Introduction The PPG 313 analog switch 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. The 313 is an improved version of the Moog 962 with 4 channels/steps rather than 3 and the ability to skip and repeat the step.
Brochure Description The PPG313 Sequential Switch is an electronic switch that connects one of four input signals to a common output. These signals can be audio signals, control voltages and trigger impulses. The main application is in conjunction with the 314 Sequencer, which can provide up to 32 pre-selected voltages, one after the other. The PPG313 has four stages which are sequenced in series.
The signal inputs are the lower 4 jack sockets labeled SW1, SW2, SW3, and SW4. One of these four inputs can be switched to the Switch Output either manually using the buttons or by trigger signals via the jack sockets SET 1, SET2, SET3 and SET4. The four positions of the electronic switch can be used one after the other, using the front panel button or via the SHIFT jack socket.
The four rotary switches called COUNTER PROGRAMMING are used to preprogram repeating patterns for each of the four stages. For example; with all 4 switches in position 1, the 4 positions are switched through evenly one after the other, according to the impulse that is given to the SHIFT input.
The output of each position is available on four jack sockets (TRIG1, TRIG2, TRIG3, TRIG4). It is a gate output (not a trigger pulse) from a channel going on.
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. The PPG 313 uses two decade counters (4017) to sequence the 4 channels and to provide the step repeats (1, 2, 3, 4, 8). The CLOCK and INHIBIT pins are used in reverse, with the clock signal being externally provided or created via the SHIFT button. By clocking the INHIBIT input the counting is triggered on the falling edge of the clock signal. The lack of Schmitt trigger on the INHIBIT input means the shift inputs (clock in and button) need to be clean.
A 4011 quad NAND chip provides the debounce of the SHIFT button with a diode protecting the gate from the INHIBIT pull up. The 4011 also provides a monostable circuit to stop the clock of the 4017 used to sequence the channels, when a SET input is triggered or a channel button is pressed. The channels drive transistors to provide the trigger signals (gates) and the JFETs, which are used to switch the analog signals on/off into a single merged output.
The PPG design has a large main PCB which contains most of the circuitry. A smaller PCB contains the 5x buttons and a 4017 and 4011. They are connected via soldered wires and therefore difficult to take apart. The PCB’s are single sided with no solder mask and with a lot of hand wiring to the jack sockets.
The PPG design is very elegant and clever, with a small number of CMOS chips and a 741 as a comparator. Some WIMA capacitor values have been assumed, whilst others like the 1000pF polystyrene caps used in the debounce and set circuits are clearly marked. Resistor values were read from the colour accurate photos.
AMSynths Design Initially I attempted to build an exact replica of the PPG 313 with the circuit traced from an original 313 using photos and tracing paper! There are two high quality PCB’s; the PANEL PCB holds the rotary switches, buttons and jack sockets. The MAIN PCB holds all the CMOS logic and transistors. They are connected using SIL headers and easy to take apart.
However the re-engineering (July 2021) proved tricky with the following issues;
- SKIP feature refuses to work, the pulse does not get to the 4017 to move it to the next step.
- P-channel JFET’s (BF320) don’t handle negative voltages, with a 10% bleed through when switched off
- SHIFT button debounce is intermittent
I managed to correct the last two issues, but the SKIP feature proved elusive. In 2023 I restarted the project with low cost 6-way rotary switches which made the 313/314 project viable. But two problems remained;
- STEP buttons not reliably switching on
- SKIP function not working
To improve the situation I bought a set of unbuffered CMOS chips (like the originals) to see if this changed the behavior. They did improve the counter programming which was now reliable, with the required number of steps controlled by the rotary switch. However despite numerous attempts I could not get the SKIP trigger (1V, 1ms) to reset the second 4017 counter. The calculated value of the pulse is only 7.5 us, which is above the enable pulse width of 35ns for the 4017.
The design was also improved:
- Diodes added to protect SHIFT and SET inputs from negative voltages
- Channel outputs mixed with an Op Amp
- A DG412 used as the analog switch
These changes make the AM313 more versatile and not sensitive to negative voltages. The Op Amps, and modern analog switch, enable audio as well as CV signals to be processed.
2026 Update The project was paused until 2026 when I had the time to create a test bed for the CMOS switching (after an abortive ATtiny microprocessor attempt). I also discovered that Gert Jalass (Moon Modular) had recreated the 313 as early as 1980, then re-worked the circuit into the Moon 564 in the 2000’s. The circuit does not look like a clone but a re-engineered module.
