I recently completed a build of an Oberheim Synthesizer Expansion Module Voltage Controlled Filter work-alike produced by Moogah (Jeff Farr). I picked up the panel plus PCB package from synthcube.com.
I’ve always been a fan of Oberheim gear from way back in the ’70s. Geddy Lee from Rush was probably my first experience hearing an Oberheim (at least I knew it was an Oberheim that I was hearing). In fact Geddy had a Moogerheim mashup. One very cool feature of the Oberheim’s were their filters which, as I began to understand the workings of filters more and more as my modular synth experience grew, I decided I wanted to add to my modular rig.
A very cool feature about the Moogah SEM VCF was that Jeff Farr got Tom Oberheim’s blessing for his design of Tom’s circuit. The VCF is a 2-pole filter which runs at either 15 or 12 volts (based on a couple of resistors). There is no V/Oct control voltage so like the original it will only track a few octaves. There is also no control voltage for the resonance (like the original), however, there is an option to build this in. The PCB includes locations for the appropriate circuitry, however, the pre-made panels do not include a location for the required potentiometer and input jack.
Wanting my VCF to be as close to the original as possible I opted for the build without the voltage controlled resonance. There are presently two panel formats available for purchase; Eurorack and Dotcom/MU. Because my modular is Dotcom based I opted for the Dotcom panel with the 15 volt power option.
Synthcube.com, the online supplier of the board and PCB, has a nice panel designed by Caleb Condit. The panel is made from anodized aluminum and illustrated using the metalphoto process. It is manufactured in the US. While I really liked being able to purchase the panel with the board both the design and illustration left me a bit flat. The panel is painted with a flat black paint which tends to show fingerprints more than its Dotcom counterparts. The white printing is more off-white and significantly less bright than silkscreened panels. The design, while functional has more of a Eurorack disco feel to it. I like my potentiometers labeled with numbers rather than cryptic shapes. Please note that these are only my personal preference. The panel is well made and just receiving a panel with the PCB was a blessing. There’s nothing worse than having a circuit built with no panel to install it into.
The SEM VCF is by no means my first, second or even fifth build. I am not a skilled electrical engineer by any means although I do have relatively good soldering skills. That said I must point out that this project is definitely NOT FOR BEGINNERS!
Step 1 in the documentation reads “The PCB requires a trace cut to address a design issue”. YIKES! That was a new one for me and definitely a bit scary. The documentation, however, clearly shows where the two traces must be cut. It also suggests to use a continuity tester to ensure the traces are cut. I cannot recommend this step enough. Just because it looks cut doesn’t mean the connection has actually been severed.
Cutting traces often involves adding jumper wires to reroute the flow of those cut traces. This board is no exception and two jumpers are added later on in the build at step 22 of the manual. This is a bit of a tight solder to points on the op-amps as they are close to some SMT caps.
Speaking of SMT or Surface Mount Technology caps – these suckers are SMALL. VERY SMALL! Normally SMT components are installed by machines. They allow us to have very tiny devices like cell phones. Soldering them by hand is a difficult process but not impossible. It takes patience and a steady hand. As my first attempt I can say that while not perfect, I did ok. There is a handy video from Colin’s Lab showing how to solder SMT devices. In addition – get smaller tweezers than the ones shown below!
The PCB quality of the the SEM VCF is quite good, however, unlike several other boards I’ve used the components aren’t marked by their reference numbers (e.g. R1, R2, C15, etc…). They are, however, marked with their component values. This makes it a bit trickier locating their positions especially as some of the labels are obscured by the through-holes drilled into the board. Several of the labels are marked with dollar signs ($) to indicate that they are required for building the version of the board which includes the resonance control voltage. Others are marked with asterisks (*) to indicate they are to be excluded for the CV resonance version. Overall though I found it easy to locate the component spots and to populate the board.
The documentation for the project is quite good with one exception; it is only for the Eurorack build. While there is reference to which resistors to install depending on your power supply’s voltage, 12 or 15 volts (step 8) there is no warning about soldering the potentiometers to the board for the Eurorack panel (step 18). I didn’t catch this at first and had to desolder my potentiometers. What a freaking nightmare. All went well though and I attached flying wires to the three potentiometer locations for the pots. While a separate documentation is not necessary for the 5U build of this project it would be helpful if the author could mention the slight differences in the builds for the reader.
Obtaining parts for the build was not too tough, however, there are three 2N4302 transistors which are discontinued and a bit tricky to find. I was able to find work-alike versions of these transistors from Small Bear Electronics. Incidentally, Small Bear is an AWESOME source for components for many DIY projects!
Another VERY important note is that if you are using the provided spreadsheet Bill Of Materials there is an error on row 25. The Poly Cap listed is 150nF (nano-farad). It should be 150uF (micro-farad). Big difference! This is corrected in the Bill Of Materials contained in the build document, however, not on the spreadsheet.
As I mentioned earlier, having the panel to install the board onto immediately after it was done was a tremendous advantage. Once completed I tested the unit with my bench power supply. I cannot stress enough the need for a bench power supply. If a mistake was made on the board I would much rather blow up a bench supply then the power supply in my modular and also risk taking some modules with it. I use a synthesizers.com QPS2 desktop power supply. I’ve also modified one of the power connectors to support a MOTM/Oakley style connector. This allows me to test both Dotcom and MOTM/Oakley modules with no muss nor fuss. The VCF utilizes an MOTM/Oakley style power connector. Once tested I built MOTM to Dotcom connector convertor and installed the unit in my modular.
So there you have it! A very cool module built in about two days.