AM8171 Sequential Switch (100M)


Roland 173 Module

Overview The AM8171 is an expansion of the Roland 173 module from the System 100M, that was introduced in 1983 as part of the third and last wave of 100M modules. It sold just a few hundred examples before the whole 100M production ceased in 1984. Analog modular died out as the DX7 and, D50 and M1 brought digital synthesis into the market. The 173 module was cheap at £125 but was really a bit limited.

Original Design Roland put four signal gates into one module, which switch an audio or CV signal on and off via two gate inputs. There is a LED for indicating an on state but no switch to turn the gate on/off manually. There are also a ton of multiples to fill the panel up. The switching is done by audio quality JFETs. It is surprising that Roland didn’t add any mix capability or on/off switches as they did in the Roland 723 module.

Behringer 173 Module

Behringer 173 In 2020 Behringer introduced a complete set of replica modules including the 173, and copied the Roland 100M design using modern SMD components. They are mounted on the rear and front of the PCB. The front panel is dominated by the multiples, and the signal gates are rather squashed into the top of the panel. At £80 this is not a cheap module, but is works fine.

AMSynths AM8171 I originally planned a Eurorack AM8173 module back in 2018 and started to layout the PCB. The arrival of the 173 as part of Behringer System 100 resulted in the project being stopped. However in late 2022 I restarted the project after locating a manufacturer that can make powder coated panels and realising that the 173 design could be improved upon.

The AM8171 features the same four signal gates but adds in the features of a sequential switch whereby each gate can be switched on in sequence. The output of each gate is mixed together and provided as a MIX OUT, in addition to separate outputs. The sequential switch can be controlled by the on board LFO or an external LFO (for example the Behringer 140 and 150).

A 3-way slide switch for each gate sets how it operates:

  • TOP – permanently switched on, as a preview mode
  • CENTER – controlled by the gate inputs
  • BOTTOM – controlled by the sequential clock

AM8171 Module

The design uses a modern analog switch chip (DG212) rather than individual JFET’s to reduce PCB space. A 4017 CMOS chip does the sequencing with some additional control logic and an analog VCO for the clock (LFO). The clock has a rate potentiometer (varies from 0.03Hz to 3 Hz), a sequence step selector switch (2,3,4) and stop/start (RUN) button as well as a STEP button. There is a clock output for daisy chaining multiple 171’s and a clock input that combines with the intermal LFO (when its running).

The front panel has been laid out to give more space to the signal gates and to be closer to the original Roland 173 in appearance. The front panel is powder coated aluminium with white silk screen. The grey colour is a close match to the original. The number 171 was chosen for the module, as maybe Roland considered a prototype based on the 723 Analog Switch in 1978, as 171 has always remained unused.

Outcome & Availability The AM8171 was designed in December 2022 – March 2023 and a prototype sucessfully test in June 2023. Production modules are now available, check our webstore for details.

Copyright AMSynths 2022