International photographer (Jan-Dec 1934)

August, 1934 The INTERNATIONAL PHOTOGRAl'HKK Nineteen Monitor room kW; ^0^, » Souno STAClt AMPLIFIER POOM RECORDING BRiDginl "«'*«r light _ d,,,„. i , , s AMP^iritBS IATTCNUATORS VALVE ^® P ' ' C Pl«.*oe I <•< trii. C«ll ) 1 f W film'i fl A ■H Kt Pf*T COILS © -4tH h* JaUS^fj PiSl »J B A T7f— S 4-# pFCOKDINC, ftj«~^ ROOM ^1 iwax HtCO« fl-_ 1 l«4 DEVill P . »f<o«io r 7 tTM 3] r AM PLIFIC ROOM D'RECT r,T",H' tn*z -x — *— s. ° — * UFI€" \ f!" Fig. 1. Block Schematic Diagram of Recording Channel. as prone to generate disturbing noises in the recording system as the result of mechanical shock and vibration as are the tubes in the larger amplifier. The higher gain in the three-stage amplifier is what makes it so sensitive to the mechanical vibration of the elements in its tubes, particularly in the voltage amplifier tube. When amplifiers of this two-stage type are used in the monitoring circuit, one of them is connected directly in place of the first monitoring amplifier, and another one is connected so that it is in the photo-electric cell monitoring circuit. The arrangement is such that when relay No. 1 is in its normal position (i. e., not energized) the direct monitoring circuit from the bridging bus passes through one of the two-stage amplifiers and the pushpull power amplifier. When the relay is energized, the PEC monitoring circuit from the output of one of the photo-electric cell amplifiers passes through both of the two-stage amplifiers and then through the power amplifiers. In this way, the second two-stage amplifiers make up for the much lower electrical level at the output of the PEC amplifiers. The arrangement is illustrated in Figure 2. The Horn Panel The output impedance of the monitoring amplifier is 500 ohms; and in order to match that impedance to the sixteen-ohm horn units, an impedance-matching transformer mounted in a horn panel is provided. This panel is equipped with seven tap switches, each having eleven contacts, which serve to regulate the volume of sound generated by each of the loud speakers in the installation. A key switch just below each tap switch permits any one of the horn units to be switched on or off. In the normal single channel recording system, only four of the switches are needed to control the horns. Two of the four horns are mounted in the monitor room. They were described in the chapter on monitoring equipment. The other two switches control small horns that are installed in the amplifier and recording rooms. The same type of electrodynamic-coil horn unit is used in all cases ; the difference is in the length (from diaphragm to opening of bell) of the exponential horn attached to the unit. Six horns are employed in the channel that is used for "dubbing," the additional two horns being mounted in the dubbing projection room and the dubbing room. Occasionally, one or two horns of the large type are also used on the sound stage for making "play backs." This is done most frequently when music is being recorded. The play back is made from one of the wax pECORDING Room 3> p£c c5> PfC AT T EQUATORS PEC AMPLIFIER (MO 1 tj> A < PEC No Z V A » >( i > o .o : t *a -• RfL -K • RFLAY No. 2 y% RELAY NO I Jack Arrangement , AMP 4 > N« I H* * ou+rut; 'lhp.» »1 Amp | PANEL -» PULL AMP R TWO-ST«„£ AMftir/pR Ap-IPUF I F.R RCOM Fig. 2. Alternative Monitoring Amplifier Arrangement. records — which damages the record so that it is worthless for processing — to provide the musical director with an opportunity to check the quality of the music and the recording. The volume of sound from each individual horn is adjusted by the proper tap switch to suit the comfort of the person working in the room in which the horn is installed. The large horns in the monitor theatre are set at the sound level that the monitor man on duty has found to be most satisfactory for his hearing. But in order that too low an impedance will not be presented to the output of the monitoring amplifier, a condition which would cause distortion and loss of sound quality, the combined impedances of all the horns connected to the horn panel should be made equal to the impedance that would exist if only one horn were used with its switch turned up to full volume. Horn Impedance Matching When a tap switch is on the tap marked zero, it provides full sound volume for the horn associated with that switch. The tap marked twenty disconnects the horn from the circuit in so far as impedance matching is concerned, but the horn will still produce sufficient sound volume for a small room. Using the figures given in the following table, the total indices of the switch settings for all the horns that are in use should add up to 1.00 (within plus or minus 0.10) to preserve the proper impedance match. Tap Switch Setting 0 2 4 6 8 10 12 14 16 18 20 Impedance Matching Index 1.00 0.63 0.40 0.25 0.16 0.10 0.06 0.04 0.03 0.02 0.00 From an examination of the table it will be evident that one horn with the tap switch set at zero will provide a perfect impedance match. Since the sum of the indices should be between 0.90 and 1.10, if two horns were used one could be set on point two and the other on point four, making the total 1.03. If several other (Turn to Page 25) Please mention The International Photographer when corresponding with advertisers.