Synthrotek Dual Low Pass Gate (LPG) - DIY Build

SR LPG - Completed Module

Lately, we have been on a kick with the slider series from Synthrotek and for this build, we took on the Synthrotek Dual Low Pass Gate (LPG)! If you were to ask any modular artist if they had a LPG in their rack, 9 out of 10 artists would answer with a yes. It is a staple in modular racks and this is especially so with West Coast Synthesis. Synthrotek kicks it up a notch by giving us not one, but two low pass gates in one 4hp module.

 Another badass thing about this module is that when you are not using at an LPG, you can use it as a VCA! (one can’t have enough VCAs!) When in VCA mode, it bypasses the low pass filter and resonance leaving color out of your audio input. 

The Build...

Before we jump right into the build, take a minute to deconstruct the kit itself. Synthrotek does and always has offered outstanding documentation and reference material on their website. They have also included a visual BOM with the kit to help move the process along.

Being that the Dual Low Pass Gate is apart of the slider series of modules released by Synthrotek, this module would sport the same beautiful slider pots and sleek black panel. We recommend that you have some soldering skills prior to taking on this build. Even though its not really an advanced build, it does require a very steady hand and clean iron (also the board mating can get tricky if you jump the gun).

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Synthrotek Dual LPG ... The Logic Board

Start by mounting the diodes. These are polarized components and if you place these in the incorrect position, you can total your module. Make sure the black line on the diode lines up with the silk screen provided. You can use top down soldering to make tacking these down a cinch. After the diodes comes the resistors which are not polarized, meaning you can place them in any direction you choose.

In this build the resistors are mounted standing up, so simply bend one leg of the resistor and feed it through the PCB. The other leg of the resistor will remain straight and slide right into place. You can solder the exposed leg that you bent over to keep everything in place, and then flip the board over to complete the soldering of the resistors.

Synthrotek Dual LPG - Time Lapse Build

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Capacitors and Voltage Regulator - Logic Board

As you finish up the resistors and diodes, you will move onto the voltage regulator and capacitors. The small ceramic/film capacitors are not polarized so you can place these according to the BOM.

Electrolytic capacitors are polarized and it is important that you pay attention to their orientation. The long leg of the cap goes through the hole on the PCB that has the ‘+’ sign next to it. Once these are placed, flip the board over and tack everything down.

The voltage regulator is next and its a quick 1, 2, 3 and you are done. Like other polarized components, the orientation matters on these. Also, you want to make sure and not leave your iron on this part for very long. Excessive heat on voltage regulators can cause them to malfunction.  Once you complete this step, it is time to move onto the ICs and power header. 

Power Header and IC Sockets - Logic Board

Next up are the IC sockets and power header. Take care when populating these to not bend any of the legs on the IC sockets. Line up the notch at the end of the socket with the notch on the silkscreen. This notch indicates where pin 1 of the IC is. When you have them populated, carefully flip your project over and solder everything in place.  As for the power header, just match its orientation with the silkscreen, flip the board over and solder everything down. 

Note: DO NOT place the ICs yet!

Builders Trick: A trick to getting these nice and flat is to only solder one or two pins of each socket, and then reflow the solder joint while gently pushing on the top of the socket to seat it flat against the PCB. Then continue on and solder the rest of the pins.

Trimmers and Vactrols - Logic Board

The location of the trimmer pots is simple to find. There is only one way that these things can be mounted. Simply push them into their slots on the pcb and flip the board over to solder them in place. 

The vactrols are the big round black components with 4 legs coming out of them. This thing is made up a LDR and an LED. The polarity is important here so make sure you line up the dot on the vactrol with the dot on the PCB. You will notice a hole in between the two mounting points; ignore this. Place both vactrols and solder from the top down. Careful not to leave the iron in one spot for too long as well. It is time to pat yourself on the back and set the logic board aside.

Synthrotek Dual LPG Build Gallery

The Control Board…

Synthrotek’s Dual Low Pass Gate is apart of the their slider series of modules. All of these modules use slider pots for control and this goes without saying that they look great too. These modules also consist of a 90 degree header pin that has to be mounted to both boards once the module is complete.

We are now going to start building the control board. Start with the 5×2 right angle board to board headers and make sure when you populate these that the side of the header without the bend in it goes through the control board. Once these are seated nice and flat against the board, ensure that everything will fit behind the front panel, (90 degree angle to the control board).

Keeping Things Aligned... The Control Board

Solder one of the middle pins on the header and then reflow/reseat the header afterwards. This makes the header sit where it should. Pay special attention to the amount of pressure you place on these pins because you could unwillingly push one of the pins out of place and cause alignment issues. After completing the initial seating, knock out the rest of pins and flip the board over to clip off the excess pin.

Builders Tip: Reflow after clipping the pins. By doing this, it not only makes the solder joint look better but it ensures that the solder is where it needs to be. 

Jacks and Slider Pots … Control Board.

First populate the aluminum hex standoff. You can do so by inserting a metal-finish 2.5mm screw through the bottom of the board, and then spinning the hex standoff a little to get it started. Then use a Phillips head screwdriver to gently tighten the assembly together. Place the jacks onto the PCB and do not solder all of the pins just yet. You can hold things in place by tacking down the ground pins on the jacks prior to panel placement. 

Slider pots are next up…  and take your time with these. Line them up with the PCB and then flip the board over to bend a pin on each of the pots to help keep things in place.

Switches … Control Board

Switches are quick and easy and they do not have a correct orientation. Pop these into place and place the panel over the control board components. Hand tighten the jack nuts to help keep everything where it should be. You can also go ahead and insert the socket head screw and gently tighten it down. This will also help hold things in place as you flip the board over to solder all of the pins. 

Final Board Mating…. Almost done!!

When mating the two boards, take care to make sure that everything lines up at a 90 degree angle. Use the same trick as before with the headers on the control board to make sure that the logic board is sitting as flush to the bent part of the headers, and is coming straight off the control board.

Solder just one pin of the header, and then re-heat and straighten until you are satisfied with how it looks. Once everything is lined up, solder the rest of the pins and pat yourself on the back for completing the Synthrotek LPG!

Synthrotek Dual LPG - Completed Module

Features and Specifications


  • Two channels
  • CV over resonance
  • VCA and mixer function
  • Input 1 normals to input 2


  • Width: 4HP
  • Depth: 57mm
  • Current draw:
    • +12V: 45mA
    • -12V: 45mA
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Synthrotek Dual LPG - Purchasing Options

Synthrotek has a couple different options when it comes to purchasing the Dual Low Pass Gate module. The 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!

LPG - Assembled
Assembled Module
LPG - PCB:Panel
Panel/PCB/IC Combo
White Synthrotek Logo

Synthrotek Dual LPG - Documentation

Our Final Thoughts . . .

Upon finishing up the build of the Synthrotek Dual Low Pass Gate module, we quickly calibrated the module with our Synthrotek TST module. Once the testing was complete, we popped the module in our performance case and began patching it up.

The fact that this module is a dual LPG as well as a VCA, gives it an added bonus. Since the LPG is vactrol based, the module creates naturally sounding decays. VCA mode bypasses the low pass filter and resonance and doesn’t color the sound of your input audio which allows for a clear and sharp output.

The aesthetics of the module look sharp and the illuminated sliders really bring life to the the module. This LPG will definitely be a staple in our our performance rack due to its versatility and dual function. This one is a no brainer… head over to Synthrotek and pick one up for your rack!

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

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