Doepfer A-135-4A/B VC Mixer Module

The A-135-4A main module

A-135-4A works in principle like the A-138p module, but here all parameters - with the exception of gain - are voltage-controlled. The front panels of A-135-4A and A-138p are very similar, the main difference being that the functions of Level, Pan and Aux are additionally indicated by LEDs on the A-135-4A. Without the LED indicators you would have no visual control over the actual value of the parameter concerned, as it is not only derived from the position of the manual controller but the external control voltage is added or subtracted if necessary. A-135-4A contains four voltage-controlled amplifiers (VCA) per channel: one for the main volume (gain), two for the left/right panorama and one for the aux path. In total, the module contains 16 VCAs. High-quality voltage-controlled amplifiers from Curtis (CEM3381) are used. The LEDs roughly indicate the current gain of each of the 16 VCA's. The A-138o module is used as output unit. A-135-4A and A-138p can also be operated in combination with a common output module A-138o (e.g. four purely manual and four voltage-controlled mixer inputs).

A-135-4B for external control voltages

The external control voltages are supplied to the main module via module A-135-4B. The following parameters can be controlled separately for each channel:

• Level (main volume)
• panorama
• Aux Way
• mute

A socket and a polarizer are available for each control voltage, so that the influence of the respective parameter can be set either positively or negatively via the externally supplied control voltage and in selectable dosage (the polarizer function is described in more detail in module A-138c). The external control voltage is added to the manual setting of the respective parameter on the main module A-135-4A and the sum of the control signal is displayed there with one (Level, Aux) or two LEDs (Panorama)
The CV input for Mute works in principle like the CV input for Level, but with inverse polarity
.
The mute control input is also much more sensitive.

Typically, the control for the mute CV input is set to the right stop.

The connection between A-135-4A and A-135-4B is made internally via a 20-pin ribbon cable.

The cable can also be longer, so that the module for the CV inputs does not necessarily have to be arranged next to the main module (but e.g. also in the row below or above).A-135-4A and A-135-4B are only available together, since a single operation is not useful.