AM1050 Module - ARP 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 lamps (no LED's) 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 - candidates for improvement!

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

• Digital PCB with all the digital logic and VCC
• Analog PCB with analog switches and Op Amps

I initially planned to replace the TTL logic with a PIC micro-controller and develop a software solution, but this takes time - so I have gone 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
• Remote control of the 3-way 4-pole switching for Counter/Mixer
• Obsolete 1339 Op Amps upgraded to modern versions

Front Panel The panel is 5U MOTM format, but with the jacks/connections mounted on a separate panel. There simply isn't space on ARP 2500 front panels as they used matrix switching.

Build History PCB's are still in design stage, I'd expect the prototype to be ready in March 2009.
 

AM1050