Journal of the Society of Motion Picture Engineers (1930-1949)

It is of interest to note that in this standard unit the erase head forms the principal load for the 60-kc erase and bias oscillator with the record head bias coil being a relatively small series impedance in the circuit. It was therefore possible to add the bias winding for the second recording head in series with the existing erase and record heads without any appreciable net change in bias or erase currents. Using the above-described arrangement it was then only necessary to supply proper pole pieces, and to reconnect the internal wiring to the heads to accommodate the second recording channel. Plug and receptacle arrangements are so chosen as to automatically maintain channel identity in the interunit cables. With the exception of the nameplate, there is no apparent difference between a binaural Magnecorder and a normal single-track unit unless the magnetic head covers are lifted to allow a view of the half-track pole pieces which do the recording. The half-track pole piece consists simply of a normal fulltrack pole piece with approximately ^ of the Mu-metal cut away and a brass insert soldered into its place in order to fully support the tape. The Amplifier Unit The development of a binaural recording and reproducing amplifier was essentially a specialized packaging job which involved building a new portable dual amplifier unit, each amplifier having all the characteristics of existing unit standard amplifiers. The packaging was accomplished with only a minor increase in space and weight for the dual amplifiers over that required by a similar existing single-channel amplifier. The latest techniques in the use of miniature tubes and components were employed in the manufacture of this equipment. Individual illuminated VU (Volume Unit) meters were provided for each recording and reproducing channel as well as individual gain controls. A unique problem in the design of this unit arose due to the necessity for providing an overall or master gain control which controlled simultaneously the gain of both channels. This was accomplished through the use of a special dual potentiometer with matched rotational ohmic accuracy in the order of plus or minus 5%. Provision was made for headphone monitoring from the front panel of the amplifier through the use of specially built Permoflux binaural headphones having an effective response to over 12,000 cycles/sec. These headphones are provided with large foam-rubber ear cushions in order to exclude extraneous noise and to reduce the wellknown head fatigue that comes from the use of ordinary earphones. Dual monitor speakers close together on a small panel would not yield any useful binaural effect and might be dangerously confusing for monitoring use. Therefore, in addition to the binaural headphones, a single small monitor speaker is provided behind a flocked screen panel on the front of the amplifier. A unique control is included for this speaker which is so arranged that it is "off" when set at its center position. Maximum volume for one channel is obtained by turning the control to the extreme right, and maximum volume for the other channel, by turning the control to the extreme left. The amplifier tube lineup for a single amplifier channel consists of two 5879 tubes followed by a dual triode 12AX7, the second half of which is used as an inverter driving a pair of push-pull 6AQ5 tubes. The same amplifier is used for playback as well as recording. A multiple section (shielded between sections) ganged selector switch is used to switch equalizer and gain functions for the dual amplifiers when changing from the record to playback positions. In order to provide freedom from hum in the low-level stages of the amplifiers, direct current is used on the filaments of Otto C. Bixler: Binaural Recorder 113