ProReplicas - vintage audio equipments for recording
ProReplicas T4B Opto-Attenautor.
The T4B Opto-Attenautor, is a heart of compressor LA-2A. This module determines about the unique nature of compression in units of LA-2A, is responsible for time of attack and releace. Although, that this module is a very simple to build - requires only the EL panel and two photoresistors - it however, is very important, application the right kind of EL panel, but primarily selecting of appropriate photoresistors about the appropriate characteristics. We have a specially designed tool and software to stripping characteristics of these photoresistors, so now, we can easly see, if the given resistor will be suitable for our T4B cell. We can use in our T4B two types of EL panels, fast and slow, but in standardly we produce of our photocell with the fast EL panel, like in primary T4 cells from the UA.
* method of matching photoresistors
This screen below, from our software, shows the most important test of our T4B cell. Here is given a short test pulse of 1 kHz, which results a graph, showing response of T4B cell. On this picture is shown, two curves; burgundy, for first photoresistor responsible for a control of compression in path audio, and green, for second photoresistor, which is to control of VU-meter in mode a gain reduction. The most important curve is burgundy, because from this photoresistor is dependent attack and releace time, and also indirectly ratio of compression. The geen curve, from second photoresistor, should be relatively close at burgundy curve, this determines the accuracy of indications of the VU-meter in GR mode. This burgundy curve is a model; here the attack time is a fast enough, and release have a right curve of time tempering. This screenshot shows scale of view only for first 2 seconds, for the better visibility of the most important a moment of response from the T4B cell. The tail of release, can extends to, even to tens of seconds at heavy compression.
The next snapshot, which look like a some contour line of geologic, show the curves of responses from the one batch of 20 photoresistors. Here can see, that best characteristics are located in the vicinity of the center (highest density), and from this place we take the photoresistors, which will be responsible for a control of compression in path audio, they will be used as first photoresistor, from two, in our T4B modules. The photoresistors with curves a bit away from the center, they will be used as second photoresistor in our T4B, to control of VU-meter in mode a gain reduction. The photoresistors with curves, which are shown as extreme will be rejected.
The batch of photoresistors NSL-5910 Silonex. These photoresistors are today a standard at production of T4B cell.