The main types of construction of dynamics processors are mainly six: optical, FET, VCA, variable Mu, diode bridge and PWM; from this article and in the next ones we will analyze them in detail.
Optical compressor
The simplest wiring diagram to make a compressor is to connect two resistors, one in series at the input of the circuit and the other at the end that leads to ground. By replacing one or both resistors with elements that can be modulated by the input signal, it is possible to modulate the attenuation of the system with the amplitude of the musical signal. This principle was already known towards the end of 1800, with the introduction of resistors that vary with light (photoresistors). In principle we can say that a light bulb, driven by the musical signal, which illuminates the photoresistor, inside a sealed dark container, constitutes the operating principle of an optical compressor.
The advantages introduced by this type of construction are remarkable: the control circuit is galvanically separated from the compression circuit and, the diaphony is significantly reduced. Of course the "disadvantages", if we can talk about them, are that the system has a relatively slow response, due to the latency of the lighting element and the response time of the photoresistor, however the action of the optical compressor will be for the softer on musical signals.
An evolution in construction occurred when a LED and faster photoresistors were used instead of the light bulb, to have a faster attack and release time, as in the case of the Teletronix LA-2A where there is one resistor that only takes care of gain reduction and the other measures the magnitude of the signal displayed on the front panel VU meter.
The Teletronix LA – 2A is a leveler valve powered and consists of the following controls: switch that allows the passage from compressor to limiter (in limiter mode it sets a ratio of 60:1), a rotary control on the back that adjusts the pre-emphasis circuit making the compressor more and more sensitive to high frequencies, a gain potentiometer that acts at the output on the compressed signal, a peak reduction potentiometer that increases compression up to a maximum from 40 dB and finally a potentiometer that allows you to view the output signal or the amount of reduction applied on the VU meter. The attack and release times are fixed and, as in other famous models, depend on the characteristic elements of the circuit. Finally, neither the machine bypass switch was provided as there was only the ignition switch which remained the only way (at the risk of damaging the power supply) to bypass the leveler, nor the balanced connections on the rear panel and, this being a monophonic compressor, in order to connect two at the same time, to work on stereo signals, the machines had to be physically jumpered using the terminals in the rear part of the link.
Another compressor developed in the second half of the Years' 60, very similar to the previous one is the Teletronix LA 3A, also a monophonic leveler with the same compression circuit as the LA-2A, but with transistor power supply.
Here too the attack and release times are fixed, there is a photoresistor dedicated to the analysis of the signal that is displayed on the meter, where this time you can only see the amount of reduction or the output signal, the two potentiometers have the same functionality of the model previously described and, this machine also compresses up to a maximum of 40 dB. There is no bypass and for the link of two machines the same is true of the LA-2A.
