PPG 314 Sequencer

Introduction The PPG 314 analog sequencer dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company. The first PPG sequencer in the 100 series was a straight copy of the Moog 960, but the 300 series sequencer was a more advanced design. It was still only 8 steps and 3 CV’s but it had dedicated controls for step timing and for moving around the sequence itself. Accurate switched step timing was also used in the Roland 717 and the Synthanorma around the same time.

In 1976 I was about to go to the University of Essex and study electronic engineering, whilst I had heard analog sequencers on records by Klaus Schulze and Tangerine Dream, I had never seen or owned one. I just had my trusty PE Minisonic synth! The PPG 314 which sold for £2,000 was way out of my reach, it was a years salary!

How Does it Work The arrival of CMOS logic chips in the 1970’s really opened up sequencer circuit design, thanks to its low power consumption compared with TTL (ARP 1050). The PPG 314 uses a decade counter at its core (4017) producing 8 steps, to drive 3 sets of CV’s that are summed using 741 Op Amps.  The value of each of the 3 CV’s is set by a potentiometer for each of the 8 steps.

The single sided Controller PCB is hard wired into the Main PCB and contains the 4017 counter chip and five 4011 NAND logic gate chips that buffer the 8 step positions into the CV’s and provide the logic control. There are also individual gate output for each step, which goes directly to the jack sockets.

The Main PCB is doubled sided (with no silk screen) and it holds all the potentiometers, LED’s and rotary switches, with the 6.35 mm jack sockets mounted directly to the panel.

Voltage Controlled Clock This circuit lives on the main PCB and is based on another 4011 configured as a VCO with variable pulse width. Whilst Emu Systems and Roland were using analog VCO’s in their sequencers, PPG opted for a simple CMOS version. There are Rate and Pulse Width (0 -100%) potentiometers and a LO/HI Range switch, as well a switch to turn the external control of sequencer rate OFF or ON. A green LED shows clock rate, and there are START and STOP push buttons which are mirrored as trigger input jack sockets. This enables remote control of the 314.

Step Timing The timing control for each step is preset via a 6-way rotary switch, with these timing divisions; 1, 1/2, 1/4, 1/8, 1/16 and 1/32. This is a great feature for setting up accurate rhythmical sequences, as the Moog 960 uses a potentiometer which makes timing accuracy difficult. With separate timing control the 314 retains three rows of CV control, without scarifying one row for timing. The timing is controlled via a precision voltage patched by the switches into the clock frequency CV input.

Programming The lowest row of 6-way switches are for programming how each step works, and provides another level of real time control. The six positions are;

• LAST – previous step values are used for this step
• GO NEXT – skips the step
• GOTO 1 – moves to step 1, and starts again
• STOP – stops at this step, and repeats
• SEL TRIG – sends a trigger to the SEL TRIGGER output
• NORMAL – sequences step by step

On the front panel the lettering is a bit cryptic with a solid circle as STOP and an arrow as SKIP. The GOTO 1 is not marked as available on Step 1 and 2 for obvious reasons, but its still wired up but it means you never move off Step 1.

AMSynths 314 As part of my “Big Moog” project I wanted to replicate the PPG modules that Klaus added around 1976/77 to his Moog IIIP. These consisted of a set of analog voice modules (VCO, VCF, VCA, etc.) and a pair of 314 and 313’s. I was lucky enough to have been given some photos of the inside of some of these modules including the 314, which meant I could reverse engineer most of the circuits.

The core of the sequencer is the standard 4017 decade counter but with the unique feature of being able to skip a step. Usually designers had to use a ladder of flip flop circuits to enable jumping seamlessly to the next step. Wolfgang Palm achieved this just using the 4017 which makes for a very efficient design. The 314 clock is based on a CMOS 4011 chip, and the timing switches use precision resistors and feed a voltage back into the Clock VCO. I have recreated all of the six function modes including LAST, SKIP and GOTO1.

The Euro Rack AM314 has condensed all the controls and circuitry onto 2 PCB’s, a Panel PCB and a Main PCB (Clock and Sequencer Controller). The prototype PCB’s were laid out in Spring 2021 and ordered in July. A 68HP wide panel was also mocked up to ensure all the switches and pots would fit and the lettering was in the right place. This leaves space for the 16HP 313 Sequential Switch in an 84HP wide case.

2023 Rework The AM314 project was paused in 2021 after problems with both the original CMOS clock and a VCO clock (from the System 700), neither would go down to low frequencies. The high cost of the rotary switches also meant the project was not financially viable at £500 for the AM314. This changed in 2023 when I located a much cheaper source of rotary switches and a new panel partner that could do silk screen printing. This will bring the price down.

The project was restarted and a corrected panel PCB manufactured in June 2023, using a precision voltage source for the timing circuit. The channel voltage switches were replaced with 3 way slide switches to make the design cheaper but also the panel shorter at 60HP. I also bought a set of unbuffered CMOS chips (like the original) to see if this improved the behavior of the design. The CMOS clock immediately started working at low frequencies, in fact too low! The fastest rate was 0.25Hz, and then it stuck at 2Hz.

2024 Rework I increasing doubt that I can recreate the behavior of the 45 year JFETs and CMOS, and that I should re-engineer some circuit to get to a production 314. The CMOS VCO has been replaced with a SSI2131 VCO chip with a triangle core, this can achieve a wide range of low (and high) frequencies. The step circuits from the Roland 717 have been adopted. They use JF|ET buffers to avoid any interaction between step voltages and controls. They also have a step input CV that can take the place of the sequencers CV, this is different to the Moog 960 and the PPG 100 clone where the input is a trigger CV to set the step active.

Here is the feature list:

• SMD components to enable automated assembly
• SSI2131 based VCO for super low frequencies
• Analog input CV for each step (Roland 717)
• JFETs used to buffer step CV’s (Roland 717)
• Resistor ladder per step, with +10V voltage reference (Roland 717)
• Time CV has a buffer op amp on every step
• +10V voltage reference for each step CV
• Gate outputs have buffer op amp on every step
• Trimmer on each CV channel to set accurate 10V, 5V and 2V ranges
• Revised momentary switches that are smaller (12mm square)
• Low cost Moog style control knobs (15mm & 20mm diameter)

A new SMD PCB was designed in the Autumn of 2024 and a dark grey powder coated panel with white silk screening ordered in November. The project objective is to launch the AM314 (and AM313) in 2027.