Fairchild 670 - General Description

MODEL 670 STEREO LIMITER
GENERAL DESCRIPTION
The FAIRCHILD MODEL 670 incorporates on one chassis two independent
limiters which can limit either two independent signals, such as the
left and right channels of a stereo signal, or the vertical and lateral
components of same. The latter is accomplished by first bringing the
two stereo channels through a matrixing network, dividing them into
their vertical and lateral components, limiting them independently,
and recombining them through a second matrixing network into left and
right channels.
Each half of the MODEL 670 uses only a single push-pull stage of 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 extremely 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 0.3
seconds to 25 seconds in six steps. Two of these have release times
which are automatic functions of the program material, providing fast
recovery for short-duration peaks and an automatic reduction with very
long recovery time of overall gain should the program level remain
high.
In the circuitry used, each half of the amplifier consists of two
separate amplifiers: one is the Automatic Gain Amplifier, and the
other provides the necessary power for producing the DC control voltage.
Because of this, the controls are somewhat different from those of most
other limiters. Essentially there is one control common to both channels
which will switch between independent limiting and vertical-lateral
component limiting, besides four controls for each channel: a Switch
for Metering; an Input Level Control; a Limiting Threshold Control;
and an Attack and Release Time Switch,
Owirg 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 with provision for connecting a remote meter to the terminals
exposed at the rear of the amplifiers.
The MODEL 670 is designed tobe placed into any normal line level
circuit and can be set to have a 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.
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