DST-2

The DST-2

The DST-2 is the second version of the DST distortion pedal series. Its main goal was to make the pedal much more modular and easier to debug, since I want all the future pedals to be compatible with one another.

The modularity comes from the use of JST connectors and modules like the TMC-1 and the TMF. These modules aim to standardise the pedal-making process and make all the future pedals compatible.

The debugging aspect of the board comes from the use of test pads labeled accordingly, so that when a problem arises, it can be easily debugged and fixed. The test pads I added mainly target features like power supply and signal line. I also connected the four screw holes in the corners of the board to GND, so that when debugging I won’t have to look for a ground test point.

Another feature that I introduced with this board is the use of SMD components rather than THT ones (except for a couple diodes and the transistors). This change aimed to minimize the board size, to ensure it is going to fit alongside the other modules in the metal enclosure.

The Schematic

The schematic is made up of a few blocks: a buffer stage, an initial amplifying stage, a variable gain stage, a clipping stage, a filtering stage, and a final buffer stage.

The buffer stage

The buffer stage is a voltage follower made up of the transistor Q1 and R1, R2 and C1, C2.

This stage has a voltage gain of (ideally) 1, so that the voltage at the collector of Q1 is exactly the signal but 180º out of phase (basically the signal but inverted, lmao maths). But the real advantage of the buffer stage comes from the high-impedance input and low-impedance output of the stage.

The resistor R1 biases the transistor whilst the resistor R2 provides thermal stability (you could add another capacitor in parallel with R2, so that it shorts at higher frequencies providing better stability thus a better frequency response). The capacitors C1 and C2 are decoupling capacitors. They eliminate the DC characteristic of the signal so that I can bias it however I need to (again, lmao maths).

The initial amplifying stage

The initial amplifying stage is made up of the transistor Q2, the resistors R3 thru R6 and the capacitors C3 thru C5.

The resistor R3 and capacitor C3 make up a high-pass filter with a cutoff frequency of about 33 Hz. The capacitor C3 is also used to decouple the signal, and the resistor R4 is used to bias the base signal of the transistor Q2.

The resistor R5 and the capacitor C4 make up the feedback loop of the stage, whilst the resistor R6 gives the amplifying ratio of the stage (its ratio is roughly R6/internal resistance of the BE junction).

Finally, the capacitor C5 is used to decouple the signal before it reaches the next stage.

The variable gain stage

The variable gain stage is made up of a few components, mainly a double op-amp SOIC-8 IC (even tho the TL072 or the LM358 work fine, I'd strongly recommend some better op-amps such as the OPA2134) with some support components such as resistors and capacitors.

The PCB

Yappa yappa PCB yappa yappa type shi

After initial testing

After initial testing, the pedal seems to work but just barely. There is a whole lot of induced noise. The values for the filter are all over the place and make the pedal sound too muffled. Overall just a bad tone. I've since learned that JST connectors are really really unreliable and that the use of separate "modules" for the footswitch and the jack connectors is a bad idea. It adds complexity and are really really prone to failure / noise. I plan on redesigning it without all that fuss, one board to rule them all. The footswitch placement is still an issue tho. I can’t mount it directly on the PCB since I can’t reliably predict the final height of the whole assembly (it can be soldered higher or lower). Therefore, it needs to remain separate, with wires manually routed to it. The same applies to the jack connectors. The barrel connector on the other hand, i can have it on the PCB no problem since it needs some stability to work properly. The potentiometers need to be routed manually also.

Conclusions:

-new pcb -manually route wires to pots/conns/switch -change filter values -pick another IC for the board (OPA2134)