Free Modular

Output Assembly Instructions

See general assembly instructions

Note: this design has 3 PCBs that are stacked into a sandwich. The small one (top) and the one with the large jacks (middle) both face UP, with their components facing the faceplate (as indicated by the silkscreen). The last PCB (bottom) faces down, with most of the components mounted on the side facing away from the faceplate. All three are connected together with a single pair of headers at the top and a set of screws or offsets at the bottom.

Components

Most parts are available on Tayda (cart link, quick-order CSV).

See general components notes for more info about acquiring parts.

Interactive BOM: front, middle, back

  Board Reference Part Value Source Comment
  Front R1, R2 Resistor 100kΩ    
  Back R3, R11, R19, R20 Resistor 20kΩ   Some of these resistors overlap the mounting holes. I recommend putting in the standoff first and then putting the resistors over/around the bolt head.
  Back R4-R6, R12-R14, R21-R26 Resistor 10kΩ   See above
  Back R7-R10, R15-R18 Resistor 10Ω   Adds output impedance to headphone output; protects against shorts when plugging in headphones.
  Back R27-R30 Resistor 100Ω   Output impedance for line level output.
  Middle R31, R32 Resistor 22kΩ   Controls the decay of the peak detector display. A lower value will make the display follow the audio signal more closely. A higher value will show signal peaks for longer. 20kΩ is fine here.
  Middle R33 Resistor 33kΩ   R33-R38 provide the voltage reference for the volume meter. To get an accurate meter, they all must be the exact values given at 1% tolerance. But, if you don’t have all the correct values, only the display will be effected; not the audio. Alternatively, if you want to use a different scale for the meters, you could use your own choice of values here. They are arranged as a voltage divider ladder from 12v to 0v.
  Middle R34 Resistor 2.2kΩ    
  Middle R35 Resistor 1.5kΩ    
  Middle R36 Resistor 2.4kΩ    
  Middle R37 Resistor 820Ω    
  Middle R38 Resistor 470Ω    
  Middle R39-R48 Resistor 10kΩ   Controls LED brightness for display. Higher values mean dimmer LEDs. Different color LEDs might have slightly different brightnesses, so you might want to experiment tuning these values to make all your LEDs uniform.
  Front C1, C2 Capacitor 150nF   Combines with R1-R2 to make a high-pass filter for the input. 100nF is fine here too.
  Back C3, C4 Capacitor 10uF   Power supply noise filtering capacitors.
  Back C5-C14, C19-C24 Capacitor 100nF   Power supply noise filtering/decoupling capacitors.
  Middle C15-C16, C25-C28 Capacitor 100nF   Power supply noise filtering/decoupling capacitors.
  Front C17, C18 Capacitor 100nF   Power supply noise filtering/decoupling capacitors.
  Back C29, C30 Capacitor 100pF   Op-amp feedback stabilizers.
  Middle C31, C32 Capacitor 10uF    
  Back C23, C34 Capacitor 100pF   Op-amp feedback stabilizers.
  Middle D1, D2 Diode 1N4148 Tayda  
  Middle D3, D8 LED RED (3mm) Tayda Must have long enough leads to reach from middle PCB to faceplate (1 inch). When not illuminated, these LEDs are exposed to 12v reverse current. Technically, this is outside the spec of most LEDs and could result in damage to the LED over time. If you are worried about this, you could look for LEDs with high reverse-voltage tolerance, or add a Zener diode or other high reverse voltage diode in series with one of the legs of each LED. But, after testing this design for a while with the cheapest Amazon LEDs, it seems like they were totally unaffected, so I wouldn’t worry about it.
  Middle D4, D9 LED AMBER (3mm) Tayda See above
  Middle D5-D7, D10-D12 LED GREEN (3mm) Tayda See above
  Front J1, J2 3.5mm Jack THONKICONN (a.k.a PJ398SM or PJ301M-12) Tayda, Thonk  
🔴 Front J3-J5 1/4in Jack ACJS-MN-3 Mouser, Tayda* The current version of the PCBs are designed for ACJS-series jacks. Any of the ACJS-MN-3, ACJS-MN-3S, ACJS-MN-5, ACJS-MV-3, ACJS-MV-3S or ACJS-MV-5 will work. The PJ-629HAN sold on Tayda can work but it isn’t an exact fit. The blades are wider so they would need to be snipped/filed in order to fit into the holes in the PCB. I believe the polarity is also reversed (so left and right channels on the headphones would be flipped). I plan to make a future revision updated to use the cheaper Tayda jacks.
  Back J6 IDC connector 2x5 Tayda Eurorack power header. Can use two rows of male pin headers or a shrouded connector (recommended).
  Back J7 Unused -   Expansion connector for future modules. Exposes an AC-coupled and buffered but unscaled copy of the input signal.
🔴 All J11-J23 Stacking Pin Headers 1x10 Amazon (double-sided), Amazon (single-sided) All 3 PCBs are connected by a single set of headers. There are a few ways to do this. You could use a stacking male/female header on the middle and use standard male/female headers on the top and bottom, use double-sided male headers on the middle and standard female on the top and bottom, or just use extra-long male headers and solder everything in place. The only thing that matters is that the offset between the front and middle PCBs is the right distance to align the large and small jacks.
🔴 Front RV1, RV2 Potentiometer B50kΩ Dual gang Thonk Linear, dual gang. Any value is fine.
  Front U1 Op-amp TL072 Tayda, DigiKey  
  Back U2, U7 Op-amp TL074 Tayda, DigiKey  
  Back U3-U6, U8-U9 Op-amp NE5532P Tayda, Mouser The NE5532P is a low-noise, high output-drive Op-amp, used here for driving the headphones and line outputs. The NE5532AP is an extra low-noise version, although it’s probably not worth the cost. The SA5532 is identical except it has a higher temperature range, which isn’t necessary. If you have a particular op-amp you prefer, you can use it here instead (as long as it follows the standard TL072-style pinout). A TL072 should work fine too, although it might not have the best distortion characteristics or lifespan depending on what load you are hooking it up to. U5 and U6 are connected in parallel for extra driving power and distortion resistance on the headphone output. You can just leave them out if you don’t need the extra power.
  Middle U10-U12 Op-amp TL074 Tayda, DigiKey  

🔴 = Missing from Tayda BoM