Headphone Amp & Stereo Sim

V2021 schematic — power supply top-left, allpass cascade across the middle (IC1 = TL074, four stages A→D), volume / mix / output amp at the bottom

Input and volume

The mono input arrives at POT1 (100kΩ logarithmic VOLUME pot), wired as a voltage divider to ground. The wiper feeds C7 (1µF, AC-couple) into a node shared by the +IN pins of both halves of IC3 (TL072). R15 (1MΩ) ties this node to virtual ground for DC bias, presenting a 1MΩ input impedance to the source — light enough not to load anything reasonable in front of it.

Twin signal paths — IC3B (mono) and IC3A (stereo-prep)

IC3B is wired as a unity-gain voltage follower — its output buffers the input directly, with no shaping. This buffered mono signal goes through R19 (10kΩ) to the off-board X_MO pad. It is the dry path for one channel.

IC3A is wired as a non-inverting AC amplifier with adjustable gain. R27 (100kΩ) sets the input-side leg, and the feedback path is R26 (10kΩ) in series with R28 (100kΩ trim, configured as a rheostat). C18 (10µF) on the −IN side blocks DC. The closed-loop AC gain is:

This is the stereo width / depth trim: it sets how loud the phase-shifted signal is relative to the dry mono signal. Turn it down for a subtle widening; turn it up to make the effect more pronounced. IC3A's output drives the first allpass stage of the cascade.

Four-stage allpass cascade — IC1 (TL074)

Each TL074 gate (IC1A through IC1D) is wired as a classic 1st-order active allpass:

• Input → series resistor (R14, R11, R12, R13 — all 10kΩ) → −IN
• −IN ↔ OUT via feedback resistor (R3, R4, R5, R6 — all 10kΩ)
• Input → series capacitor (C6, C3, C4, C5 — all 10nF) → +IN
• +IN → bias resistor (R7, R8, R9, R10 — all 20kΩ) → VR (virtual ground)

Each stage's centre frequency — where the phase shift passes through exactly 180° — is set by the +IN R/C network:

With all four stages identical at R = 20 kΩ and C = 10 nF, every stage transitions through 180° at the same f₀ ≈ 796 Hz. The cumulative phase shift sweeps from 0° at DC to 4×360° = 1440° at high frequencies — the strongest phase difference relative to the dry mono path occurs around the mid-range, which is where the stereo illusion is most convincing for a guitar.

Channel mixer and stereo / mono switch

The four-stage allpass output (at IC1D's output) goes through R16 (10kΩ) to X_ST; R18 (10kΩ) ties X_ST to ground. The dry mono path (IC3B's output through R19) reaches X_MO; R17 (10kΩ) ties X_MO to ground. Both R16/R18 and R19/R17 form simple half-amplitude attenuators (×0.5), so the level into IC2 is the same whether the source is the wet or dry path.

An external SPDT switch sits between three off-board pads — X, X_ST, and X_MO. The switch's common goes to X, and the two throws go to either X_ST (stereo simulation enabled) or X_MO (mono only). X feeds the left channel of the power amp through C10 (1µF) and R23 (10kΩ bias). The right channel always gets the mono path through C11 (1µF) and R22 (10kΩ bias).

Power amp — IC2 (TDA2822M)

The TDA2822M is a dual stereo power amp in a DIP-8 package, designed for low-voltage portable applications. Each half drives one headphone channel. The closed-loop gain is set by R20/R21 (2.2kΩ) in series with C8/C9 (100µF) from each −INPUT to ground — these are the standard application-circuit gain-set components per the datasheet. Higher resistor values reduce the gain (and the heat dissipated by the chip — see the troubleshooting section).

At each output, R24/R25 (4.7Ω) in series with C12/C13 (100nF) form a Zobel snubber: a high-frequency damping network that prevents the amp from oscillating into the inductive load presented by some headphones. The Zobel cuts in around 339 kHz — well above audio.

C14/C15 (220µF electrolytic) AC-couple each output to the headphone jack, blocking the chip's DC offset. C16/C17 (1µF MKT film) are placed in parallel with the big electrolytics to provide a low-ESR path for high frequencies — this stops the electrolytics' parasitic inductance from taking the treble down.

Power supply

D1 (1N4001) is reverse-polarity protection. C1 (100µF) is the supply bulk decoupling cap. R1/R2 (10kΩ each) form the virtual-ground divider; C2 (47µF) decouples VR. C18 (10µF) provides additional local decoupling at IC3A's −IN node.

Allpass cascade — kit default values

All four stages are identical at f₀ ≈ 796 Hz. The cumulative phase shift varies from 0° at DC to 1440° at high frequencies, with each stage contributing 180° at f₀ and the full 360° well above f₀.

Width / depth trim — IC3A AC gain

R28 (100 kΩ trim) sets the closed-loop AC gain of IC3A, which drives the first allpass stage. Higher trim setting = wider stereo image; lower setting = subtler effect.

Output coupling and load response

The 220µF output caps (C14, C15) form a high-pass with the headphone load. The corner depends entirely on what you plug in:

For low-impedance earbuds the 220µF cap is just enough to keep the bass; for high-impedance studio cans, the response is essentially flat through the entire audio band.

Other filters

• Input HPF — C7 (1µF) + R15 (1MΩ) to VR → 0.16 Hz (DC-block, nothing audible)
• Channel input HPFs (left and right into IC2) — 1µF + 10kΩ → 15.9 Hz each
• Zobel network — R24/R25 (4.7Ω) + C12/C13 (100n) → 339 kHz (HF stability damper, well above audio)

PCB silkscreen — V2021 layout with all resistors lying flat

Populated PCB — three ICs, R28 trim bottom-right, off-board pads bottom-centre (X_ST, IN, X, X_MO)

Populate the board in order from lowest to highest profile. The pot mounts on the back of the PCB so it has to go in last; everything else fits flat on the front side.

Diode and resistors

Mount D1 (1N4001) — black band marks the cathode, match it to the silkscreen. Then all 27 resistors. None of them have polarity. Take care to put the right value in each position — it's easy to swap a 10k for a 20k since most of the board is 10k. Re-check by colour code as you go.

IC sockets

Insert the DIP-14 socket for IC1 and the two DIP-8 sockets for IC2 and IC3. Notch on the socket = notch on the silkscreen. Solder one corner pin first, check the socket is flat, then solder the rest. Do not insert the ICs yet.

Box film capacitors (low profile)

Fit C3-C6 (10n), C12, C13 (100n), and C16, C17 (1µ MKT — these are taller than the others). No polarity. The 1µ MKT caps are noticeably larger than the 10n and 100n; check the silkscreen footprints.

Trim pot R28

Solder the 100kΩ trimmer in. Set to roughly the middle position — you'll dial it in by ear once everything is built and powered up.

Electrolytic capacitors

C1, C8, C9 (100µF), C2 (47µF), C7, C10, C11 (1µF), C14, C15 (220µF), C18 (10µF). Polarity matters — long lead is positive, body has a stripe on the negative side. PCB silkscreen marks positive pads with a +.

Pot — mount last, from the back

The 16 mm right-angle VOLUME pot (POT1) mounts on the back side of the PCB. Cover the pot's metal back with a small piece of insulation tape (insulating PVC tape is fine) so the body cannot short any solder pins coming through the board from the front side. Insert from the back, push fully home, and solder from the component side.

Pot mounted on the back of the PCB — note the white insulator between the pot body and the soldered pins on the back of the board

Clip the bracket off the pot

The Alpha 16 mm right-angle pot has a small mounting notch / bracket projecting forward from the body. Clip this off with side cutters before mounting the assembly in the enclosure — it interferes with the pot sitting flush in a panel hole.

External wiring & insert ICs

Wire the off-board components: input jack (¼″ mono), output jack (¼″ stereo TRS — tip/ring/sleeve), DC jack, and the SPDT toggle. Then carefully insert IC1 (TL074, DIP-14), IC2 (TDA2822M, DIP-8), and IC3 (TL072, DIP-8) into their sockets — notches matching the silkscreen.

External wiring overview — input jack on the left, SPDT toggle (red) in the centre, headphone TRS jack on the right, DC jack lower-left

SPDT switch — STEREO / MONO toggle

The PCB has three off-board pads: X, X_ST, and X_MO. The SPDT switch wires between these three pads:

The right channel always gets the dry mono signal regardless of the switch position — that's why the switch only routes the left channel between two sources. Position the switch so that "STEREO" on your panel label corresponds to the X→X_ST connection.

Output jack — must be stereo TRS

The headphone output is a stereo TRS jack — three connections: tip (HEAD-L), ring (HEAD-R), and sleeve (HEAD-GND). Don't use a mono ¼″ jack here — its sleeve shorts what would have been the ring to ground, which on this circuit means shorting the right channel output of the TDA2822M to GND through the cap. The chip survives but no sound comes out the right side.

Input jack and power

Standard mono ¼″ jack for the input: tip = IN, sleeve = GND. The DC jack is centre-negative 2.1 mm — sleeve = +9V, tip = GND, exactly as a Boss-style pedal supply.

Setting the stereo width — by ear

Once everything is wired up, plug in a guitar (with a cab sim and some drive in front), put on headphones, and engage the stereo toggle. Turn R28 (the trim) slowly — at one extreme the stereo image is narrow (almost mono); at the other it's noticeably wide. Find the position that sounds best to you, lock it in, and forget it. There is no "right" setting — different headphones and different sources favour different widths.

Since this circuit was first released years ago, a handful of recurring issues have shown up. The V2021 rework addresses some of them in the layout; the rest are component-related.

The TDA2822M gets warm or hot

The TDA2822M dissipates more power into low-impedance loads (16Ω earbuds) than into high-impedance ones (250Ω studio cans). Some warmth in normal operation is expected — if it gets uncomfortable to touch, the chip is being driven harder than it likes:

• Reduce the overall gain by raising R20 and R21 from 2.2 kΩ to 3.3 kΩ or 4.7 kΩ. This drops the closed-loop AC gain of the power stage and the chip runs cooler.
• Turn the VOLUME pot down. Long-running operation at very high volume into low-Z headphones is the usual cause.
• Try a different TL072 for IC3 — see below.

TL072CN vs TL072IP

In some builds, replacing the TL072 (IC3) with a TL072IP (industrial-temperature variant) instead of the more common TL072CN fixes the heating issue on the TDA2822M, and also clears up some intermittent noise. The exact reason is unclear — the IP variant seems to bias more cleanly in this circuit. If your build is on the warm side, try a different TL072 first; it's a 30-second swap if you used a socket.

No sound on one channel

If the right channel is dead but the left works, almost always the cause is a mono jack on the output instead of a stereo TRS jack. Swap to a TRS jack and the right channel comes back. If the left is dead, check the SPDT switch wiring — the centre lug must go to X, not to X_ST or X_MO.

Volume drops at one switch position

If the stereo position sounds quieter than mono, try turning R28 up (clockwise, increasing R28's resistance) — the stereo path picks up its level from IC3A's gain, and at minimum trim the gain is only +0.8 dB above unity. The mono path goes through R19 + R17 attenuator, the stereo path through R16 + R18 — both the same divider ratio, so at trim ≈ 50% the levels should match closely.

Hum or buzz in the headphones

This circuit is sensitive to the quality of the 9V supply because the TDA2822M's PSRR is only moderate. A clean regulated wall-wart is essential — battery operation is the cleanest. If you must use a daisy-chain pedal supply, add an extra 100µF on the +9V pad, and ensure the supply provides at least 100 mA per channel.