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Midikraken hardware

This page describes the hardware itself. A build manual is available here.

Midikraken boards are meant to be stacked using M3 spacers and screws. In such a setup, one "master" exists that holds the microcontroller with the USB port, and zero or multiple "slaves" are daisy-chained with their upstream ("from master") or downstream ("to slave") ports.

Photo of Midikraken

The photo shows an (almost) fully equipped Midikraken including:

  • 1 DIN connector board (the large MIDI connectors).
  • 1 TRS connector board (MIDI via headphone jacks).
  • The main microcontroller
  • The UI consisting of a display and a rotary encoder.

Midikraken boards can be stacked almost arbitrarily; the only limit is (currently) a maximum of 16 MIDI in/out port pairs.

Two board types are available:

  • The DIN board with four DIN MIDI inputs and four outputs
  • and the TRS board with eight TRS inputs and eight outputs.

DIN boards may or may not house the main microcontroller, so every stack needs one. Note that the DIN board can be used to house only the microcontroller, if desired. No actual MIDI ports need to be equipped, making a TRS-only stack possible.

The TRS board supports both TRS-MIDI-A and -B (where roles of tip and ring are swapped).

Note that while it would be possible to design boards with only inputs or only outputs, the current boards do not support switching an input port to output mode or vice versa. If desired, however, inputs can be left unmounted; in this case, the appropriate LEDs and opto-couplers can be left out, too.

Board variants

Both boards can be built in different variants. For the DIN board, the options include:

  • It can either house the microcontroller (A1) or act as a slave *(A2).
  • It can hold angled MIDI connectors (B1, the default), or not have any connectors (B2). (This is useful when used as user interface board only). (Additionally, there is a somewhat hacky way to mount upward-facing MIDI connectors to it for use in Eurorack setups, but that's dirty and untested.)
  • If the board is configured as a master, it can or can not have a user interface (C) consisting of a display and a rotary knob with a push button.
  • If the board holds a microcontroller, it can support chaining more boards to it (D1), or save some components at the cost of losing the chaining ability (D2). (not recommended).
  • In this case, MIDI can be driven with 5V (E1) or with 3.3V (E2, saves another chip, not recommended).
  • As a the buffer U1, a 74HCT245 (F1) or 74HCT541 (F2) can be used.

For the TRS board, two options are available:

  • Either 8 in/out port pairs are mounted (X1)
  • or only one side is equipped with only 4 in/out pairs (X2)
  • Angled TRS jacks (PJ-324) (Y1) or upright TRS jacks (Y2) can be used.

The following components need or need not be mounted on the PCB:

  • (A1): mount A1, J1, U1; leave J3 unmounted.
  • (A2): mount J3, leave A1, J1, U1 unmounted.
  • (B1): mount J10x and J20x, U101, U102, U2, U3 and Dxxx.
  • (B2): do not mount the components from (B1), instead run a copper wire across TODO.
  • (C): mount or do not mount J8 and SW1.
  • (D2): Close JP1-4, and close JP5-8 according to your selection in (E).
  • (E2): Do not equip U1.
  • (F): Configure JP9 accordingly.
  • (X2): Close JP102 and do not mount Jx05-Jx08, and all LEDs and resistors on this side. Also do not mount U205-U208.
  • (Y): Solder in what is needed, the PCB can hold both.

Note: While this sounds complicated, in the recommended default configuration which is reflected by the BOM and schematics, only JP9 needs to be closed to select the IC type used in U1. All other solder bridges are left open and no deviations from the schematic need to be made, except for (A).

Currently, two different hardware revisions exist. They are described here from most recent to oldest.

Revision 01

Bill of material

din5_pcb/bom.html, interactive BOM

trs_pcb/bom.html, interactive BOM

Errata

During powerup, the MIDI ports send garbage bits. To fix this, you need to disconnect the 74HC595s' output enable pins from GND using a sharp knife (and some wire to reconnect all GNDs that should stay connected), and connecting them like this instead:

+5V --- Capacitor 10µF --- OE pin --- Resistor 10k --- GND

This delays the output enable enough such that the shift registers can be filled with proper data after the microcontroller has booted up.

The exact fixing procedure is described in this document.

Eurorack compatibility

Please note that Midikraken was designed as a standalone adapter box. However, where possible, efforts were made to make Midikraken as Eurorack compatible as possible:

  • The PCBs are 110mm in height, fitting tightly into any Eurorack setup.
  • The boards are prepared for upright connectors. This is sort of hacky, but it might work. The TRS board can hold PJ392 sockets that are usually meant to have wires soldered to their legs. The DIN board can hold DIN sockets similar to the NYS325, with a hack: The outermost two solder tail must be removed, and the remaining three must be bent so that they fit into the PCB.
  • Alternatively, any socket can be used and wires can be soldered to the PCB.

Enclosure

Midikraken can either be built half-open as shown in the photo above, or can be protected from dust and metallic parts with a 3d-printed enclosure. The enclosures reflect Midikraken's modular nature and itself can be stacked onto each other. Currently, three enclosure modules exist:

  • The display shell which is put on top of the DIN5 PCB and holds the display. It terminates the Midikraken stack to the upper side.
  • The DIN5 shell which is put on the opposite side of the DIN5 PCB and covers the MIDI ports, leaving cutouts for the LEDs and the USB port.
  • The TRS shell which covers the ports of the TRS PCB and terminates the Midikraken stack.
  • Additionally, the display top plate and back plate are alternatives to the display shell and should be used in the half-open, caseless configuration to support the display and protect the circuitry.

The enclosure light guides that cover the LED holes and make it easier to see the LEDs from an angle. These guides are to be printed with transparent material and are snapped in from the inside. (DIN5 needs 4 of these and TRS needs 8 of these)

Yet to be done:

  • A terminating DIN5 shell with a bottom plate for builds without a TRS shell.
  • A non-terminating TRS shell for builds that use more than one TRS board. Note that currently, the firmware only supports a maximum of 16 MIDI ports, which rules out a two-TRS-board-solution for now.

Ghettokraken on perfboard

Schematics for the first prototype board were created with KiCAD and are located in perfboard.

A 3D-printed enclosure (which needs some adjustments to fit well) was designed in FreeCAD and resides in perfboard/cad/. The enclose provides the DIN ports with additional stability which the usual perfboards lack.

Note: You need to use the firmware from 7072a0c6eb or earlier due different pin assignment; or you reassign the pins.

This roughly resembles the (D2, E2) configuration