Erica Synths Modulator II - DIY Build

Completed Front - Modulator

Following our last build of the Erica Synths Mixer II module, we decided to go back to back by taking on the Erica Synths Modulator II. This DIY module is essentially an LFO with simultaneous triangle and square wave outputs. It is a clockable S&H module with external CV input and noise source. It is also one of the new kits currently being offered by the gang over at Synthrotek.

Apart of the DIY series from Erica Synths, the Modulator II is unique in that it not only has one noise source, but two. Noise sources come in the form of “backpack” PCBs that one can swap for different tones of noise. You have the choice of transistor-based or Zener diode-based noise generator for massive, full spectrum white noise.

As you start this build, it can easily feel overwhelming upon cracking open the kit being that there are multiple PCBs and a bunch of components. Synthrotek did a great job on this kit by providing a clear BOM which we have scanned and linked below. They also separated all of the components for each backpack PCB so that the parts could be located quickly and partnered with the correct board.

IMPORTANT: Before we jump into the build, take note that this is not a build for novice builders. This is due to the very tight soldering pads, poor layout of the silkscreen and one’s ability to read a BOM. You do not want to go into this build not knowing how to read or build from a BOM, being that there is little to no step by step assembly instructions.

The Erica Synths assembly guide is really generalized compared to most DIY build guides we have seen from other companies (but hey, thats why we are writing this up and recording our videos).

Let's Get On With The Builds... (plural)

Transistor Based Noise Board:

PCB 1 of 2, this board is the smaller of the two. If you read the silk screen you will see that it states “Noise” on it. This is the board we will be building first. First start with the ferrite beads and resistors, and do not worry about the orientation of these components as their polarity does not matter. 

Builders Note: Things are going to get very tight, keep in mind that your iron can damage other components if you keep in one place too long!

Once this has been completed, pop in the IC socket, the single turn trim pot and capacitors. Take note that the only capacitor that you have to be aware of orientation is the electrolytic. Make sure line up the white stripe on the part with the screen print. After these are complete, populate the headers on the back of the board, and the voltage regulator on the front (make sure you match the layout on the silk screen).  Set this board aside and grab the Acid board. 

Zener Diode-Based ACID Board: 

With the ACID board, everything is the same except for the number of components and how the resistors mount. They are vertical or stand-up mounted to allow for more parts to fit on the board. This is where you have to be very careful on reading the board and the BOM. The stand-up resistors can be confusing on where they mount since there are a few other holes on the board (all over the board). One way to make sure you did everything correctly it so not populate anything in the holes labeled “TPx” (x being a number). 

Electrolytic capacitors need to mount to match the PCB layout and the ones not labeled with a white stripe are mounted with the long cap leg through the square hole. This backpack board gets very tight, so make sure you are flipping and clipping leads to allow more access to the pads. We used a combo of top down to hold parts in place and clean up soldering on the backside of the board where it was too tight to come at the part from the top. After you complete this board, it is time to move onto the main board. 

Erica Synths Modulator II - Time Lapse Build

Play Video
fin Divider

Modulator II Main PCB

Twenty eight resistors (including one stand-up) and 17 diodes start off this build. The orientation of the diodes is important just like before or it will render the module useless. Follow these up with the ferrite beads since you can easily access these via top down soldering. When mounting the resistors, it is pretty easy to see where they go.

We used our resistor tester to ensure we were using the correct ones (just to be on safe side). Synthrotek did an amazing job sorting parts and lining them all up on the tape, but sometimes things get shifted in packaging. It is never bad to double check before placing. Go ahead and tack down the IC socket why you are here. 

Caps and more caps…

Next up are the ceramic capacitors and electrolytic capacitors. The ceramic ones are quick and easy and their orientation does not matter. Simply plug and tack. As for the electrolytic, these mount on the backside of the PCB along with the power header. Go ahead and place and tack all of these parts down. Mind the orientation and double check the power header orientation before placing it. This can fry the module if you mount it incorrectly. Check out our completed rear photo if you have questions about which way to mount it. Why you are on the backside of the board, go ahead and tack down the stand-offs for the backpack PCBs. 

Fuses, voltage regulators and the Alpha IC (the weird circle thing)

The fuses are quick and easy and the orientation doesn’t matter. Just place and solder. Voltage regulators on the other hand need to match the layout of the holes and silkscreen on the PCB. We recommend flipping the board over and soldering and clipping one leg at a time on the voltage regulators being careful not to keep the iron on the leg for too long. This is due to the closeness of the legs and the ability to fry the part with excessive heat. When this is complete, it is time to move onto the weird circle thing…. ie. the Alpha IC. 

The Alpha IC or Jellyfish

The Alpha IC is the weird round thing with all the legs hanging off of it. It kinda looks like a metal jellyfish. This thing is tricky, so be patient with it. You can start by carefully bending out the legs to make a “bigger” circle of legs to match that of the PCB as best you can. Slowly place one leg at a time in a circular motion around the part. We found that clipping a few of the legs shorter as we were placing the part made it easier to mount.  When you complete the tedious task of mounting the part, flip it over and tack it down. 

The LED, pot and jacks…

Upon completing the tiny parts, it is now time to conquer the bigger ones. The pot mounts quick and easy and as for the jacks, there is only one way they can mount. Just follow the silkscreen and line everything up. 

Builders Tip: Tacking down the ground pin on each jack holds them in place before you place the front panel on. 

As for the LED, you do not want to solder it down until the front panel has been placed and everything has been lined up. Orientation is important here too. Make sure you line it up with the PCB. The bulb of the LED will be pushed through the front panel and then tacked down to keep it in place. Go ahead do this and pat yourself on the back … you are almost done.

The IC and knob...

Last but not least is the IC and the knob. Remove the panel and locate the IC socket. The main distinguishing feature of the TL074 Op-Amp is that they incorporate high-voltage JFET and bipolar transistors which helps the transistor to have very high input impedance and low bias current. This is exactly what the module calls for as do most LFOs. We used our leg bending tool to line up the legs on the IC to match up the socket. You don’t have to use one of these tools, but we think they are kinda cool and simple to use. 

Reinstall the front panel and grab your handy dandy Synth DIY toolset to tighten down the nuts on the jacks and pot. Once this is complete, turn the pot all the way counter-clockwise until it no longer turns. This will be your line up point with the pointer on the knob. Using a small flathead screwdriver tighten the pin on the knob and call it done. 

Erica Synths Modulator II - Completed Module

Features and Specifications


  • LFO output amplitude: -5v/+5v
  • LFO frequency range: 0.1 – 20Hz (configurable)
  • Noise output level: 10Vptp
  • S&H CV input level: up to 20Vptp
  • S&H circuit based on high-quality 1100CK2 S&H chip
  • S&H CV and clock inputs


  • Width: 8HP
  • Depth: 35mm
  • Current draw:
    • +12V: 27mA
    • -12V: 22mA
fin Divider

Erica Synths Modulator II - Purchasing Options

Synthrotek offers a several options when it comes to purchasing the Erica Synths Modulator II module. Their prices are excellent and the quality of this build is way worth it! If you are interested in buying the kit, the assembled module or just the PCB/Panel/IC, click on the images below and you will be redirected to their site to purchase! Make sure and tell the gang at SR that we sent you!

Testing and choosing the right backpack for you.

Now comes the part where you can choose which backpack PCB to mount to the completed module. We chose the ACID board due to its complex levels of noise. These boards can be switched at any point as long as you turn off the power and remove the main module from the power source. 

If you are unsure of what type of sound you want your module to produce, check out this quick video that explains the difference between the two. We used our Synthrotek TST module to test the power of the module. Since there was no smoke or explosions, we knew it was good to go.

Our Final Thoughts . . .

Sadly, we haven’t had a lot of time to play with the module as of this build write up, but we can say that we are partial to the ACID board over the other transistor based noise board. It is due partially to the wider spectrum of sound that board produces. Being that this is a clockable module, we are anxious to see what it can do with regard to drums and noise filtering.

The build was an advanced hole-through build that requires a very steady hand due to the tight fit of the components. If you feel like you might struggle with the build, pick up the fully assembled module, either way, several purchasing options are available. Go get one today!

Comments or Suggestions?

If you have questions about the build process or suggestions/feedback about this article, feel free to let us know by contacting us  We look forward to hearing from you!

Until the next build … 

~ f i N

fin Divider