A-135-4 modules in detail:

A-135-4A is the main module. It is very similar to module A-138p, but with voltage control for all parameters (except gain). Also the front panels of A-135-4A and A-138p are very similar. The only difference are the additional LEDs which are used to display the magnitudes of all voltage controlled parameters. This is necessary because the positions of the manual controls do not necessarily correspond to the resulting parameter value because these are composed by both the manual controls and the applied external control voltages (via A-135-4B). Without LEDs one would be left in the dark in regard to the actual parameter value.
A-135-4A contains four high quality voltage controlled amplifers (VCA) for each channel: one for the main level, two for panorama left/right and one for aux. The control scales for all parameters are linear. Altogether 16 VCAs are included in the module. High end linear VCAs manufactured by Curtis/USA are used (CEM3381). The 16 LEDs display roughly the amplification of the VCA in question.
Module A-138o is used as output module for the A-135-4A. A-135-4A and A-138p can be connected together to the same A-138o to obtain e.g. four fully voltage controlled channels (A-135-4A) and four manually controlled channels (A-138p).

A-135-4B

This module is used to route the 16 external control voltage to the main module A-135-4A. These control voltages are available for each channel:

• Level (main loudness)
• Panorama
• Aux
• Mute

Each control voltage has an input socket and a polarizer control available (the polarizer function is described more detailed on the A-138c info page). That way it's possible to adjust the depth and polarity of the external control voltage which affects the parameter in question. If the same CV (e.g. LFO or ADSR) is used e.g. for two panorama control inputs the result may be the opposite if the polarizers are adjusted accordingly (i.e. one positive and the other negative). The external control voltage is added to the voltage that is generated by the manual control on the A-135-4A main module. The sum of both voltages is used to control the VCA in question and is displayed with the corresponding LED on the A-135-4A module. The picture on the left side below shows one channel of A-135-4A/B as block diagram.

The mute CV input works in principle in the same way as the level CV input but with reverse polarity and higher sensitivity (about factor -2). Typically the mute polarizer control is adjusted fully CW. Then a (gate) voltage of ~ +2.5V or more fully mutes the channel in question. If the mute function is not used the mute CV input can be used as a second CV input for level (with twice the sensitivity and inverted polarity compared to the normal level CV input).

A-135-4A and A-135-4B are connected internally via a 20 pin ribbon cable. The cable can be longer so that the CV input module A-135-4B needs not to be mounted next to the main module A-135-4A (e.g. to the row above or below).

Modules A-135-4A and A-135-4B are available only as a module combo because the single operation of each module does not make sense !

A-135-4C

Module A-135-4C is a four-fold envelope follower with ajustable attack and release time for each channel. The main function of the module is to derive four envelopes from the A-135-4A audio input signals. These voltages can be used to control parameters of the main module A-135-4A via the CV input module A-135-4B2. Typical applications are ducking (e.i. the signal of one channel is used to mute another channel more or less), as well as compressor and expander functions. For this the desired envelope output of the A-135-4C is patched to the CV input of the A-135-4B which is assigned to the parameters that has to be modified by the envelope signal. Intensity and polarity of the envelope influence to the parameter in question are adjusted by means of the corresponding polarizer control.
The module has four audio inputs and four envelope outputs available. The inputs can be normalled to the inputs of the A-135-4A by means of an internal 10 pin ribbon cable. One can choose between direct input (= input sockets of the A-135-4A), post gain and post main VCA (3 different connectors).  The cable can be longer so that the envelope follower module A-135-4C needs not to be mounted next to the main module A-135-4A (e.g. to the row above or below).
As soon as a patch cable is connected to an input of the A-135-4C the internal connection to A-135-4A is interrupted and the module can be used independent from the A-135-4A for other applications (e.g. as a simple Attack/Release module with gate signals as inputs or as envelope followers for other signals).
The four envelopes generated by the module are displayed by means of LEDs.
The attack/release time can be adjusted in the range from about 0.01s to 0.5s.
A-135-4C is not necessary to run the modules A-135-4A/B but is required only for the additional features described above (e.g. ducking, compressing, expanding).
The time constant of each envelope follower is about 10 ms and determined by the value of a capacitor. By changing the value of the capacitor the time constant can be lowered (for faster response) or increased.

Additional technical notes

Module A-135-4A has an additional internal connector available that can be used to normalize the four audio inputs. If e.g. the position of the inputs socket at the top of the modules is unfavorable under certain conditions this internal connector may be used to connect 4 external sockets which are positioned elsewhere. If there are sufficient inquiries we may offer such a passive input module (A-135-4?).
The audio inputs of the A-135-4A are suitable only for high level signals of the A-100. Low level audio sources (e.g. microphones, electric guitars) cannot be connected directly. In these cases external pre-amplifiers are required.
The audio inputs and outputs of the A-135-4A are AC coupled ! It is not possible to process DC signals or slowly changing control voltages with this module.

The following document explains the internal connections between the three modules:  A135_4_connection.pdf

Note: Black versions of the small grey knobs used in these modules are available as spare parts. If some of the grey knobs (e.g. Pan or Aux) should be replaced by black ones they can be swapped easily by the customer. If applicable pay attention to order the knobs with 11.5mm diameter (A-100KSB in the price list / section spare parts).