MFOS Sound Lab Mini-Synth is a cool analog monophonic synthesizer. The original version has been created by Ray Wilson from Music From Outer Space.
It’s composed of two VCO (Saw and Saw+PWM), one Envelope Generator (AR, re trig-able), one LFO, one VCF, one VCA and one Noise source. All the elements are internally routed. LFO and AR Envelope can modulate (On/Off) all the other elements.
See the diagram below for a graphical representation of the internal wiring.
You’ll find more details and explanations of the differents circuits, the complete theory of operation, etc. on MFOS website.
The SMT Edition, version 1.2
This is an updated version. You can also read SMT SLMS previous posts.
The SMT Edition has been specifically designed to fit in a Eurorack System (Power input: -12, 0, +12 volts) and it removes any extra wiring. It’s a one board solution.
Also, while the original is designed for beginners (with some technical backgrounds), this version is in Surface Mount Technologie (SMT) and thus, requires some experience in soldering such small components.
The electric circuit is identical to the original SLMS. Also, it does not include the later optional mods proposed by Ray.
I strongly recommend to read the original documentation.
Cross reference between the two version is easy: the part references and values are identical. R1 in SMT version is R1 in Ray’s original version, and so on.
… with a few adaptations
The conversion from Through Hole to SMT required some component updates:
- The FET transistors (MPF102) Q5 and Q6 have been replaced by two BF545.
- The four NPN transistors (2N3904) Q1, Q2, Q3 and Q4 have been replaced by two NPN arrays BC847BS. They are closely matched and remove the need to hand match transistors.
Adaptation to Eurorack Power Bus is made with the following:
- The power input is buffered with two linear regulators 78L09 and 79L09 (U101 and U102). They lower the input voltages from -12 and +12 volts to -9 and +9 volts. 100nF capacitors C101 and C102 provided for stability.
The assembly is pretty straight forward. No kludge, bodge, correction, mod or whatever is required to make it works.
The Surface Mount Components
SMT soldering is easier than what it looks like. No need for special tools or special soldering iron or tips.
I recommend the use of thin solder wire. 0.3mm is good. 0.5mm is ok.
Get a good pair of fine, non magnetic, tweezers.
To help you with SMT component placement, I created a Component Map, one page by value, showing in RED where each component goes.
Check also this Interactive Pick and Place Map.
Usually, it’s recommended to start by soldering the smallest parts. Tall components can block you. Start with resistors for example. And start with the largest group of same value resistors, for instance, the 100k group. Then continue with decreasing group sizes.
Next step is SOIC packages. The white dot mark on PCB represents pin #1.
Continue with SOT transistors and SOD diodes. Prefer SOD 323 over SOD 323F. Attention, D8 is the Zener Diode.
The most challenging parts to solder are the two transistor arrays Q1 and Q2 (SOT 363 package). The BC847BS doesn’t have a polarity (the part is in fact symmetrical) so simply focus on pin alignment to the pads. Check for possible shorts between pins.
The capacitors require a special attention. Ceramic caps are fragile. They can break easily if bent, twisted, squeezed, heated too much… You wont notice a broken cap, until it fail either open or worse, closed (short circuit!)
Also, there’s no marking on ceramic caps. Do not mix values!
Finish with the aluminum capacitors.
The Through Hole Components
Now it’s time to solder the Through Hole Components. Here’s my recommended order:
Flip your board and solder the two regulators. 78L09 is U101 and 79L09 is U102.
Continue with either Q7 or Q101. 2N3904 are good Noise sources. Use Q101 for 2N3904 or Q7 for BC547. Use only one of them, not both.
Flip again your board and solder the 10 pin Eurorack Power Connector K101.
Now is a good time to solder the (Tempco) resistors R60 and R61. They are mounted on TOP of the Transistor Arrays Q1 and Q2. Place them as close to the arrays as possible.
The Bottom side of the board is now complete (connector S1 is optionnal). It’s time to solder the remaining TH components.
Starting as usual with the smallest ones.
RM1 and RM3 come first. Orient the screw head toward the outside.
If you already have the Front Panel, you can use it now. It will help you align correctly the controls.
Solder the four jack connectors.
Then, the potentiometers. Before soldering them, I highly recommend to remove the protruding tabs with pliers. You might need to remove the nuts and washers.
Finally, solder the Switches. Pay attention to S16. It’s the only 3 position switch.
If your switches come with 2 washers and 2 nuts, only keep 1 of each.
First tests and calibration
Before plugging in any power source, check twice your job! If there’s a short circuit anywhere on the board, the two regulators will switch off quickly, but some damages can occur nevertheless.
Power your board with -12, 0, +12 volts (see “power connector” diagram on the board). Also, a thick white band indicates the -12V rail (usually the “red stripe” on your ribbon cable).
The Front Panel
You can find the CAD file download link in the Downloads section below.
Michele (Modular Audio Devices) has designed this amazing panel, using KiCad. Contact him if you want one:
For a complete list of components, suggested sellers and parts, sorted by values, with refs and comments, see Google Calc document or below in Downloads section.
There’s also a PDF document showing Component positions and an interactive BOM available.
Temperature Compensation Resistors
Temperature Compensation Resistors R60 and R61 can be found at Thonk Tempco Resistor – Akaneohm 1% 3300ppm
However, it’s possible to use simple 50ppm/°C resistors instead. Your VCO will work, track the 1V/octave CV input but will be out of tune as soon as the temperature will change.
All switches are SPDT (Single Pole, Double Throw) with the exception of S16, which is a “3 position” SPDT with an additional central position (No Contact).
Sources for pots and jacks:
https://erthenvar.myshopify.com/collections/9mm-potentiometers/products/smooth-shaft-vertical-pot(Erthenvar shop is now closed)
10 mm height Jacks (PJ398SM / “Thonkicon”):
https://erthenvar.myshopify.com/collections/accessories/products/3-5mm-inline-jacks(Erthenvar shop is now closed)
- Schematics (PDF) (updated 20181223)
- Front panel design, Black/natural engraving (euro slms 33hp 20180821.fpd)
- SVG version of Front Panel (euro-slms-33hp-20180821.svg)
- Pick and place map for Surface Mount components (See also the Interactive BOM)
SLMS made by other users
Csaba made this gorgeous aluminum panel with the Toner Transfer method: