Headphone Mini Amp V1.0b — Build Documentation

Overview & Features

This is the add-on / OEM-style version of the TH Custom Effects headphone mini amp. Unlike the V1.0a 1590A "all-in-one" build, V1.0b strips the design back to the bare PCB and a handful of external connections — input pad, output pads (HEAD-L / HEAD-R / HEAD-GND), and 9V power. It's meant to live inside another pedal or rack project, sharing the host's power supply and existing input/output infrastructure.

V1.0b assembled with 16mm pot, battery clip, and TRS output jack

Final assembly — 16 mm pot mounted on the back of the PCB, simple wiring to a 9V battery clip, ¼″ input jack, and ¼″ stereo output jack

Drop-in module

A handful of pads to wire in. No on-board jacks, no LED, no enclosure constraints. Wire it to whatever input source and headphone jack the host project provides.

Adjustable gain

R1 / R2 are specified as 2.2 kΩ to 3.3 kΩ — pick the value that suits your noise floor. Smaller R = more gain (and slightly more chip-noise). 3.3 kΩ is the quietest option.

Optional reverse-polarity protect

D1 (1N4001) is on the PCB but only needed for standalone use with a separate DC jack. If you're tapping into a host pedal's already-protected supply, skip D1 with a wire jumper.

Mono → both ears

The volume pot wiper feeds both inputs of the TDA2822M directly — same mono signal in both ears. No stereo width, but no surprises either.

About the load on the source: The 100 kΩ volume pot loads the source signal directly — there's no input buffer in front of it. This is fine when feeding the amp from the output of an effects pedal (which is already buffered to a low impedance). It also works well with passive guitar pickups thanks to the 100 kΩ value being light enough not to dull the highs much. But don't expect a 1 MΩ input impedance like a conventional guitar amp — the pot will load a passive source somewhat.

Circuit Theory

V1.0b schematic — single TDA2822M (IC1), passive volume pot at the input, optional D1 for reverse polarity protection

Schematic vs BOM: The schematic image shows R1 and R2 = 1 kΩ5 and the volume pot as 50 kΩ-log — these are values from an earlier revision. The BOM is authoritative for V1.0b: R1 and R2 are now 2.2 kΩ to 3.3 kΩ (selectable for noise/gain trade-off) and the volume pot is 100 kΩ-log on a 16 mm body. The PCB silkscreen positions are unchanged.

Signal path

Audio enters at the IN pad. C6 (4.7 µF electrolytic) AC-couples the input to one end of the VOLUME pot. The pot is wired as a voltage divider to ground: pin 1 grounded, pin 3 at the cap, and the wiper (pin 2) carrying the attenuated signal. R4 (10 kΩ) ties the wiper to ground for DC bias — this is what determines the load on the source when the cap settles. From the wiper, the signal feeds both non-inverting inputs of IC1 (TDA2822M) — pin 7 (+INPUT1) and pin 6 (+INPUT2) — so the same mono signal drives both channels of the power amp.

TDA2822M power stage

The TDA2822M is a low-voltage dual power amp in a DIP-8, designed for portable battery-powered use. Each half drives one headphone channel. The closed-loop AC gain is set by external components on the inverting inputs:

R1 (2.2 – 3.3 kΩ) in series with C3 (100 µF) from −INPUT1 (pin 8) to GND
R2 (2.2 – 3.3 kΩ) in series with C4 (100 µF) from −INPUT2 (pin 5) to GND

The exact gain depends on R1 / R2: smaller resistor = higher gain. Start with 2.2 kΩ. If the resulting noise floor is audible at low signal levels with quiet headphones, swap R1 and R2 for 3.3 kΩ. The trade-off is a small reduction in maximum output level — usually a worthwhile exchange for a quieter background.

At each output, a Zobel snubber (4.7 Ω + 100 nF: R5/C7 on output 2, R6/C8 on output 1) damps any high-frequency resonance from the inductive load presented by some headphones. The Zobel rolls in around 339 kHz — well above audio. C9 and C10 (220 µF) AC-couple the outputs to the headphone jack, blocking the chip's internal DC offset (typically half supply, ~4.5 V).

Power supply — D1 is optional

D1 (1N4001) provides reverse-polarity protection. It's listed in the BOM as "only useful if used standalone — leave off otherwise." If this module is being added to an existing pedal that already has its own reverse-polarity diode on the +9V rail, omit D1 and bridge its PCB footprint with a wire jumper. C1 (100 µF) is the supply bulk decoupling cap; C2 (100 nF) provides high-frequency decoupling.

There's no virtual-ground divider in this design — the TDA2822M biases its own inputs internally and runs from the single +9V rail directly.

Filter Analysis

This is a simple amplifier — no signal-shaping filters, only DC-blocking caps and one stability snubber. All calculated corners sit well outside the audio range.

Input HPF

C6 (4.7 µF) AC-couples the source to the volume pot. The HPF corner depends on the load presented after the cap — primarily the 100 kΩ pot.

f₀ = 1 / (2π × 100 kΩ × 4.7 µF) ≈ 0.34 Hz

Far below audio — pure DC block, no roll-off in any frequency you can hear.

TDA2822M gain network HPF

The R1 + C3 series network on each −IN pin sets the AC gain. Below f₀ the gain rolls off to a lower flat value (the chip's open-loop gain dominates DC).

f₀ = 1 / (2π × R × 100 µF) where R = 2.2 kΩ → 0.72 Hz, R = 3.3 kΩ → 0.48 Hz

Both options are far below audio — full closed-loop gain across the entire audio band regardless of which R you fit.

Output coupling and load response

The 220 µF output caps (C9, C10) form a high-pass with the headphone load. The corner depends on what you plug in:

Headphone load	HP corner f₀	Behaviour
16 Ω (earbuds)	45.2 Hz	Mild bass roll-off below 50 Hz
32 Ω (most consumer headphones)	22.6 Hz	Full audio bandwidth
250 Ω (DT-770 Pro etc.)	2.9 Hz	Flat through the audio band
600 Ω (DT-880 Pro 600 etc.)	1.2 Hz	Flat — output cap is overkill but harmless

Other

Zobel network — R5/R6 (4.7 Ω) + C7/C8 (100 nF) → 339 kHz (HF stability damper, well above audio)

Choosing R1 / R2 in practice: If you have headphones across a few different impedances, leave the R1 and R2 pads accessible so you can swap between 2.2 kΩ (more output, slightly hotter noise floor) and 3.3 kΩ (quieter, slightly less output). With 32 Ω consumer cans, 2.2 kΩ is usually fine. With sensitive 16 Ω earbuds at low listening levels, 3.3 kΩ is the better default.

Bill of Materials

Ref	Qty	Value	Colour code	Notes
Resistors — Metal film, ¼ W, 1%
R1, R2	2	2k2 – 3k3	Red · Red · Black \| Brown · Brown	TDA2822M gain-set resistors. Start with 2.2 kΩ; if too noisy, swap for 3.3 kΩ. Smaller R = higher gain
R4	1	10k	Brown · Black · Black \| Red · Brown	Volume pot wiper bias to GND
R5, R6	2	4R7	Yellow · Violet · Black \| Silver · Brown	Zobel snubber resistors at IC1 outputs 5R1 OK
Capacitors — Electrolytic
C1, C3, C4	3	100µF		C1 = supply bulk decouple; C3 / C4 = TDA2822M gain-network caps. 16 V or 25 V rated, observe polarity
C6	1	4µ7		Input AC-couple from IN pad to volume pot, observe polarity
C9, C10	2	220µF		Output AC-couple to headphone jack, observe polarity
Capacitors — Film
C2, C7, C8	3	100n		C2 = HF supply decouple; C7 / C8 = Zobel network shunt caps at IC1 outputs — box film
Diode
D1	1 (opt.)	1N4001		Optional. Only useful if running standalone with its own DC jack — provides reverse polarity protection. If you're tapping into a host pedal's already-protected supply, leave the position empty and bridge with a wire jumper
Semiconductor
IC1	1	TDA2822M		Dual stereo power amp in DIP-8
Pot
VOLUME	1	100k-log		16 mm body, A-taper (logarithmic). PCB-mount, fitted to the back of the board
Hardware (off-board, builder-supplied)
IC socket	1	DIP-8		Standard dual-wipe DIP-8 socket — no special "precision" requirement on this version since the board has more clearance
Input jack	1	¼″ mono		Or whatever input route fits the host project — wires to the IN pad
Output jack	1	¼″ stereo TRS		Tip = HEAD-L, Ring = HEAD-R, Sleeve = HEAD-GND. Don't use a mono jack — it shorts the right channel to GND

Build Guide

PCB silkscreen — VOLUME pads at top, off-board pads on left edge (HEAD-R, HEAD-L, HEAD-GND) and bottom-left (+9V, GND), IN pad at top-right

Populated V1.0b PCB — TDA2822M centred, electrolytics on the front side. The three "1k63" resistors visible bottom are 1.5 kΩ values from an earlier build; your kit ships with the V1.0b 2k2 / 3k3 values

The order is straightforward — lowest profile first, tallest last. The 16 mm pot mounts on the back of the board, so it goes on at the very end.

Diode and resistors

If you're including D1 (1N4001), mount it first — black band marks the cathode and must match the silkscreen. If you're skipping D1, bridge the position with a piece of resistor leg or a wire jumper. Then all the resistors. None have polarity. Pick R1 and R2 = 2.2 kΩ for now (you can swap to 3.3 kΩ later if it turns out noisy in your particular host project).

IC socket

Insert the DIP-8 socket. Notch matches the silkscreen. Solder one corner pin to fix it flat against the board, then the rest. Don't insert the IC yet.

Box film capacitors

Fit C2, C7, C8 (all 100 nF). No polarity. Low profile so they don't interfere with anything.

Electrolytic capacitors

C1 (100 µF), C3 / C4 (100 µF), C6 (4.7 µF), C9 / C10 (220 µF). Polarity matters — long lead is positive, body has a stripe on the negative side. The PCB silkscreen marks the positive pad with a +.

Pot — back-mount, last

The 16 mm VOLUME pot mounts on the back of the PCB so the shaft sticks out the back side. Cover the pot's metal body with a piece of insulating tape (PVC tape is fine) so it can't short any solder pins coming through the board from the front. Insert from the back, push fully home, and solder from the component side.

Insert IC1

Push the TDA2822M into its socket — notch matching the silkscreen.

Wiring & Integration

A handful of off-board pads to connect — that's it.

Power — +9V and GND

Connect +9V to the host project's +9V rail (or a 9V battery red wire if standalone). Connect GND to common ground (battery black wire if standalone). If you fitted D1, the input is reverse-polarity protected. If you skipped D1, make sure the host already has reverse-polarity protection upstream.

Input — IN pad

The IN pad expects an audio-frequency signal at line level or below — typically taken from the output of an effects pedal that lives in the same enclosure, or from a dedicated ¼″ jack if standalone. Don't tap into a node carrying DC bias; the cap C6 will block any DC anyway, but it's cleaner to take the signal from a properly AC-coupled point.

Output — HEAD-L, HEAD-R, HEAD-GND

Three wires to the headphone jack:

Tip → HEAD-L
Ring → HEAD-R
Sleeve → HEAD-GND

Use a stereo TRS jack — a mono jack will short HEAD-R to ground via its sleeve and you'll lose the right channel. ¼″ TRS or 3.5 mm TRS both work; pick whichever matches your host project's panel.

Volume pot

The pot is on the PCB itself (back-mounted). Drill a hole in the host enclosure where the shaft can poke through, and fit a knob. No external wiring needed for the pot.

Where to take the signal from: If you're adding this to an existing pedal, the cleanest tap point is right after the output volume of the host pedal (before its output jack). That way the host's volume knob still works as the main volume, and this module's volume knob acts as a "headphone volume trim". Alternatively, tap directly off the output jack tip — that's the same node, just on the other side of the wire.

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Disclaimer & Licence

PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.