15
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-
3A. 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-3A 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.
Side-Chain Circuit
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. The term side chain comes
from the process of “splitting” the audio into a side path to be used for sensing and controlling loud volume
peaks. The LA-3A is a feedback style compressor. This is due to the fact that the signal that is used to drive
the side-chain circuit is affected by the gain-reduced signal. This signal is first fed into the side-chain drive
circuit and in turn controls the compression threshold and amount of gain reduction. A pre-emphasis circuit is
provided on the output of the side chain amplifier circuit. Originally designed for broadcast, the LA-3A allowed
for side-chain equalization, which allowed the operator to make the compression more or less sensitive to the
voice frequency bands. For musical applications, this equalization is usually set to a flat frequency response.
