Universal Audio LA-3A - Manual (Page 17)

14
The T4 Cell
DIAGRAM OF THE T4 ELECTRO-OPTICAL CELL
The cell T4 is comprised of an electro-luminescent (EL) panel and a photoelectric 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 photoelectric cell. A photoelectric
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 photocell will have.
The photocell is used to control the gain of the circuit. Essentially, the photocell acts as the bottom leg in a
voltage divider circuit. The lower the resistance of the photocell, the lower the signal voltage will be at the out-
put 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 photocell 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 photocell exhibits very low resistance. If the
resistance of the photocell becomes zero (a dead short), then the signal would be grounded and there would be
no output. In reality, the photocell resistance cannot go completely to zero and hence there will always be some
signal present.
The T4 electro-optical device is the heart of the compressor and its gain reduction characteristics. Its unique
characteristics affect the overall sound and character of the LA-3A.
In addition to the compression curve, the combination of the EL panel and the photocell determine the attack
and release characteristics of the LA-3A. This is one of the most important contributors to the sound of the
LA-3A. Unlike other compressors that allow the user to adjust these parameters, the attack and release of the
LA-3A are completely determined by the T4.
There are several important characteristics of the T4 that play crucial roles in the sound of the LA-3A. The first
is the attack. The LA-2A was the first electro-optical compressor to use an electro-luminescent panel for the
light source. Previous attempts at electro-optical compression employed either neon or incandescent lights.
Both of these took time to light up, and this delay resulted in slow attacks. The electro-luminescent panel
resulted in a faster attack than exhibited by other contemporary devices.
The next important aspect is that of the release of the compressor. This is determined almost entirely by the
characteristics of the photocell. The LA-3A the uses the same cadmium-sulfide photocells. 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