AM1050 Mix Sequencer

Overview This module is a replica of the Mix Sequencer module from the ARP 2500 synthesizer, the ARP 1050. This is a straight forward analog switch with some rather cool features. The original was designed back in December 1970 using TTL logic chips and FET transistors to switch the signals off and on. This module pre-dates the arrival of CMOS logic, analog switch chips. LED’s and even decent Op Amps. Pearlman was pushing the limits of 60’s analog electronics!

Description This module has sequential switching for the rapid selection of preset waveforms and signals into a conventional audio mixer format. There are two four-input mixers with electronically gated inputs, an eight-step counter and clock, and associated logic and switching circuitry. Each input has its own attenuator and each output has a master gain control. The module can be used as an eight input mixer with two adjustable outputs or as two separate four input mixers. A column of illuminated push buttons indicates which inputs are gated on. The switches are pushed to change the state of an input, push-on/push-off. A column of “Exclusive-on” push buttons will turn on a particular input while simultaneously turning all the others off.

Original Circuit The 1050 has a now obsolete TTL logic core of a 74155 dual 2 of 4 decoder driven by a 4-bit binary counter 7493 chip, and clocked by a internal pulse generator. This logic provides the eight step counting. A set of 7474 flip-flops provide the manual latching of the square push buttons, and four 75451 peripheral driver chips light the incandescent 12V lamps and drive the analog switches. The analog switches are built around FET’s. Additional logic gates provide individual control of the switches from another ARP 2500 module, via an internal cable. The audio signals are mixed into the 2 separate audio outputs using four 1339 Op Amps.

AM1050 I obtained a full set of schematics in 2006, and transcribed the circuit into CAD and have designed 2x PCB’s:

  • Digital PCB with all the digital logic and voltage controlled clock
  • Analog PCB with analog switches, Op Amps and Audio Outputs

I initially planned to replace the TTL logic with a PIC micro-controller and develop a software solution, and then I went on to design a EuroRack version, but in 2015 I went back down the hardware route. The original circuit has been retained with the following upgrades:

  • LS TTL logic to reduce power requirements
  • SSM2402 Analog Switch chips to replace the discrete FET’s
  • Digital I/O – 16 pin DIL with 8 steps in and out
  • Obsolete 1339 Op Amps upgraded to OPA2134

Front Panel The panel is 5U MOTM format, but with the jacks mounted at the base of the panel.

Build History With renewed energy in the summer of 2015 I carefully laid out the two PCB’s. The main digital PCB was tested in September and after a few issues it was nicely sequencing the lights. However the momentary push button response was poor, and I made changes to the voltage levels that it switches, so it drives the 74LS74 chip correctly. In October I had a panel manufactured and fitted the digital board.

In December I wired up all of the digital board and switches and ordered the Analog PCB, however whilst the clock worked the switching of the channels was error prone and the whole lot locked up. I will try non LS TTL chips but even if these work the current consumption is high, so I don’t really like this hardware design.

Build Update In spring of 2018 I started working on a software version using the PIC18F46K22, which has sufficient I/O pins to control most of the logic, except the external input and control which is done in CMOS hardware. Some use of BCD encoding and decoding is needed to keep within the 40 pins. The analog design remains the same using SSM2402 analog switches and high quality Op Amps. The hybrid design means this can all fit on a single PCB.

Copyright AMSynths 2017