The T4 opto-attenuator unit is the key component that gives the LA-2A its unique smooth, natural compression charactersics and is therefore the key to the unit's unique sound. Since the LA-2A's introduction and acceptance by the pro audio community there have been at least three major versions of the opto-attenuator used in them: the T4A, T4B and T4C. There are several variations of each version, as the manufacturer apparently attempted to simplify the unit and make it easier to produce. From my brief experience with some very early (and very special) LA-2A's, the early T4 units are the best, and impossible to duplicate. Here is some info I've gathered on the difference between the units:
The above photo shows an early T4B unit on the left and an extremely early T4A unit on the right. Both of these units use Clairex CL-505L glass-encapsulated photocells, which is the earliest Clairex part I know of that was used in T4 units. According to Al Bomchill of Clairex, whom I spoke with in 1997 or so, Clairex cannot produce these cells any more because the EPA (Environmental Protection Agency) would not approve the manufacturing process, due to the use of liquid flourine (if memory recalls - I don't remember exactly which element was used) kept at -400 degrees farenheit, or something like that. Anyway, the point is, there will never be any new versions of these photocells so if you've got some, either hold on to them or sell them to me!
You can clearly see the differences in construction between the later T4B and earlier T4A units. Most obvious is the use of a circuit board in the case of the T4B, and the additional CL705 photocell wired in parallel with the photocell used for audio attenuation. Apparently they didn't feel the need (or couldn't find the real estate) to use an additional cell in the metering circuit. Also, on the T4B there is a fine steel mesh sandwiched between the photocells and the EL panel. For years I've debated the reasoning behind the use of this mesh. It has to be one (or both) of two reasons. First, it could be used to provide electrostatic shielding to help minimize any capacitive coupling between the EL panel and the photocells. The voltage across the EL panel could be high enough to cause voltage variations across the photocells. Second, the mesh could help to diffuse the light given off by the EL panel and/or reduce the amount of light seen by the photocells, in order to allow the voltage across the EL panel to increase to a higher level before attenuation takes place.
The T4A doesn't have any of these things and doesn't need them; it's sounds incredible just the way it is. In this unit the photocells are placed in direct contact with the EL panel, held in place with the aid of a paperclip, which can be seen more clearly in the photos below. The early T4A and T4B units used the same Clairex photocells and Grimes electroluminescent panel.
More about T4A units -->
More about T4B units -->