Universal Audio LA-610 Channel Strip - Manual (Page 15)

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electro-luminescent panel. This signal controls the gain of the compressor. After the
gain reduction circuit, the signal is sent through an Output Gain control and a two-stage
output amplifier, followed by the output transformer.
As mentioned previously, compressors are devices that vary their gain in a manner which
is dependent upon the level of the input signal. In order to do this, the compressor must
first have some method of determining the level of the signal, and must then be able to
use this to control the gain. There are many different schemes to accomplish these tasks.
In the case of the LA-610, both of these functions are performed by the T4, which is an
electro-optical element.
Electro-luminescennt
Panel
Photo-Electric Celll
Figure 5 : Diagram of the T4 electro-optical cell.
The cell T4 is comprised of an electro-luminescent (EL) panel and a photo-electric cell
housed in a light proof box or can. The EL panel is essentially a night-light. As you
would expect, the larger the signal that is applied to it, the brighter the light that is
generated. This light shines upon the photo-electric cell. A photo-electric cell is a light
sensitive device whose resistance changes depending upon the intensity of light to which
it is subjected; the brighter the light, the less resistance the photo-cell will have.
The photo-cell is used to control the gain of the circuit. Essentially, the photo-cell acts as
the bottom leg in a voltage divider circuit. The lower the resistance of the photo-cell, the
lower the signal voltage will be at the output of the gain reduction stage. To see why this
is true, we can look at the extreme cases. If the resistance is extremely high (this is the
case when there is a small input signal and the light is off) then the photo-cell does not
affect the circuit and there is no gain reduction. The second case we can look at is when
there is a large signal present. In this condition, the light shines brightly and the photo-
cell exhibits very low resistance. If the resistance of the photo-cell becomes zero (a dead
short), then the signal would be grounded and there would be no output. In reality, the