UralTone Eurorack Series - Tube Sounding Micro Mixer build guide

We received inquiries from the first Micro Mixer builders about the suitability of the mixer for a Eurorack case. This challenge was easy for us to take on, leading to slight modifications in the design. From the beginning, the mixer's circuit board included the option to use a three-position switch for the clip function, allowing routing to the aux output as an alternative to clipping. In the original mixer, designed to fit into an effects pedal enclosure, these expansions were left optional to favor battery operation.

Due to the Eurorack format, the front panel has more space, and battery operation is no longer necessary. The increased height has been utilized by adding 6.3mm jacks with switches for inputs and 6.3mm stereo jacks for output. This makes it easy to use the mixer with devices that utilize either connector type. The same input linking system remains. The space previously occupied by the battery now features a stereo output (A+B), a mix-bus input (for chaining mixers), an AUX output, and a B-output selection (B-BUS / AUX). The power switch and DC jack can also function as a 9V output for an external guitar effect or similar device.

The core circuitry and philosophy remain unchanged: "This product was designed entirely for our own needs. Working simultaneously with different sound sources, synths, loopers, and guitars requires a mixer. A full-sized mixing console is often overkill. The dream was a device that sounds good, without needing to be Hi-Fi. We built the mixer around a well-proven MOSFET circuit. The sound and simplicity are familiar from the tube world." We also added a few clever features:

Clipping: Each channel has its own (soft) Clipper switch, allowing distortion to kick in significantly earlier. Before distortion, compression begins, meaning the switch can also be used to achieve tape-like compression. 
Insert: A must-have feature when working with different effects. The insert point allows you to route any studio or guitar effect into the signal chain. The insert jack can also function as a channel-specific output (e.g., for an audio interface). 
Linking: Channels are internally linked so that if only channel 1 is used, the remaining channels are connected in parallel. If only channels 1 and 2 are used, then channels 2, 3, and 4 are linked in parallel, and so on. This enables parallel effects on the same track, such as blending distortion and a clean signal on a bass track.
Sound: This is not meant to be a Hi-Fi device but rather a great-sounding audio tool for both electronic music and guitarists. Each track can be compressed or distorted smoothly as needed.
AUX: Instead of clipping, the signal can be routed (pre-fader) to a dedicated AUX output.

Once again: The UralTone tube-sounding mixer delivers an exceptionally rich tone. It is not a transparent standard mixer but is designed to add compression and saturation!

Width: 22HP (111.4mm)
Power consumption: +12V, 40mA (without power output)
Input: 0 - 15Vpp
Output level: 4Vpp (max 6Vpp with distortion)
Frequency response: 20 - 20kHz (-0.4dB to -0.7dB)

Tips on soldering
How to identify components
Bill of materials

Follow the assembly order shown in the images below. We strive to keep the component models the same, but the component market changes rapidly, so we sometimes have to switch manufacturers or series due to availability issues. If any component looks slightly different in color, don’t worry—each component has its required values marked on its body.

Start by soldering the resistors onto the board. Resistors R26 and R36 are not labeled on the PCB, but they are located in their respective channels in the same logical positions as R16 and R46.

Solder the IC socket onto the board.

Solder the ceramic and plastic capacitors, as well as the diodes, onto the board.

Solder the electrolytic capacitors and the transistor onto the board. Pay attention to the installation direction of the electrolytic capacitors: the board is marked with the capacitors' + terminal.
C10 has been printed twice on the board. The one under the regulator is actually C9.

Solder the potentiometers onto the board. Use the front panel as an alignment aid.

Solder the jacks onto the board. Again, use the front panel as an alignment aid.

Solder the switches onto the board in the same way.

Cut off the regulator’s mounting tab and solder it onto the board. Install the IC into its socket.

Solder the IDC connector onto the back of the board.

Insert the LEDs into place, but do not solder them yet. Slightly bend the LED leads to prevent them from falling out. Adjust the switch nuts to the same height as the jack housings so that the front panel sits evenly. Secure the front panel using the jack and switch nuts. Push the LEDs through the front panel holes and solder them in place.

Attach the 6.3mm jacks to the front panel. Mono jacks go to the inputs, and stereo jacks go to the outputs. Solder the wires from the jacks to the JACK solder pads on the PCB as shown in the image. If you want to remove channel linking, leave the red wire from the inputs unconnected.

Finally, prepare the power cable. Crimp 10- and 16-pin connectors to the ends of the flat cable using the appropriate crimping tool, a screw press, or pliers. The position of the red stripe is marked on one side of the connector with a small triangle. Fold the cable over the connector and push the strain relief into place.

Measure the input (pin 1) and output (pin 3) of the IC2 regulator with an ohmmeter relative to ground. If the meter reads zero or a short circuit, check the wiring. Once everything looks good, mount the device in the rack and give yourself a pat on the back for a job well done!