International photographer (Jan-Dec 1934)

Thirty T h INTERNATIONAL PHOTOGRAPHER Jutv.lvM MOTION PICTURE SOUND RECORDING (Continued from Page 25) fiers. Jacks on the amplifier panel permit the plugs on the cords connecting with the ammeters of the meter panel to be plugged into any filament or plate circuit, the one set of meters thus serving to read the filament or plate current drawn by any individual tube in the several amplifiers. The Bridging Bus The output of this main amplifier feeds into a branching circuit called the bridging bus. Four of the six branches of this circuit connect to amplifiers known as bridging amplifiers, the fifth branch feeds the volume in An amplifier of the rack type that is used for sound reproduction — not for recording. Courtesy Bell Telephone Laboratories. Inside view of the amplifier illustrating the manner of wiring. The same construction is used in recording amplifiers. Courtesy Bell Telephone Laboratories. dicator panel, and the sixth branch supplies energy to the monitoring amplifiers and through them to the monitoring horns. It is customary to maintain this bridging bus at zero level in decibels, or reference level, by balancing the amount of gain (in the amplifiers) and loss (in the monitoring controls, or attenuators) in the circuit between the microphones and the bridging bus. The volume indicator meter aids in keeping this level at the bridging bus more or less constant. The Bridging Amplifiers The bridging amplifiers each have only a single stage of amplification with transformer coupling at the input and output, but the circuit is of the push-pull type with two tubes. These are power tubes operating at a plate potential of 350 volts. The input impedance of a bridging amplifier is quite high ; so it is truly "bridged" across the circuit. As a result, the disconnection of any of the bridging amplifiers have no effect on the impedance match existing between the remaining amplifiers and the bus. Where large power output is required of an amplifier, two tubes are usually used in the last stage. They are connected either in parallel (the two grids being connected together and the two plates connected together) or in push-pull. In the push-pull form of connection, the secondary of the input transformer is tapped. Each end of the secondary winding is connected to a tube grid and the bias battery to the paralleled tube filaments. The primary winding of the output transformer is likewise tapped at the center, the tap connecting to the positive terminal of the plate supply and each end of the primary winding connecting to the plate of a tube. The push-pull arrangement is more satisfactory than the parallel connection of power tubes, because when operated in push-pull the tubes function alternately, each tube operating on its own half of the alternating speech current cycle. This results in a decided reduction in the harmonic distortion always present in the output of an audio amplifier, and the output power is nearly double the power that would be obtained with a single tube. The output circuit of each of the bridging amplifiers connects to one of the recording machines. In the normal installation, there are two wax recording machines and two film recording machines, with a bridging amplifier for each machine. These four recording machines are installed in a recording room located quite close to the amplifier room. Four pairs of shielded leads connect the bridging amplifiers with the recording machines. A variable attenuator is introduced in each line between the output of the bridging amplifiers and the recording device. Although these attenuators are installed in the recording room and are really a part of the recording equipment, it would be well to examine their use at this time. The Recording Machine Attenuators The attenuators are of the T-type, which means that they have three "arms", or resistance elements, arranged in the shape of a T, as shown in Figure 1. There are two series arms, A and B, and a shunt arm C. A threebladed switch is arranged so that it varies the amount of resistance in the three arms simultaneously where it is turned. To increase the amount of attenuation, or loss, presented to an alternating current passing through this attenuator, the resistance of the shunt arm, C, is reduced and the resistances of the series arms, A and B, are increased. To decrease the amount of attenuation presented by the attenuator, the resistance of the shunt arm is increased and the resistances of the series arms are decreased. The three-bladed switch accomplishes this in steps of one decibel each when it is rotated. The input and output impedance of the attenuator remain constant at 500 ohms regardless of its setting. The recording room attenuators have a range of zero loss to twenty decibels loss. The bridging bus is operated at a level of zero decibels and the bridging amplifiers have a gain of ten decibels, so the output of each bridging amplifier is normally at a level of plus ten decibels. (Decibels represent electrical power: reference level, or zero decibels, is 0.006 watt. Minus ten decibels is one-tenth that power, or 0.0006 watt ; and plus ten decibels is ten times zero decibels, or 0.06 watt. Decibels are logarithmic quantities, so they may be added or subtracted. For that reason, plus twenty decibels is 100 times — not twenty times — zero decibels, or 0.6 watt.) If the attenuator switch were set at mid-scale, which would cause a loss of ten decibels, the ten decibel gain in the bridging amplifier would be just balanced and the output of the attenuator would be at the same level as the bridging bus, or zero decibels. Since he can move the attenuator switch in either direction from this position, the recording machine operator is provided with a wide range of control over the level of the speech current applied to the recording device. By turning the attenuator switch toward the zero loss position the speech current at the recording device may be built up to a level as high as plus ten decibels or by turning the switch toward the 20 decibels loss position the level of the speech current may be reduced to as low as minus ten decibels. A change in the setting of the attenuator is made whenever necessary just before recording commences. center tap of the winding is connected through the gridPlease mention The International Photographer when corresponding with advertisers