Free Modular

Output 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	Resistor	2kΩ		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	Resistor	1kΩ		See above
	Back	R7-R10, R15-R18	Resistor	10Ω		Adds output impedance to headphone output; protects against shorts when plugging in headphones.
	Back	R19, R20	Resistor	2kΩ		Any value is fine.
	Back	R21, R22	Resistor	1kΩ		Any value is fine as long as they are 1/2 R19, R20.
	Back	R23-R26	Resistor	10kΩ		Any value is fine as long as they are all equal.
	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.
	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*	Only 3 poles are used but there are 5 holes on the PCB so the switched versions (ACJS-MN-3S or ACJS-MN-5) will work as well. ACJS-MV-3 is exactly the same except that it has a spiked washer, so that will work too (as will ACJS-MV-3S or ACJS-MV-5). Frustratingly, the PJ-629HAN sold on Tayda is almost an exact clone, but switches the left and right channels. You can edit the PCB to swap the channels back or just shell out for the original part. Or just live with your headphones reversed.
	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