Free Modular

Offset/Atten Assembly Instructions

See general assembly instructions

Components

See general components notes for more info about acquiring parts.

Interactive BOM: front, back

Board Reference Part Value Source Comment
Front R1-R4 Resistor 33kΩ   Should be ~5/7 times the resistance of R2/R4.
Front RV1-RV4 Potentiometer B50kΩ Thonk Linear. Any value is fine. Determines the input impedance of the module, so a higher value will mean a better attenuation range for some inputs (100k might actually be better).
Front J1-J4 3.5mm Jack THONKICONN (a.k.a PJ398SM or PJ301M-12) Thonk  
Front D1-D11 LED 3mm bipolar ebay This is a bipolar/bi-color LED. That means it is essentially two LEDs wired in opposite directions in the same package. Reversing the connection reverses the colors. When the module is outputting a positive voltage, the current will flow “forwards” through the LED, i.e. the square “cathode” hole towards the top of the module will be negative and the round “anode” hole towards the bottom of the module will be positive. When the module is outputting a negative voltage, the current will be reversed. The LEDs I bought were red in their “forward” direction and green in reverse, so I put them in backwards to get a green light for positive voltages. If you only have normal unipolar LEDs, just solder another led or any diode in reverse across the legs to protect the LED from the reverse current.
Back R5-R8, R11-R14 Resistor 10kΩ   Any value 10k-100k is probably fine as long as they all match.
Back R9, R16 Resistor 10kΩ   Determines LED brightness. Any value between 220Ω-10kΩ might be appropriate depending on which LEDs you have and how bright you want them.
Back R10, R15 Resistor 1kΩ   Determines output impedance
Back C1-C2 Capacitor 10uF   Power supply noise filtering capacitors
Back C3-C6 Capacitor 100nF   Power supply noise filtering capacitors
Back C7-C10 Capacitor 330pF   Stabilizing cap for op-amps. Depending on the model of op-amps used, these probably aren’t strictly necessary. 100pF should also be fine.
Back U1, U2 Op-amp LM324N DigiKey Any quad op-amp would probably work here, as long as it follows the standard pinout and can handle 24V power supply. A TL074 would work fine, but it can’t handle rail-to-rail inputs, so it might give unexpected behavior if the input or output approaches -12V. Therefore, it is better to use a rail-to-rail-capable op-amp like the LM324.
Both J5-J8 Pin headers 1x7, 1x8 Amazon Solder the two boards directly together using the male headers or make them detachable using a male/female pair (recommended).
Back J12 IDC connector 2x8   Eurorack power header. Can use two rows of male pin headers or (recommended) a shrouded connector.