MFOS Mini-Controller (+/-9V to +/-15V)

Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
We retain the content because it reflects a valuable point of view representing that time and place.

Article by Ray Wilson
This is an intermediate to advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building, troubleshooting and electronics experience is assumed. Additionally, electronic equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

Value Change Bulletin

The value of R7 has been changed from 1M to 10M. Doing so reduces the current through R7 and subsequently increases the precision of the bus voltage when a key is pressed. A voltage divider exists between the resistor ladder and R7. By increasing the value of R7 by a factor of 10 the output of the resistor ladder is loaded less (by 10X) when a key is pressed.

Features

  • The MFOS Mini-Controller has a twenty nine note stylus keyboard (C - C - C to E)
  • It can be used to play your Sound Lab Mini-Synth or 1V/Oct modular.
  • The MFOS Mini-Controller supports four CV outputs (2 with portamento 2 without portamento) My prototype (shown) only used two.
  • There is one trigger output and one gate output.
  • Just the coolest thing EVER!

Quirks

  • Rare, rare (did I say rare) mis-sampling that grabs the raw buss voltage. Remember you are jabbing a PC trace with a piece of metal (slightly bouncy). But even so this thing is really a lot of fun to play with.

MFOS Mini-Controller MP3 Samples

Short song using the Mini-Controller.

Introduction

If you are into the synth hobby scene this is a "must have" you will be playing with this thing into the wee hours of the morning especially if you built a Sound Lab and don't already have a keyboard controller or you are planning on building a Sound Lab. This thing has added a new dimension to the fun I have with my Sound Lab. My son built one for his Sound Lab and he says he can't put it down.

More Photos

MFOS Mini-Controller Schematic Page 1 PDF

This circuit is identical to the one in the Single Buss 1V/Octave Keyboard Controller. See the circuit description there. This board has the addition of the resistors used in the keyboard voltage divider. Whereas in typical keyboard pressing a key brings the resistor divider in contact with the main buss we bring the bus to the divider via the probe used to touch the PCB pads that represent the keyboard.

MFOS Mini-Controller Schematic Page 2 PDF

This is the full resistor divider that is used to produce the 1V per octave voltage necessary to control the Sound Lab or other moddular synthesizer.

Approx. Current Consumption
+12V 30mA
-12V 30mA
+15V 32mA
-15V 32mA

MFOS Mini-Controller PCB Parts Layout Legend View (Parts Side Shown) PDF

MFOS Mini-Controller PCB Parts Layout Value View (Parts Side Shown) PDF

MFOS Mini-Controller PCB Bottom Copper (Parts Side Shown)

MFOS Mini-Controller PCB Top Copper(Parts Side Shown)

MFOS Mini-Controller PCB Top Silk Screen

MFOS Mini-Controller Front Panel and Wiring PDF

As you can see in the photos above I combined the MFOS Wall Wart Power Supply with the MFOS Mini-Controller and also fed the +12V, -12V and Ground out via banana jacks so I can power my Sound Lab Mini-Synth. It is very convenient. You only connect the PCB points shown here and of course KBUS to the banana jacks so you can connect the wand to them. There are several PCB connection points that are not used unless you score a real keyboard and then use this board to build a regular keyboard. Then you might want to use TLA (Trigger LED Anode), GLA (Gate LED Anode), and KBTP (keyboard top) as described in this article Single Buss 1V/Octave Keyboard Controller. As far as the Mini-Controller circuit is concerned when you plan to use the touch pads everything is already pre-connected on the PC board and you just leave these as they are.

KBUS Is Where The Wand Gets Connected
You connect the KBUS to the banana jacks so you can connect the wand to them.

Old Meter Probe Makes A Good Wand

I have several old meter probes lying around and they make a great wand. I suggest that you dull the tip.

Not All Board Connection Points Are Used
You only connect the PCB points shown here. There are several PCB connection points that are not used unless you score a real keyboard and then use this board to build a regular keyboard. Then you might want to use TLA (Trigger LED Anode), GLA (Gate LED Anode), and KBTP (keyboard top) as described in this article Single Buss 1V/Octave Keyboard Controller. As far as the Mini-Controller circuit is concerned when you use the key-pads everything is already pre-connected on the PC board and you do not need to connect the PCB connection points you see here with no wires going to them. Additionally if you use this PCB for a keyboard you would remove the LEDs, and the keyboard voltage ladder resistors.

MFOS Mini-Controller Project Parts List

MFOS Mini Controller Parts List

Qty. Description Value Designators
1   TL071 Op Amp   TL071   U1  
2   TL082 Dual Op Amp(s)   TL082   U2, U6  
2   TL084 Quad Op Amp(s)   TL084   U5, U4  
1   LF444 Quad Op Amp (do not sub)   LF444   U3  
11   1N914 Sw. Diode(s)   VALUE   D5, D4, D1, D2, D3, D6, D10, D7, D11, D8, D9  
1   2N3906   2N3906   Q1  
2   2N5457 N(s)   2N5457   Q2, Q3  
2   LED(s)   LED   LED1, LED2  
1   Capacitor Ceramic   .001uF   C18  
2   Capacitor Ceramic(s)   .0047uF   C25, C24  
1   Capacitor Ceramic   .01uF   C1  
15   Capacitor Ceramic(s)   .1uF   C6, C7, C9, C11, C10, C12, C13, C14, C20, C2, C3, C4, C5, C16, C22  
1   Capacitor Ceramic   22pF   C23  
2   Capacitor Ceramic(s)   470pF   C21, C17  
1   Polystyrene or Polycarbonate (low leackage) Capacitor   .01uF   C19  
3   Electrolytic Capacitor (S)(s)   10uF   C8, C15, C26  
1   Potentiometer   1M   R11  
28   Resistor 1/4 Watt 1%(s)   100 ohm   R37, R38, R39, R40, R41, R42, R43, R44, R45, R46, R47, R48, R49, R50, R51, R52, R53, R54, R55, R56, R57, R58, R59, R36, R60, R61, R62, R63  
4   Resistor 1/4 Watt 5%(s)   100K   R23, R9, R32, R33  
3   Resistor 1/4 Watt 5%(s)   10K   R15, R13, R21  
1   Resistor 1/4 Watt 5%   150K   R22  
2   Resistor 1/4 Watt 5%(s)   1K   R27, R25  
6   Resistor 1/4 Watt 5%(s)   1M   R1, R19, R20, R10, R30  
1   Resistor 1/4 Watt 5%   20 Ohm   R8  
1   Resistor 1/4 Watt 5%   200K   R34  
3   Resistor 1/4 Watt 5%(s)   20K   R24, R26, R16  
1   Resistor 1/4 Watt 5%   3.9K   R2  
1   Resistor 1/4 Watt 5%   300K   R31  
1   Resistor 1/4 Watt 5%   30K   R12  
1   Resistor 1/4 Watt 5%   39K   R4  
2   Resistor 1/4 Watt 5%(s)   3K   R28, R29  
1   Resistor 1/4 Watt 5%(s)   10M   R7 (changed from 1M to 10M Apr 2009)  
1   Resistor 1/4 Watt 5%   4.7K   R18  
1   Resistor 1/4 Watt 5%   475 Ohm   R5  
1   Resistor 1/4 Watt 5%   47K   R35  
1   Resistor 1/4 Watt 5%   82K   R6  
1   Resistor 1/4 Watt 5%   200K   R14  
1   Resistor 1/4 Watt 5%   39K   R17  
1   Multi-turn Trim Pot   1K   R3  

Miscellaneous