed to a tone rectifier which rectifies the A.C. tone producing a D.C. voltage. The value of this D.C. voltage is at all times proportional to the strength of the control tone, becoming higher as the control tone gets stronger. The variable D.C. voltage is then applied to the grid circuit of the first amplifier stage of the variable gain amplifier as a bias voltage. The first stage of the ampli fier consists of a balanced pair of RCA 6K7 tubes In a push-pull circuit. The RCA 6K7 is a remote cut-off, super control pentode tube so constructed that the gain of the tube may be varied over wide limits by changing the bias voltage. Two ordinary amplifier stages follow this variable gain stage to further amplify the signal before passing it on to the power amplifiers. Controls are available on this amplifier to change the expansion characteristics so that varying degrees of volume changes can be ob tained for a given change in control signal. By this ar rangement the equipment can be adjusted to give any overall volume range that the size and acoustics of the theater will permit. By the proper combination of these two new pieces of sound reproducing equipment with standard Photo- phone amplifiers, speakers, and power supply units, a very versatile system has been created which now brings