Power Amp V1.1 — Build Documentation

Overview & Features

The Power Amp is a compact solid-state amplifier that takes line-level audio and turns it into up to 10 W of speaker power. It runs from any 9 V or 12 V centre-negative wall wart rated for at least 1 A, and uses the same audio IC twice — once as a voltage-doubling oscillator that lifts the supply rail to roughly 16 V, and a second time as the audio amplifier itself.

You'll have rocked the house with an iPod and a 2×12" cab. This thing gets loud. It's equally at home as:

Pedalboard amp

Drop it into any 1590-series enclosure. Direct-drive a small speaker for amp-less stage use or silent home rehearsal.

External practice amp

Feed it from any preamp, multi-FX or modeller. Pair with an 8 Ω cabinet for a self-contained practice rig.

Portable iPod amp

Plug a phone or media player directly into the IN pad and party. Wide bandwidth, hi-fi quality from a tiny board.

Power supply requirements: use a wall-wart rated at least 1 A at 9 V or 12 V — anything less will sag under transients and clip badly. If combining this with other 9 V circuits on a pedalboard, run the Power Amp from its own separate supply — the doubler oscillator radiates switching noise that will couple into shared rails.

Circuit Theory

Power Amp V1.1 schematic showing TDA2003 voltage doubler oscillator (IC1), 7815/LM2940 voltage regulator (IC3), and TDA2003 audio power amplifier (IC2)

Schematic — three TO-220 ICs in a row: IC1 doubler oscillator, IC3 regulator, IC2 audio power amp. Power flows left-to-right, audio flows right-to-left at the top.

Active devices

Three TO-220 packages do all the heavy lifting:

IC1 (TDA2003) — re-purposed as a Schmitt-trigger relaxation oscillator. Drives the voltage-doubling diode pump.
IC3 (78xx-family voltage regulator) — takes the doubled, unregulated +18 V rail and produces a clean +15 V supply for the audio amplifier. The exact part depends on supply voltage — see §06.
IC2 (TDA2003) — the audio power amplifier itself, in standard non-inverting configuration.

The clever bit — same IC, two completely different jobs

Why two TDA2003s and not one big amp? A 9 V supply alone can't drive a TDA2003 above ~2 W into 4 Ω before clipping — the supply rail itself becomes the limit. By using a second TDA2003 as a voltage-doubling oscillator, the supply gets pumped up to nearly twice its input voltage, and a small linear regulator cleans up the result to 15 V. The audio amp then has the headroom for full 10 W output. It's the same chip BOM line, used twice: simple, cheap, single-supply, and effective.

Stage 1 — Voltage doubler oscillator (IC1)

IC1 is wired as a free-running square-wave oscillator. The TDA2003's high open-loop gain plus positive feedback from the OUT pin to the +IN through R1 (2M2) turns the chip into a comparator with hysteresis. R2 (330 k) from +IN to GND fixes the trip-point ratio, and the timing capacitor C1 (10 n) on the −IN charges and discharges through the chip's low-impedance output stage. The OUT pin then swings between roughly GND and the supply rail at a frequency in the audio-to-low-RF range.

That square wave drives a classic two-stage Schottky charge pump:

• On the OUT-low half-cycle, C4 (2200 µF / 40 V) charges through D1 (1N5817) to roughly +V_supply − V_F(D1).
• On the OUT-high half-cycle, C4's bottom plate is lifted to the supply rail, so the top plate sits at ~2× V_supply. D2 (1N5817) then transfers that charge into the bulk reservoir C5 (2200 µF / 40 V).
• Result: the +18 V (T) rail at IC3's input sits at roughly 2× V_supply − 2× V_F(D1/D2) ≈ 16 V from a 9 V wall wart, ≈ 22 V from a 12 V wall wart. Schottky diodes are essential — their low ~0.4 V forward drop preserves voltage that ordinary silicon diodes would waste.

Why a power IC for an oscillator? The TDA2003's OUT can sink and source amperes, which is exactly what the diode-pump needs to charge the 2200 µF capacitor banks at audio-rate switching frequencies. A 555 timer or CMOS gate would be the textbook choice but couldn't deliver this current; the TDA2003 happens to do both jobs effortlessly.

Stage 2 — Voltage regulator (IC3)

The doubled rail at C5 is unregulated and ripples with the oscillator switching, so it can't directly feed the audio amp without producing audible noise. IC3 (LM2940-15 for 9 V supply, or 7818 for 12 V supply) drops the rail to a clean, regulated DC voltage. C6 (100 n) is the input bypass; C7 (100 µF / 25 V) is the output reservoir. D3 (LED) with current-limit R3 (6k8) sits before the regulator and shows whether the doubled rail is alive — a useful debug indicator.

Stage 3 — Audio power amplifier (IC2)

IC2 is a textbook TDA2003 application circuit:

• Input coupling: C12 (10 µF) couples the input signal to the +IN pin. The signal can come straight from any line-level source — guitar preamp output, mixer line out, mp3 player, modelled-amp simulator.
• Gain network: R4 (220 R) from OUT to −IN and R5 (2R2) from −IN to GND set the closed-loop voltage gain. C9 (470 µF) in series with R5 makes the gain frequency-dependent: at DC the loop gain is unity (no offset multiplication); above the corner frequency it rises to the full 1 + R4/R5 ≈ 101× (40.1 dB). See §03.
• Output coupling: C8 (2200 µF / 40 V) blocks DC from reaching the speaker. With the ½-supply DC bias on the OUT pin, this cap is essential — without it, the speaker would carry a continuous DC current that would burn out the voice coil.
• Output stability network — Boucherot/Zobel: R6 (47 R) + C10 (47 n) at IC2 OUT prevents oscillation by presenting a defined load to the output stage at ultrasonic frequencies (where the speaker's inductive impedance climbs steeply). A second R7 (1 R) + C11 (100 n) shunts any residual radio-frequency feedback at the speaker terminal.

Gain & Frequency Analysis

The Power Amp is not a filter circuit — it's a wide-band amplifier. The "frequencies" of interest are the corner points where gain droops or coupling caps roll off, plus the gain itself.

Closed-loop voltage gain (IC2)

Audio amplifier gain — IC2

Non-inverting feedback configuration

R4 (feedback)

220 Ω

R5 (gnd leg)

2.2 Ω

101×

linear

40.1

P_out

W max

Av = 1 + R4 / R5 = 1 + 220 / 2.2 = 101

A typical line-level signal of 200–400 mV_peak at the IN pad is amplified to 20–40 V_peak, which is more than enough to swing the regulated 15 V rail rail-to-rail and produce full output power into a 4 Ω or 8 Ω speaker.

Low-frequency gain corner

C9 sits in series with R5, so at low frequencies its impedance increases and the effective ground-leg impedance rises — which lowers the closed-loop gain back toward unity. The −3 dB corner of this gain droop is set by the time constant R5 × C9:

LF gain corner — feedback network

Where Xc(C9) = R5 → gain has dropped 3 dB from mid-band

2.2 Ω

470 µF

f_{−3 dB}

154

slope below

−6

dB/oct

DC gain

1×

unity

f_LF = 1 / (2π × R5 × C9) = 1 / (2π × 2.2 × 470 µF) ≈ 154 Hz

Below ≈150 Hz the closed-loop gain begins to fall. This sounds like bass roll-off but in practice it is dominated by the speaker's own low-frequency limit — small guitar speakers don't reproduce useful output below 80–100 Hz anyway. To extend bass response, increase C9 to 1000 µF (corner ≈72 Hz) or 2200 µF (corner ≈33 Hz).

Output coupling cap and high-frequency stability

C8 (2200 µF) and the speaker form a series HP filter. R6 + C10 (the Boucherot/Zobel network) and R7 + C11 (speaker-side Zobel) prevent ultrasonic instability when the speaker's inductive impedance rises with frequency.

Output coupling — C8

Series HP — speaker low-frequency limit

2200 µF

Speaker

4 Ω / 8 Ω

f_{−3 dB} @ 4Ω

f_{−3 dB} @ 8Ω

slope

−6

dB/oct

2200 µF is generous — well below most guitar speakers' useful response. Substituting 1000 µF (the value on the schematic) raises the corner to 40 Hz / 20 Hz — still below most cabinets' rolloff.

Boucherot / Zobel — R6 + C10

HF stabilisation at IC2 OUT

47 Ω

C10

47 n

corner

kHz

slope

−6

dB/oct

role

load

Above ≈72 kHz the network presents a near-resistive 47 Ω load to the OUT pin. This swamps the rising inductive impedance of the speaker and stops the feedback loop from going unstable at supersonic frequencies. Do not omit — chip will oscillate, get hot, and likely die.

Power output is supply-dependent: the headline 10 W figure assumes a 12 V wall wart (which the doubler lifts to ≈22 V before the 7818 regulator clamps to 18 V) into a 4 Ω cabinet. From a 9 V wall wart with the LM2940-15, expect roughly 5–6 W into 4 Ω or 3–4 W into 8 Ω — still plenty for home practice and small stages, just below the spec ceiling.

Bill of Materials

Resistors are 1 % metal film ¼ W unless noted. Power resistors (R7, 1 W) handle the speaker-side Zobel current. Box-film caps are 5 mm pitch; electrolytics need to fit the marked PCB diameters.

Ref	Qty	Value	Colour code	Notes
Resistors — Metal film 1 % ¼ W (R7 = 1 W)
R1	1	2 MΩ2	Red · Red · Black \| Yellow · Brown	Metal film ¼ W. Oscillator positive-feedback resistor
R2	1	330 kΩ	Orange · Orange · Black \| Orange · Brown	Metal film ¼ W. Oscillator hysteresis ratio
R3	1	6 kΩ8	Blue · Grey · Black \| Brown · Brown	Metal film ¼ W. LED current limit. Test with your chosen LED first — ultra-bright types may need 10 kΩ to avoid retina-piercing brightness
R4	1	220 Ω	Red · Red · Black \| Black · Brown	Metal film ¼ W. Audio-amp feedback resistor — sets gain with R5
R5	1	2 Ω2	Red · Red · Black \| Silver · Brown	Metal film ¼ W. Audio-amp ground-leg resistor — sets gain with R4. tuning — see §03 for gain-modification options
R6	1	47 Ω	Yellow · Violet · Black \| Gold · Brown	Metal film ¼ W. Boucherot/Zobel network at IC2 OUT — HF stability
R7	1	1 Ω · 1 W	Brown · Black · Black \| Silver · Brown	1 W metal film or wirewound — not ¼ W. Speaker-side Zobel — sees the full output current at HF. The colour-band scheme above is the standard 5-band 1 % code; physical size is roughly twice that of a ¼ W resistor
Capacitors — Film box (non-polarised)
C1	1	10 n		Box film. Oscillator timing capacitor on IC1 −IN
C2, C6, C11	3	100 n		Box film. Supply-rail and regulator-input bypass capacitors. Note: the original BOM document lists C1, C6, C11 as 10 n — schematic confirms only C1 is 10 n; C6 and C11 are 100 n along with C2
C10	1	47 n		Box film. Boucherot/Zobel cap at IC2 OUT
Capacitors — Electrolytic (polarised)
C3, C7	2	100 µF / 25 V		Polarised electrolytic, max 5 mm Ø footprint. C3 = +9 V supply bulk; C7 = regulated +15 V output reservoir
C9	1	470 µF / 25 V		Polarised electrolytic, max 8 mm Ø footprint. Audio-amp feedback bootstrap. tuning — increase to extend bass response
C12	1	10 µF		Polarised electrolytic. Audio input coupling — observe polarity (+ toward IC2 +IN)
C4, C5, C8	3	2200 µF / 40 V		Polarised electrolytic, max 12 mm Ø footprint — measure carefully before ordering. Note: the schematic shows 1000 µF / 40 V; the BOM revised them to 2200 µF for cleaner pump operation and lower output ripple. 1000 µF will work but you'll hear more switching residue at low listening levels. C4 = pump cap, C5 = doubled-rail reservoir, C8 = audio output coupling
Semiconductors
D1, D2	2	1N5817		Schottky rectifier, 1 A / 20 V. Substitutes: SB120 1N5818 1N5819 — any 1 A Schottky with V_F ≤ 0.5 V at 1 A. Do not use 1N400x silicon — too much forward drop, doubler will deliver insufficient voltage
D3	1	LED 5 mm		Power-on indicator. Standard 5 mm LED — colour of choice. The LED is fed from the +18 V (T) doubled rail before regulation, so it lights only when the doubler is alive and pumping
IC1, IC2	2	TDA2003 (V)		5-pin TO-220 audio power IC. Substitutes: TDA2002 TDA2003V UTC TDA2003 — pin-compatible 10 W audio amps. Same part used twice — once as oscillator (IC1), once as audio amp (IC2). Both must be heatsinked.
IC3*	1	see §06		Choice depends on supply voltage: • 9 V supply → LM2940CT-15 (low-dropout 15 V regulator). Mouser P/N `LM2940-15/NOPB` • 12 V supply → µA7818 or LM7818 (standard 18 V regulator) • A normal 7815 will not work from 9 V — its dropout voltage is too high to regulate from the ≈16 V doubler output. The schematic shows "7815" as a generic placeholder; use the part listed here
Hardware
Heatsink**	1	3-up TO-220 sink		Required — running without a heatsink will destroy one or more ICs. Any heatsink that accommodates three TO-220 packages on a single face works; common Fischer / Aavid types around 6–10 K/W are sufficient. Alternative: use a metal enclosure (1590B or larger) as the heatsink — see §07
Speaker pads	1	LS1, LS2		Wire to a 6.35 mm or banana speaker connector of your choice. Cabinet impedance 4 Ω or 8 Ω; lower-impedance loads draw more current — confirm your supply can deliver before connecting 4 Ω
Input pad	1	IN, GND		Wire to a 6.35 mm mono jack (guitar/line) or 3.5 mm stereo jack (mp3 player — sum L+R through 10 kΩ resistors externally if mono mixing is desired)
Power pad	1	+9 V, GND		Wire to a 2.1 mm centre-negative DC jack. 1 A minimum supply rating — anything weaker will cause supply sag and audible clipping under transients

* See section 06 for the full regulator selection logic and supply-voltage trade-offs.
** See section 07 for heatsink mounting options including using the enclosure itself as a thermal sink.

Build Guide

Power Amp PCB silk-screen layout showing component reference designators and TO-220 IC positions along the top edge

PCB layout (silk-screen). The three TO-220 ICs sit along the top edge — IC1 left, IC3 centre, IC2 right — so a single straight heatsink bar can clamp all three. The silk reads "10W Power Amp V1.0" but this is the V1.1 build doc; the PCB artwork was unchanged between revisions.

Build order is bottom-up — flat low parts first, large electrolytics and power devices last.

Resistors and small diodes

All seven resistors first. Watch R7 — it is a 1 W part with a larger physical body than the others; the PCB hole spacing accommodates it but lay it flat against the board for stability. Then D1 and D2 (1N5817) — both Schottky, observe the band-side cathode marking.

Box-film capacitors

C1 (10 n) followed by all three 100 n parts (C2, C6, C11) and C10 (47 n). Film caps are non-polarised — orientation does not matter.

Small electrolytics — observe polarity

C12 (10 µF), then C3 / C7 (100 µF / 25 V), then C9 (470 µF / 25 V). Long leg = positive. The PCB silk-screen shows the + position — do not skip this check.

LED — D3

Insert with long leg (anode) to the marked position. If you plan to panel-mount the LED on an enclosure, install short leads now and extend with hookup wire later.

The three TO-220 ICs — careful alignment

Insert IC1 (TDA2003), IC3 (regulator) and IC2 (TDA2003) into their PCB pads but do not solder yet. Lay a straight edge across the three packages and gently bend their legs at exactly the same height so all three mounting holes line up on a single horizontal line. Tack-solder one pin per IC; verify alignment is still correct, then solder the remaining pins.

Why this matters: the heatsink is a single straight bar bolted across all three IC tabs. If the three holes are not at exactly the same height, the heatsink either won't fit cleanly or will mechanically stress one of the packages — leading to cracked dies and silent failure later.

Large electrolytics — last

C4, C5, C8 (2200 µF / 40 V). These are the tallest parts on the board and want to sit upright. Polarity is critical — these caps see 16–22 V and reverse-connection will destroy them spectacularly. Check the silk-screen + position twice before soldering.

Off-board pads

Solder header pins or flying leads at IN / GND, +9 V / GND, LS1 / LS2 (speaker), and optionally external LED leads. Use stranded wire ≥ 22 AWG for the speaker connection — solid-core can fatigue and break under cabinet vibration.

Heatsink mounting

Bolt the heatsink to the three TO-220 tabs using M3 hardware. Use mica/silicone insulating washers and shoulder bushings if the heatsink is metallic and connected to anything other than the IC's own tab voltage — the TDA2003 tab is internally connected to GND, so direct metal-on-metal contact is electrically safe but mechanical insulation is good practice. Apply a thin film of thermal compound between IC tab and heatsink. Tighten evenly — do not over-torque or you will crack the package.

First power-on — without speaker

Apply 9 V (or 12 V) supply. The LED should light immediately; if not, check D9 polarity, D1/D2 polarity and the diode-pump direction. Measure the +18 V (T) test point with a multimeter — should read ≈16 V (from 9 V supply) or ≈22 V (from 12 V supply). Measure the IC3 OUT pin — should read 15 V or 18 V respectively. If both rails check out, power down and connect a speaker through a 100 Ω in-line resistor for the first audio test (limits damage if anything is wrong).

Reference photo — built board

Power Amp prototype photograph showing three TO-220 ICs with test heatsinks, three 2200uF axial electrolytics, and the LED power indicator

Prototype build — three large axial electrolytics dominate the layout. The black heatsinks shown here are testing only; permanent installation needs larger thermal mass.

Power & Regulator Options

The regulator IC choice depends entirely on what supply voltage you intend to feed the board. The two supported options:

9 V supply path

Standard pedalboard adapter

Doubler output ≈ 2 × 9 V − 2 × 0.4 V ≈ 16 V on the +18 V (T) rail. A standard 7815 has a dropout voltage of ~2 V — 16 V minus 2 V dropout = 14 V output, which means the 7815 fails to regulate cleanly and the audio rail sags under signal.

Required IC3: LM2940CT-15 — a low-dropout 15 V regulator with only ≈0.5 V dropout. Operates correctly from a 16 V input. Mouser P/N LM2940-15/NOPB.

Doubler V

≈16

Audio rail

12 V supply path

Higher-voltage adapter — more headroom

Doubler output ≈ 2 × 12 V − 2 × 0.4 V ≈ 22 V on the +18 V (T) rail. Plenty of headroom for a standard 78xx-family regulator, and the higher rail allows the audio amp to swing larger output voltages → more clean power.

Required IC3: 7818 or LM7818 — standard 18 V positive regulator. Drops 22 V to 18 V cleanly; ≈10 W into 4 Ω is achievable.

Doubler V

≈22

Audio rail

Do not use a normal 7815 from a 9 V supply. The schematic shows "7815" as a generic placeholder symbol, but the actual installed regulator must match your supply voltage as described above. A 7815 fed from the 16 V doubler output will partially regulate, sag under signal, and the amp will sound noisy and weak. The mismatch is silent — no visible failure — so it's an easy mistake to ship.

Supply current rating

The Power Amp draws short-duration peaks of 1.5–2 A under transient signal. Always use a wall wart rated at least 1 A continuous. Smaller adapters (200 mA, 500 mA) will sag during loud passages, causing audible clipping and a dirty, distorted sound. The doubler oscillator also presents a switching load that some regulated supplies don't enjoy — a simple linear or basic switching wall wart works better than a "smart" adapter with output current foldback.

Shared supply caution: if you must run the Power Amp on the same supply as other 9 V pedal circuits, the doubler oscillator's switching transients will couple back into shared rails as audible whine. Use a dedicated supply, or at the very least a separate isolated output on a multi-output pedalboard PSU.

Heatsinking & Enclosure

Heatsinking is not optional. A TDA2003 dissipates several watts as heat under signal — and you have two of them, plus a regulator that drops 1–4 V at the same audio current. Without a heatsink one or more ICs will reach internal-thermal-shutdown temperature within minutes, and prolonged operation in that state will eventually destroy them.

Heatsink options

Discrete bar heatsink

Aluminium bar across all three TO-220 tabs. Common Fischer / Aavid 6–10 K/W parts work. Cheapest option; needs M3 hardware and thermal paste.

Enclosure as heatsink

Use a metal 1590B (or larger). Drill three holes in the box wall, mount the ICs directly to the enclosure with M3 screws and thermal paste. The whole box becomes the thermal mass — works well and saves space.

Bolted-through approach

Mount the PCB so the IC tabs face an external metal panel. Bolt through the panel with insulating shoulder washers if any IC tab is electrically live (the TDA2003 tab is GND so this is generally safe).

The PCB component layout was specifically chosen so the three TO-220 mounting holes are accessible to a screwdriver or finger from above — making heatsink installation straightforward whichever option you choose.

Enclosure recommendation

The board itself fits comfortably in a Hammond 1590B (112 × 60 × 31 mm) with the heatsink-as-enclosure-wall scheme. For loud-listening builds, step up to a 1590BB or larger — the extra metal area handles heat better at sustained high output.

Speaker connection

Wire LS1 (signal) and LS2 (return) to a panel-mount 6.35 mm jack or banana posts. Use ≥ 22 AWG stranded wire for the run from PCB to connector. Confirm cabinet impedance before connecting — 8 Ω is the safe default; 4 Ω works but draws more current and stresses the IC tabs more thermally.

Usage Notes

The Power Amp accepts any line-level audio signal. With the gain set at 40 dB internally, you can drive it from:

Guitar preamp output

Direct from a tube preamp, modeller, or pedal chain. Most preamps deliver 200 mV–1 V peak — well within the Power Amp's input range. Pair with a 1×10" or 1×12" cabinet for a compact amp head.

iPod / phone / mp3 player

Direct from headphone or line out. Phone outputs are usually 0.5–1 V peak. Adjust source-side volume for a clean output — the Power Amp has no volume control of its own, so the source becomes your master level.

Mixer / interface line out

Standard −10 dBV (consumer) or +4 dBu (pro) outputs both work. With +4 dBu sources you may need to attenuate at the source — full +4 dBu is 1.7 V peak, near the upper limit of clean operation.

Pedalboard final-stage amp

Drop it into the end of any FX chain to drive a speaker without a separate amplifier head. Especially useful with the All-in-One signal processor (MIC/LINE B output) for a complete amp-less stage rig.

Modifications to consider

Goal	Change	Effect
More bass	C9: 470 µF → 1000 µF or 2200 µF	LF gain corner from 154 Hz down to ≈70 Hz or ≈33 Hz — full bass response into a full-range cabinet
Less gain (line out)	R4: 220 R → 47 R	Gain drops from 101× (40 dB) to 22× (27 dB). Useful if you only need a buffered line driver, not full speaker drive
Add input attenuator	Add 100 kΩ A-log pot at IN	Gives a master volume — useful when feeding from sources that don't have their own volume control
Higher output power	Use 12 V supply + 7818	Doubler reaches ≈22 V, regulator gives 18 V — full 10 W into 4 Ω possible

You've built a serious power brick: 10 W into 4 Ω from a wall wart. Have fun, make noise, and respect the thermal limits — keep an eye on heatsink temperature for the first half-hour of any new build.

—

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.