Rat King Modular TONE - Eurorack Modular Build

Tone DIY Kit

For our first build of September, we were honored to receive one of the first Rat King Modular TONE kits from our friends over at Synthrotek

A relative newcomer to the Eurorack Modular scene, the Rat King is quickly proving to be a master of modular synths. Being the third module in the series, we have to say that the TONE module pairs well with its brethren kin; the EVENT and the Random SEQ

So, what makes the Rat King Modular TONE different from other VCOs?  Well, besides the beautiful mustard gold PCB and the matte black panel with gold laced graphics, the TONE uses the Alfa AS3340 chip (a CEM clone), made famous by its outstanding quality.  It is hard to go without saying that this VCO has some grit and some warm sounds that are trackable over an entire 8 octaves. 

Let’s get onto the build … All Hail The Rat King! 

Time Lapse and Build Notes

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Contents may include...

Like the other previous Rat King Modular builds, the TONE instantly had us excited to break out the trusty iron. PCBs of these kits are gorgeous and the quality of workmanship is outstanding. Solder just grips the board like it was made to go where it needs to.

The TONE kit came with a printed BOM and everything you needed to begin the build. Assembly instructions and an online BOM were also available via the Rat Kings website

Note: At the time of this build, the BOM was missing labels for the transistors/voltage regulators. Here are the correct placement locations for the transistors provided by the Rat King himself.

Transistor/Voltage Regulator PCB Placement Guide:

– LM78L05 = U3 on the PCB

– L78L12 = U4 on the PCB 

– L79L12 = U5 on the PCB

Another Important Thing...

It is easy to miss a very important step in the build … one of the 1M 1% Resistors (there are 2 of them) needs to be soldered on the back side of PCB prior to installing the 3340 IC socket. The other 1M 1% will be soldered into the R9 position on the board.  You can see the placement of the resistor in the build gallery below. (its the small blue resistor on the back of the board)

The build continues...

Following the BOM, we were able to truck right though the build. As we mentioned previously, the golden colored PCB just soaks up the solder and top down soldering is recommended for both the resistors and the diodes. We also used top down soldering to mount the 1 of the legs of the jacks to help keep things in place as the board was flipped over. 

Before soldering the potentiometers, dry run panel mounting so that you can see if everything is lined up. It’s really important to ensure that everything is flush, including the HF and scale trimmers. Once everything is where it should be, flip the module over and solder the pots into place. Mount the black Davies clone 1900H knobs and onto calibration.

Upon completing the module, we had to go through the calibration process. This can be very confusing to the novice builder or enthusiast. In fact, it can almost make someone want to get out of the modular scene altogether. TONEs calibration procedure is listed on the Assembly instructions and they can seem daunting. Luckily, the guys over at Synthrotek put together the video below to help calibrate your new VCO. 


VCO Calibration Guide by Synthrotek

The chip makes a world of difference... TRUST US!

Features and Specifications

TONE Features:

  • Simultaneously available triangle, saw and pulse outputs
  • Frequency range: 5Hz to 28kHz
  • FM input
  • PWM input and attenuation control
  • 1V/o input, tracks 8 octaves
  • Hard sync

Power and Specs:

  • Width: 8HP
  • Depth: 22mm
  • Power draw:
    • +12V: 20mA
    • -12V: 19mA

Purchasing Information

Panel and PCB Combo

  • Black Matte Panel 
  • Mustard/Gold Dual Sided PCB
  • List Price: $25.00 USD
  • Click on the image to be redirected to the Synthrotek Store Page


  • Includes all components, PCB and Panel for assembly
  • List Price: $65.00 USD
  • Click on image to be redirected to the Synthrotek Store Page

Assembled TONE Module

  • Fully assembled and calibrated 
  • List Price: $115.00 USD
  • Click on image to be redirected to the Synthrotek Store Page

Rat King Modular TONE - The Final Word

Overall Rating:

After completing the build of this new RK Module, we have to say that it was a fun module to build. Its aesthetics and the warm 3340 tone of the chip really bring some warmth to our rack. Like the saying goes, “you can have enough VCOs” … and the Rat King Modular TONE is a must have. 

If you have questions or comments about the Rat King Modular TONE build, please feel free to reach out via our Contact page.

Interested in picking up the Rat King Modular TONE? Click on the links above to head on over to Synthrotek (it’s exclusive) and let them know that we sent ya!

Until the next build … 

~ f i N

Completed Front View