Deepstomp: Low-Cost High-Performance DIY Multi-Effects Pedal/Stompbox Platform

Figure 1. The Low Price of DSP Microcontroller Advertised in An Online Store (Upper) and DeepStomp Digital Pedal Mock-Up (lower)

Introduction

The advance of today’s digital electronics bring a new hopes for many things, because now there are many choices for low cost and high performance DSP microcontrollers. For example, a small board, popularly called “blue-pill”, is advertised in an online store for only US$ 2.25 (Figure 1.-upper part). It has 72MHz 32-bit processor core with some DSP instruction sets (ARM Cortex M3), 20kB RAM, multi channel 12-Bit ADC (can be expanded to 15-bit by multichannel oversampling), 4 PWM output, various communication peripherals, and many digital  IO pins. This ridiculously cheap development board is very tempting not only for just develop new product but also for embedding it into the final product as its core component. To take benefit of this digital technology for everyone, I had decided to design an open platform (open source hardware and software) for DIY digital stompbox/effect pedal which is based on “blue-pill” as its core, and I name it as Deepstomp. Now the hardware is officially released at Deepstomp’s official website, and here is the schematic diagram of the current version (1.2), shown in the Figure 2.

Figure 2. Deepstomp Main Board Circuit (released under hardware version 1.2)

Bill of The Materials

 

  • Resistors
    • VR1,2 ……………………………………………………………. 2 x 10K Potentiometer
    • VR3 ……………………………………………………………….. 1 x 10K Multi-turn trimmer potentiometer
    • R1,22,25 ……………………………………………………….. 3 x 1K
    • R2,3,4,5,6,7,10,16,17,18,19,20,41 ……………. 13 x 150R
    • R8,29 …………………………………………………………….. 2 x 100K
    • R9,28,40………………………………………………………… 3 x 10K
    • R11, R12 ………………………………………………………. 2 x 330R
    • R13,14,15,39 ……………………………………………….. 4 x 100R
    • R21 …………………………………………………………………1 x 220K
    • R23, R30 ………………………………………………………. 2 x 33K
    • R24 ………………………………………………………………… 1 x 68K
    • R26,27 …………………………………………………………… 2 x 15K
    • R31 ……………………………………………………………….. 1 x 4K7
    • R32,33,34,35,36,37,38 ……………………………….. 7 x 10R
  • Capacitors
    • C1,2,3,4,15,16,17,18,19,27…………………………. 10 x 47uF/16V Electrolytic
    • C5,7,10,11,21,22,23,24,25,26 ……………………. 10 x 100nF Mylar or 1uF Multilayer Ceramic
    • C6,9 ……………………………………………………………….. 2 x 4u7/16V Electrolytic
    • C20 …………………………………………………………………. 1 x 22nF Mylar
    • C8,14 ……………………………………………………………… 2 x 1nF Mylar
    • C12 …………………………………………………………………. 1 x 8n2 Mylar
    • C13 …………………………………………………………………. 1 x 470pF Mylar
  • Transistors
    • Q1,2,3 ……………………………………………………………. 3 x 2N3904
  • Diodes
    • D19 ………………………………………………………………… 1 x 1N4002
    • D20,21 …………………………………………………………… 2 x 1N4148
    • D1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 ……. 18 x LED (3 mm RED)
  • Integrated Circuits (IC)
    • U1 ………………………………………………………………….. 1 x LM324
    • U2 ………………………………………………………………….. 1 x LM358
    • U3 ………………………………………………………………….. 1 x 555
    • U4 ………………………………………………………………….. 1 x 7805
    • U5,6 ……………………………………………………………….. 2 x 741
  • Switches
    • S1,2,3 …………………………………………………………….. 3 x SPDT Toggle Switch
    • S4 ……………………………………………………… Rotary Encoder with Push Button
    • CHECK ………………………………………………………….. 1 x 3PDT Stomp Switch
  • Connectors
    • IN …………………………………………………………………….. 1 x Mono audio plug port
    • OUT ………………………………………………………………… 1 x Mono audio plug port
    • Serial Communication ………………………….. 1 x Stereo audio (3.5mm) plug port
    • POWER ………………………………………………………….. 1 x DC power plug port
    • CPU/Core board …………………………………………….. Blue-pill board compatible female header

Input/Output Conversion Depth

The audio input/output conversion is done using the built-in ADC and PWM generator. Using multichannel expansion oversampling, its 8 channel reading of 12-bit ADC should theoretically result in 15-bit final resolution for the input conversion. The output conversion is done using cascaded 8-bit PWM with automatic calibration to produce 16-bit resolution with no missing-steps nor over-steps error.

Power Supply

The circuit requires 9-12V DC power supply, and the required current wold be around 100 mA, so it is recommended to use at least 300 mA rated power supply adapter. The 555 IC is employed as the negative voltage converter to supply the operational amplifier since this chip is very cheap and widely available. Unlike other effect pedal that has switch-crafted  audio input plug socket, this pedal has a power-on toggle switch as the most convenient way to do power-on reset when dealing with the programming operation. As a power safety feature, this circuit employs a polarity protection diode to prevent any damaged by incorrect power supply polarity.

Intuitive User Interface

The main operational interface is one rotary (cw-ccw pulse) encoder with push-button function and two rows of LEDs (6 LEDs for control indicator and 10 LED for parameter value or signal indicator display). When the platform runs a single effect application, then up-to 6 parameters can be assigned directly to the 6-LED control indicator, and the value can be displayed by the 10-LED signal/value indicator When the platform runs a multi-effect application, then the last two LEDs (5th-LED and 6th-LED) of the control indicator will be assigned to the general control: preset and effect module. Only 4 effect parameters  (of the active/selected module) would be connected to the first 4-controls (which is associated with the first 4-LEDs)

By combining single-LED and double-LED with continuous and blinking lights, the signal/value indicator can intuitively display any value  from 0 to 9 in 10 steps, 19 steps, or 38 steps.

  • 10-Step Mode, one active LED will directly show the value (equal to its position)
  • 19-Step Mode, one or two LEDs will be activated to show a value from 0.0 to 9.0 in 19 steps. For example:
    • 3.0 = LED3 (continuous)
    • 3.5 = LED3 (continuous) + LED4 (continuous)
    • 4.0 = LED4 (continuous)
  • 38-Step Mode, a combination of one active LED or two active LEDs,  with continuous or blinking lights, will indicate any a value from 0.00 to 9.00 in 38 steps. For example:
    • 1.00 = LED1 (continuous)
    • 1.25 = LED1 (continuous) + LED2 (blinking)
    • 1.50 = LED1 (continuous) + LED2 (continuous)
    • 1.75 = LED1 (blinking) + LED2 (continuous)
    • 2.00 = LED2 (continuous)
  • Semitone display mode is also supported, and it would show any notes from C to B from the 1st to the 3rd octaves. A chromatic notes will be indicated by two activated LEDs, such as C#, will be indicated by LED0 (continuous) + LED1 (continuous). While the note indication is shown by one or two continuous light LED (of D0-D6), the octave position will be indicated by one blinking LED of LED7,LED8, and LED9.