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MS-420 Build Guide



The MS-420 is an easy-to-build adaptation of the classic Korg MS-20 Low Pass Filter. The circuit has been modified to run on a single 9V battery, and also to reduce component count as much as possible to keep this build beginner-friendly.





Inside the box you will find all the components you need to build MS-420. All you will need to bring to the build is

  • Soldering iron

  • Side cutters

  • 2.5mm allen key or hex driver

  • Pliers or socket set

  • 9V battery





The kit comes with a printed BOM which I've uploaded here as well. It is a good idea to mark down on your BOM which components you've soldered as you progress through the build.




As with all CCTV builds, care has been taken to ensure the process is as simple as possible. Soldering from shortest components to tallest components is recommended, so let's start with the resistors.





Let's start by soldering the 10K resistors. Each resistor needs to be removed from its carrier tape, have its legs bent sharply at a 90° angle, inserted into the appropriate location on the PCB, soldered, and then clipped. Sounds like a lot but let's take it slow.


Each footprint on the the PCB is marked with values. Resistors can be installed either way around, they do not have a polarity. Bend and insert your first 10K resistor like so. Ignore my dirty hands please.



Bend the legs on the under side outwards, so the resistor does not fall out when you flip the board over.





Solder the legs by heating up BOTH the leg and the copper ring and introduce solder to the joint.



Once both legs are soldered, use side cutters to clip down the legs.



A good solder joint should look like a little volcano.




Solder the 6x 10K resistors




Solder the 4x 1K Resistors



Solder the 3x 100K and 3x 220K Resistors



Nice!


DIODES

MS-420 uses two diodes (semiconductors that function as one-way switches), one is a general purpose diode for reverse power protection (so your filter doesn't blow up if power is applied backwards accidentally) and the other is a special kind of diode called a ZENER diode (pronounced like Zee-ner; rhymes with Cleaner). A Zener diode acts like a normal diode in the forwards direction, but develops a specific voltage drop when conducting from Cathode to Anode. In MS-420, a 3.3V Zener diode is used to create a 3.3V bias voltage, used as a ground potential.


It is very important that we install the correct diode in the correct location AND that we observe the polarity of these diodes so we do not install them backwards.


D1 is the general purpose 1N4005 diode. This one is black with a white stripe in your kit. It is installed with the stripe pointing down.


D2 is the 3.3V zener diode. This one is red with a black stripe in your kit. This one is installed with the stripe pointing up.





Capacitors


MS-420 uses 3 different capacitors, and uses 2 of each kind - Electrolytic, Ceramic, and Film. Different types of capacitors have different qualities that make them suitable for different tasks within a circuit.


Ceramic Capacitors


These small blue (sometimes orange or yellow) capacitors are bipolar (meaning they do not have a polarity) which makes them well-suited to passing audio signals that alternate between positive and negative voltages (AC or alternating current). They are small, cheap, and are available in sizes up to tens of microfarads, although higher capacities and voltages become more expensive quickly.


These two capacitors are installed at locations C6 and C1, and are both 1uF.




Film Capacitors


Film Capacitors have excellent linearity, meaning they maintain their capacitance across frequency and voltage ranges. This makes them the ideal choice for the frequency-setting capacitors in filters. We are using 680pF capacitors in positions C4 and C5, one for each pole of filtering.


Film capacitors are more expensive than ceramics, and are commonly found in values less than 1uF.


We need to bend the capacitors over, so they sit on top of the resistors. If they stick straight up, they will interfere with the panel once installed. You can either solder them loosely (not right down to the board) and then fold them over, or like I did - fold them over first and then solder.





Electrolytic Capacitors


Electrolytic capacitors are POLARIZED, meaning they work very well in one direction, and will violently EXPLODE in the other. They have a white stripe on one side which indicates the NEGATIVE leg. The printed circuit board has a plus sign (+) indicating the positive leg. C2 and C3 are the electrolytic capacitors.


Electrolytic Capacitors are typically available in values 1uF and up. Although they are polarized, they have extremely low ESR (effective series resistance). This makes them the optimal choice for power supply filtering, where it is desirable for all AC to pass straight through to ground.



Power Switch


One side of this pushbutton has a small triangle on the white square shaft. This triangle needs to line up with the triangle on the SW1 footprint. If you solder this the other way, the MS-420 will be ON when the button is OUT, and OFF when the button is IN. How embarassing.





JACKS AND POTS


The jacks and pots are easy - they can only fit one way around. 3 pots go in the CUTOFF, RES, and INPUT_VOL spots, and the 3 jacks go in the IN, CV1, and OUT spots. Take care to make sure these components are flat against the printed circuit board. I always just solder one pin of each component before checking the fit. Then if it looks good, solder the rest. If it's crooked, just heat that one pin and push the part into position.





ELL EEE DEE


The LED needs to be soldered in the LED1 spot, near the power switch. LEDS ARE POLARIZED DEVICES, so this needs to be soldered the right way around. The LONG leg of the LED is the POSITIVE leg. There is a small plus sign (+) indicating the positive hole in the footprint.



Bad photo, sorry.


Now we can discuss the HEIGHT of the LED. It needs to sit off of the printed circuit board substantially, so that it will fit through the hold in the top panel. You can use the top panel as a guide to find the optimal height, HOWEVER - the panel will actually be about 1mm above the top of the pots and jacks, so give it a bit more height than I did here. Fit one nut on either a pot or a jack to keep the panel in place while you solder the LED.




I recommend just soldering one leg, and testing again. Then you can reflow that leg and move the LED up or down before soldering the other leg.


9V Battery Clip




The red lead goes to the + spot, and the black lead goes to the - spot.


TEST TEST TEST


At this point you can connect a 9V battery and test your filter! Feed any signal (line level, guitar, or eurorack) into the input and patch the output to an amplifier, audio interface, synthesizer input, or guitar pedal. Turn up the CUTOFF and you should hear your filter functioning. Cranking INPUT_VOL should increase the signal amplitude, perhaps even overdriving if the input signal is hot enough. The RES control should introduce resonance to your filter, and even cause it to self-oscillate.


If any of these are not functioning correctly, check your solders!





CASE ASSEMBLY


Start by screwing the PCB down to the bottom plate. Insert the M3 x 16 screws in from the bottom and screw down the 4mm standoffs. Finger tight only. Plce the PCB over the standoffs and thread the M3 nuts down, again just finger tight.






Attach your battery before assemnbling the sides. The laser-cut pieces have a protective film which needs to be peeled off. Then the pieces can be inserted into the slots on the bottom plate, with the rounded feet sticking down through the bottom.




The battery wires need to be arranged in a way that they are not on top of the jacks, nuts, power switch or LED.


At this point the top panel can be placed, and nuts can be put on the jacks and pots. The pots get a washer too.




These should be tightened down gently with pliers or sockets - 8mm and 10mm.


All that's left is fitting the knobs and signing your name.











SCHEMATIC






How Does it Work?


The MS-20 filter, on which MS-420 is based on uses OTAs - Operational Transconductance Amplifiers. These are kinda like op-amps, except they have a "gain" control via a current input pin, and their outputs are a current output, rather than a voltage output. Because they have a current output that is proportional to the gain setting, they fulfill a function of the elusive "Variable Resistor".


LM13700 is the OTA used in MS-420, and it is an SMD chip, presoldered in this kit. Through hole versions of this chip are becoming increasingly rare, so the choice to use an SMD IC was one made for me. The LM13700 includes 2 OTAs and 2 buffers. The buffers perform a current-to-voltage conversion.


The filter capacitors - C4 and C5 - combine with the 'R' in each variable resistor, to provide 2 poles of low pass RC filtering. The transistor buffer present inside the LM13700 chip is used to convert the current back to voltage, and the gain of the entire RC filter +buffer block is set by R4 and R5.


Resonance of this filter is implemented by feeding back the output (filtered) signal back into C4. A potentiometer is used to sweep between the bias ground (+3.3V) and the feedback signal. Connecting C4 directly to 3.3V causes no resonance. Feeding back the entirety of the output signal causes maximum resonance.


The CVs - the input CV and the CV potentiometer are added using a passive summer - this takes an average of the two signals, and applies this sum to the resistors R11 and R12 (100K). These resistors limit the applied voltage to a current range that the LM13700 is expecting.


Bypassing and Power are combined into one switch. This switch turns power on and off to the filter to save battery, but also sends the input signal to the output jack to audition your filter against a dry signal.


INPUT_VOL (or Drive on the panel) is a 10K attenuator. This means MS-420 can accept very hot signals, by simply turning down the drive. The downside of this is the input impedance is set by this B10K potentiometer. You may find that guitars sounds a bit muddy going through this filter, unless some buffering (i.e. a guitar pedal) is used.


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