The FAIRCHILD MODEL 660 uses only a single push-pull stage or audio amplification and an extremely high control voltage, with the result that the Automatic Gain-Controlled Amplifier never produces any audible or observable thumps. Contrary to most limiting amplifiers heretofore available, this unit has extreme low distortion and noise under all conditions, both as a straight-through amplifier and under maximum limiting conditions.
The attack time of the unit is made extremely fast in order to catch short transients, and the release time is made adjustable from O.3 seconds to 25 seconds in six steps. Two or these have release times which are automatic functions or the program material, providing fast Recovery tor short-duration peaks and an automatic reduction with very long recovery time or overall gain should the program level remain high.
Because of the circuitry used, the controls on this LIMITING AMPLIFIER are somewhat different from most ether units. Essentially, four controls are provided: a Switch for Metering; an Input Level Control calibrated in l db steps; a Limiting Threshold Control and an Attack and Release Time Switch.
The unit itself consists electrically of three parts: First, a Single Stage Push-Pull Control Amplifier (Vl, V2, V3, and V4); second, a Control Voltage Fewer Amplifier V5, V6, V7, and V8) terminated by silicon bridge rectifiers (CR1 through 4); and third, a Power Supply providing a regulated filament voltage (T5 and C17) and plate voltage (V9, VlO, Vll, Vl2, T4, and T6) tor the Control Amplifier.
The unit is designed to be placed into any normal line level circuit and could be set to have unity gain at no limiting. Since all critical components are of highest quality, working well below their maximum ratings, a dependable operation can be expected.
Owing to the wide choice of attack and release time, as well as the Automatic recovery feature, this unit can be used to· limit program. material severely without producing the audible thumps or pumping so often associated with limited program material. A limiting meter is provided and provision is made to connect a remote meter to the terminals exposed at the rear or the amplifier.
INPUT LEVELS: Normal line levels (+4 or +8 VU)
OUTPUT LEVELS: +4 or +8 dbm line (+27 dbm clipping point)
GAIN: 16 db
FREQUENCY RESPONSE: 20 cycles to 20 kc 1 db
NOISE LEVEL: 70 db below +4 dbm
LIMITING NOISES: No audible thumps produced.
INTERMODULATION OR HARMONIC DISTORTION:
Less than 1% at any level up to +18 dbm output (no limiting)
Less than 1% at 10 db limiting and +12 dbm output.
ATTACK TIME (adjustable):
.2 milliseconds in positions 1, 2, and 6 .
.4 milliseconds in positions 3 and 5 .
.8 milliseconds in position 4.
RELEASE TIME (from 10 db of limiting):
Position 1: .3 seconds.
Position 2: .8 seconds.
Position 3: 2 seconds for multiple peaks.
Position 4: 5 seconds.
Position 5: Automatic function of program material: 2 seconds for individual peaks, 10 seconds for multiple peaks.
Position 6: Automatic function of program material: .3. seconds for individual peaks, 10 seconds for multiple peaks, 25 seconds for consistently high program level.
COMPRESSION RATIO: A function of the amount of limiting as well as setting of the two threshold controls which can be set to operate at ratios from 1:1 to 1:20.
POWER REQUIREMENTS: 117 volts, 50-60 cycles AC, 3 amps
STABILITY: Unit maintains stability of gain, gain reduction and balance over the range of line voltage fluctuations from 100 to 127 volts.
CONTROLS:
a) Input Gain Control Step attenuator: l db per step.
b) Threshold Control Continuously variable.
c) Time Constant Switch 6 positions, so as to provide fixed and variable time constraints for any type of program material.
d) Metering Switch 3 positions, so as to measure plate current of each of the control tubes, or as a limiting indicator.
e) ON-OFF Switch
MECHANICAL DIMENSIONS: Standard 19” rack, 10-1/2” panel space, depth behind panel 10”.
WEIGHT: 41 lbs.
TUBE COMPLEMENT: 4-6386; 1-EF806; 1-5651; 1-12AX7; 1-12BH7; 1-6BL7; 2-6V6; 1-GZJ4 (5V4).
INPUT CONNECTION: XLR MALE (Z = 600 Ohm)
OUTPUT CONNECTION: XLR FEMALE (Z = 600 Ohm)
