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10
The next important aspect is that of the release of the compressor. This is determined
almost entirely by the characteristics of the photo-cell. The LA-2A uses cadmium-sulfide
photo-cells. The first important aspect of the cell is its “two-stage decay”. After the light is
removed from the cell, it releases quickly (40-80 milliseconds) to approximately half of its
off resistance. The remainder of its release can take place over as much as several
seconds.
The next aspect is the “memory” of the cell. This results in two important aspects of the
character of the LA-2A. The amount of time it takes for the cell to recover after the light is
removed depends on how long light had been shining on it and how bright the light. In
the case of the LA-2A this results in behavior where the release time is slower if the unit has
either been in compression for a while, or the amount of compression is large. This signal
dependent release characteristic is critical to the sound of the unit.
The amount of compression, as well as the compression threshold, is controlled by the
“Peak Reduction” potentiometer. This potentiometer controls the gain of the side-chain
circuit. The greater the gain of this circuit, the lower the threshold and the greater the
amount of compression will be. Many modern limiters and compressors allow for the
direct adjustment of the threshold. Other units such as the 1176LN use a fixed threshold
and provide an input level control, which adjusts the signal level before it is applied to the
compression circuit. In contrast, the LA-2A, while also having a fixed threshold,
does not control the input level, but rather controls the amount
of side chain gain applied to the input signal.
C8
.0 3
R31
1K
R2
100K
C9
.0 2
R33
220K
1 Meg
LIM
RESP
R37
R3
1M
STEREO
ADJ
OUTPUT
R36
1K
C10
50 uF
C7D
30 uF
R35
220K
R 3 4
22K 2W
V4
6AQ 5A
IN P U T
R30
47K
.0 1
ATTEN U ATO R
D R IV E -A M P LIF IE R
C12
.001
STEREO
PARALLEL
PO W ER
SU PPLY
R32
1K
C6
12AX7A
V3B
V3A
C11
.1
Figure 6 - Schematic diagram of the LA-2A side-chain circuit.