Synthrotek XFD - DIY Builds

The gang over at Synthrotek have done it again with another crossfader module, only this time it can work with video as well. According to SR, “the XFD is a CV-controlled vactrol-based crossfader module, ideal for both audio and bipolar CV sources.

It can be used as a cv-controlled attenuator: connecting just one input allows the signal to quickly fade in a VCA-like fashion.” For our build, we chose the audio version of the module whereas both are currently available in the links below. 

The Build - Let's get started with diodes....

The XFD module is a small module at only 3HP, it fits in perfectly with the Synthrotek black slider series modules. The slider on the module does illuminate when signals are passed through the module itself. Black finish sets the modules appearance, just like its slider kin. 

Diodes are up first. Following the BOM, insert all the diodes into their proper spots, paying attention to the polarity. Diodes are polarized components, and must be populated correctly to function. Make sure to align the cathode stripe on the diode with the cathode marking on the silkscreen. Then you can flip your project over onto a flat surface and solder all the diodes in place. Then clip the excess leads

Pin, Power Headers and IC Sockets

Next up place the pin headers onto the logic board. These will want to be lined up so that when we line up the PCBs, everything is in its right place. While we are here we will install the IC sockets as well. When populating them, make sure to align the semi-circular notch in the socket with the same looking notch in the silkscreen. Once you have them in there, an easy way to get them flat is to only solder one leg of each. Then you can go back and reheat the solder joint and gently apply pressure to sit them flat and call it done. Go ahead and place the power header while you are here too. 

Synthrotek XFD - Time Lapse Build

Electrolytic Caps

Electrolytic capacitors are polarized, so make sure when you are placing them that you get the longer leg in the square hole, closer to the + symbol. Once in, flip your project over, solder and clip any excess leads.

Resistors and Transistors

Polarity matters with transistors so be careful to line things up with the PCB silk screen. Also double check your BOM here as some things could get tossed around in shipping. The resistors do not matter with orientation besides they are standing up. Double check with a multimeter if you believe anything is out of order and need to be sure. 

Ceramic Capacitors

ATTENTION! The XFD can be built with high-speed operational amplifiers if you care to use it with with video signals.  If you bought the high-speed op-amp kit then you will need to populate all of the ceramic capacitors (comes with kit).  If you purchased the normal audio version, just populate the two ceramic capacitors as shown below.  Place, turn over to solder then clip excess leads.

Next, place the trim pot into the PCB then carefully turn over to solder in place.   Then place the two vactrols in place as shown below by aligning the small “+” mark on the Vactrol with the “+” on the PCB.  These will only function if oriented properly!

IC chips

Insert the ICs into their sockets. Following the BOM, insert each IC into its respective socket, making sure to align the notches in the ICs with the same notches in BOTH the sockets and the silkscreen.

Synthrotek XFD - Build Gallery

Control Board Sockets

Take the two header pin sockets and place them into the control PCB. Make sure they are perfectly vertical, then carefully turn over and solder in place. One way to do this is to plug the the sockets directly into the logic board headers, then place the PCB on top and solder.

Jacks and the Slide Pots

Simply place the slider in the correct orientation and flip the board over to solder it into place. As for the jacks, carefully place the jacks into the PCB lining everything up where it needs to be.  You can use the panel to help align them properly.  Turn over and solder in place.

Standoffs and Panel

Gently, place and screw down the two metal standoffs onto the PCB. The 10mm standoff faces upward, connecting to the panel. The 11mm standoff faces downward, connecting the two PCBs. Place the panel onto the control board and add the jack nuts and black screw onto the header.

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Where do you get all of these wonderful toys?

Synthrotek has a couple different options when it comes to purchasing the XFD module. The prices is excellent and the quality of this build is way worth it!

If you are interested in buying the kit or the assembled module, click on the links below and you will be redirected to their site to purchase! Make sure and tell the gang at SR that we sent you!

Main Control Board Connection

Connect the main and control boards together by aligning the header pins and sockets and then screw in the screw into the standoff.


  • Move the slide pot to the the center of the throw (at the detent). If you look at the sides of the vactrol, you can see how bright the internal LEDs are. Turn the trim pot on the back until both LEDs are approximately the same brightness.
  • Patch an audio signal into the UP jack, then slide the pot all the way down. The pot should go from completely cut out to full signal when you slide it upwards.
  • Now move the audio signal to the DOWN jack. The pot should go from completely cut out to full signal when you slide it downward.
  • If this isn’t the case, adjust the rear trimmer until both the UP and DOWN inputs completely cut out properly.

Our Final Thoughts

What can we say, the XFD is a fader. It fades signals and is able to process either audio or video synthesis. This is really cool in that you could technically have several of these lined up and really put on a hell of a show with audio or video. Maybe we should pick up another or five?

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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