Inovonics 235, Инструкция по эксплуатации и техническому обслуживанию

Страница: 20 / 35

— — 18
can be seen in the curve for 0dB, “flat” low-frequency equalization. Its
effect at other equalization settings may be noted as well.
The feedback path of IC37B includes the front-panel
INPUT GAIN
control, R212. This pot affords the 15dB of continuous level
adjustment within each jumpered input gain range.
The sinewave signal level at the output of IC37B which will maintain
AGC gain at 0dB is –5dBu.
AGC AMPLIFIER (Schematic Sheet 1, Page 29)
Antediluvian
Gain-Control
Element
Explained
The next paragraph details operation of the AGC amplification stage,
but the gain-control technique used there is typical throughout the
Model 235.
IC38B is a classic inverting op-amp, but with transistors Q35 and Q36
included in the input network. It is the saturation resistance property
of these two transistors which is used to effectively shunt input signal
current to ground, thus controlling the overall gain of this stage. The
signal voltage swing, being electrically very close to a current summing
node, is negligible, far below the V
CE(SAT)
of the bipolar NPN devices
used here. The upside-down, back-to-back transistor hookup helps
linearize the circuit for a low figure of distortion. Surprisingly, gain
reduction vs. base current is a stable and predictable function.
If this circuit is unfamiliar to you, don’t feel bad. It is somewhat
unconventional and dates from one of Inovonics’ first products
developed in the early 1970s. It is also the subject of a patent, long
since expired.
Rectifier and
AGC Control
IC39B and IC40B, along with diodes CR27 and CR28, give full-wave
peak rectification of the gain-controlled input signal. R197 and C67
yield the 10-millisecond-integration for AGC response to program
material.
IC41B is a DC gain stage with a fixed offset. Its output sits at
+1.5VDC when the program level from IC38B is exactly –5dBu. The
output of IC41B is driven negative when the program exceeds –5dBu,
and is driven positive when program level drops below –5dBu.
Let’s assume for the moment that the output of IC41B is routed
through IC42A directly to IC41A, an inverting comparator with a
reference voltage of +1.5V and a tiny bit of hysteresis. The output of
IC41A is going to snap smartly to the positive rail when program audio
rises above –5dBu, and to the negative rail when it falls below this
level. This toggles analog switch IC42B which steers integrator IC40A.
The integrator is driven from its own output when the program level is
too low, and from an inversion of its output when the level is too high.
This gives a logarithmic voltage ramp which matches the logarithmic
transfer function of the gain-control stage to yield a linear dB/second
correction. The time constant of R173 and C58 determine the slow,