Synthrotek Crossfader (CF) and 2hp Buff Mult - DIY Builds

We all know that you can’t have enough VCA’s in a modular system. The gang over at Synthrotek recently released a new VCA module in their slider series and we had to pick one up. This kit is different from most of the other Synthrotek kits we have assembled in the past, in that it is an SMD kit. With that being said, this module and/or build is not for the novice builder. 

You will want to have a few hours of practice on SMD before jumping into this one. 

The slider series by Synthrotek looks great and along side the other slider modules, the VCA fits right in. As with most SMD builds, we generally use a digital microscope, but on this build we did not. We not only used our normal 60/40 solder but we also used solder paste on some of the build. Let’s get on with the build shall we?

Resistors and Diodes

There are only 3 diodes in this build and they can only be mounted one way. Make sure and follow the outline on the PCB for the correct orientation prior to tacking things down. As for the resistors, you will want to utilize your BOM as well as the markings on the component bags themselves. Resistor orientation doesn’t matter, but making sure you mount the part correctly does. 

Note: When soldering with SMD parts, an easy way to get things to line up is to place a dab of solder on one of the pads. Then heat up the solder dab while placing or moving the part into place. You will also want to use a fine tipped pair of tweezers for the build like we did. 

ICs and Capacitors

The ICs were mounted by using solder paste and the soldering iron. The trick to this is to lay a fine line of solder paste across the IC mounting pads. Then with your tweezers in one hand and your soldering iron in the other, place the part and heat the joint. This can be difficult and can take some practice, but as long as you run your iron across the soldering paste after setting the part, the component will sit right down into place with little to no cleanup needed. One thing you will want to watch out for here is solder bridges. If you happen to cause one, reheat the section and clean things up. Hence the reason why SMD can be difficult. 

Capacitors are like the resistors. Place a dab of solder on one of the mounting pads and then heat up the dab while gently placing the cap into place. It will make a clicking sound once it touches the PCB. Once you have completed one side of the cap, continue on by completing the other sides of the caps. Pat yourself on the back you have just completed all the SMD soldering and it is time to move onto hole-through. 

Synthrotek VCA - Time Lapse Build

Electrolytic Caps and Switches

After completing all the SMD caps and the ICs, we move onto the hole-through parts starting with the electrolytic capacitors. When placing the electrolytic caps, you will want to make sure you mind their orientation. The long leg on the cap will go through the whole marked with a positive or plus ‘+’ sign. Once you have set these parts into place, flip the PCB over and solder the caps into place. 

Exponential / Linear Switches are next and take little to no time to complete. Place these two parts and then make sure to trim the leads very close on the other side of the PCB. The leads should not be flush with the PCB, just sticking up a tiny bit. They need to be short enough that the slide pots can sit squarely on the board. 

Tip:  If you aren’t sure if the leads are low enough, put a slide pot on top without soldering, and see if it sits ok.

Trimmer Pots

Trim pots are up next and there are 5 of them. They only mount one way but you will want to be sure you place the correct value where it belongs. Utilize the BOM that comes with the kit to make sure you don’t get this wrong. When you are done soldering them into place, flip the board over and again, trim the leads VERY close to the underside. 

Synthrotek VCA - Build Gallery

The Power Header

Place the 10-pin power connector into the PCB by aligning the notch on the connector with the notch on the PCB silk screen. Carefully turn the project over and solder the pins in place. When complete, flip the PCB and trim the pins as close as you can to the PCB. 

Jacks and the Slide Pots

You will use the stand-off that is supplied with the kit to help keep the upper portion of the module together as you place the jacks into their appropriate places. As previously stated, any extra length on the legs of the parts that protrude through the slider section on the PCB need to be trimmed as closely to the PCB as you can.

Place the sliders into place and then temporarily place the front panel and secure it with nuts and the stand-off screw. Flip the module over and complete tacking down the rest of the unsoldered legs. Pat yourself on the back as you just completed the Synthrotek VCA Slider Series module. Time for calibration and patching… 

Where do you get these wonderful toys?

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Synthrotek has a couple different options when it comes to purchasing the VCA 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!

Our Final Thoughts

The Synthrotek VCA is a solid module that leaves a very small footprint in your modular rack. It can either be in linear or exponential mode and is aesthetically pleasing to ones eye. The sliders brighten softens or brightens depending to the amount of voltage it has running through it.

As with any SMD kit, this build is not recommended for beginners and you will want to have several hours of practice before jumping in. VCAs are VCAs. There isn’t a whole lot to know or to discuss other than you can never have enough. If you are in the need for a small 4HP VCA with a lot of umph, head on over to and pick one up. 

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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