PPG 313 Sequential Switch

PPG 313

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.

PPG 313 Buttons

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 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 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.

AM313 Mockup

AMSynths Design The AM313 is an exact replica 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.

The tricky parts of the re-engineering job were the SKIP feature and getting the channels to SET with the buttons and external inputs. The SET feature was easily corrected  from my schematic error but the SKIP feature refuses to work, the pulse does not get to the 4017 to move it to the next step.  I am reworking this part of the circuit and the SHIFT button debounce which is intermittent.

The analog switching is done using P-channel JFET’s (BF320) rather than a CMOS switch. The JFET’s work well but don’t handle negative voltages, as there is a 10% bleed through when switched off. Given the 313 was usually used with the 314, it only needs to deal with 0 to +10V outputs.

Outcomes & Availability An initial prototype AM313 was tested in late July and corrected over the next few weeks. Careful testing was conducted on the STEP repeats to ensure they were correct and stable across all channels. Rework was needed on the SKIP and STEP features, and a second prototype set of PCB’s ordered in August 2021.

2023 Revisit The AM313 circuit continued to have problems and was paused until 2023, when I identied a low cost source of 6 way rotary switches which made the 313/314 project viable. The two problems remained; STEP button not reliable and SKIP function not working. To improve the siutation I bought a set of unbuffered CMOS chips (like the originals) to see if this changed the behaviour.

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. Rather than chase ghosts, I have reverted to using an analog compator to create a stronger SKIP signal and OR gated it with the first 4017 output. More circuitry but CMOS chips are cheap these days.

The design has also been reworked with some improvements:

  • 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.

2024 Update AMSynths has moved to SMD component assembly, which reduces the cost and speeds production. To achieve this I need a bullet proof circuit for the 313, although I can still use unbuffered CMOS chips by ordering them into JLCPCB inventory. We are also using powder coated panels, and a black/dark grey panel with white silk screen will be perfect.





Copyright AMSynths 2022