The Technical Stuff
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Electro-Optical Compression/Limiting
In order to operate, a compressor must first have some method of determining the level of the
incoming signal, and must then be able to use the fluctuations in that signal to control the gain.
There are many different circuit designs which have been developed to accomplish these tasks. In
the case of the LA-610, both of these functions are performed by an electro-optical element called a
T4.
The T4 is the very heart of the LA-610’s T4 optical compressor. Identical to the T4 used in the LA-2A,
it is comprised of a small light-proof metal canister that contains two components: an electro-
luminescent
(EL) panel (a device that lights up when electrical signal is applied) and a cadmium-
sulfide
photoelectric cell (a light sensitive device whose electrical resistance changes depending
upon the intensity of light to which it is subjected). It is the unique gain reduction characteristics that
result from the interaction between these two components that predominantly gives an electro-optical
compressor its signature sound. (Note: While the “el-op” circuit design was one of the first of its
kind, and is still used to this day in other compressor products, Universal Audio has its photo-cells
uniquely manufactured to exacting specs that were originally found on the LA-2A. No other
manufacturers have access to these cells.)
The genius of the el-op circuit lies in its simplicity: the larger the signal that is applied to the EL
panel, the brighter the light that is generated; the brighter the light, the less resistance the photo-cell
(which controls the gain of the electrical circuit) exhibits. Thus, the louder the incoming signal, the
brighter the light and the more gain reduction is applied... and with virtually no harmonic distortion or
audible artifacts. (In the most extreme case, 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, photo-cell
resistance cannot go completely to zero and hence there will always be some signal present.)
Conversely, when there is a small input signal (resulting in a dim light), the photo-cell will have a
great deal of resistance and will therefore not affect the circuit at all, so there will be no gain
reduction.
