Doepfer A-171-4 Quad Voltage Controlled Slew Limiter (Polyphonic Portamento)

Module A-171-4 is a fourfold voltage controlled slew limiter. It's intended purpose is polyphonic portamento but can be used also for other applications where several slew limiters are required. The common slew time can be adjusted manually and in addition controlled by an external control voltage. The module uses light-sensitive resistors (LDR). Therefore the slew times are not exactly the same for all units due to the tolerances of the LDRs and the LEDs, which are used to illuminate the LDRs. Inputs and outputs of the slew limiters are buffered to avoid voltage drops. This is especially important for polyphonic portamento applications. Each output can drive several VCOs without the need of exernal buffers. In addition the slew limiter inputs and outputs are available at internal pin headers for internal pre-wiring of polyphonic patches. As the switching contacts of the sockets are used for the internal pre-patching the internal patch can be overridden by using the sockets at the front panel.

The module has these controls and in/outputs available:

• Control Man. : manual control of the slew limiter value (base value)

• Control CV: attenuator for the signal applied to the CV socket

• Socket CV: control voltage input 

• Sockets In 1...4: slew limiter inputs

• Sockets Out 1...4: slew limiter outputs

• LED: visual control of the slew limiter value (dark = long time, bright = short time)

Technical notes:

• Each light-sensitive resistor (LDR) forms together with an associated capacitor a simple standard RC network (Resistor/Capacitor network). Input and output of each network are buffered.

• The slew time range is about 5 ms ... 10 s (1:2000). Due to the tolerances of the LDRs and the LEDs the slew times vary a bit for each unit. By changing the capacitor values this range can be altered: by lowering the capacitor values the slew limiters can be used as 6dB low pass filters. Increasing the capacitor values result in longer slew times.

• The 4 photo resistors and LEDs are assembled within an small lighproof box. In addition the pc boards are made of lighproof black material to avoid interfering light from other modules or the bus board.

• On the module pc boards two pin headers with 4 pins each are available. These are planned for the internal pre-patching of polyphonic modules:

  • In 1...4: The pins of this pin header are connected to the switching contacts of the sockets In 1...4. Hereby an internal four-wire connection to the CV outputs of the polyphonic midi-CV-interface A-190-5 can be made. As long as the sockets In 1...4 are not used the internal connection to the A-190-5 is established.

  • Out 1...4: The pins of this pin header are connected to the outputs Out 1...4. Hereby an internal four-wire connection to the CV inputs of the polyphonic VCO A-111-4 can be realized. If more than one VCO has to be controlled by one output we offer so-called micro multiples.