An Analog Circuit Class-a Guitar Pedal Thats Cor, a ESP32 project

This project builds a Class-A analog guitar boost pedal with a character control that shapes the tone while increasing output level. The pedal features an external SEND/RETURN loop for inserting additional effects, all powered by a regulated boost-buck converter that accepts 9–18 V input. The circuit uses a single 2N3904 transistor in a carefully biased configuration to deliver smooth, natural amplification without digital processing.

The guide provides a complete wiring diagram, full parts list with sourcing guidance, and step-by-step assembly instructions covering power protection, audio signal routing, and final testing procedures. Builders will learn how Class-A amplification works in practice, how to implement a true bypass loop, and techniques for integrating protection circuits that prevent damage from reversed polarity or incorrect voltage supplies.

Wiring diagram

Gather all the parts

Qty	Component
1	2N3904 NPN Transistor 2N3904 General-purpose NPN bipolar junction transistor (BJT) in a TO-92 package. Rated for 60V collector-emitter voltage and 200mA continuous collector current with an hFE (gain) of 100–300. Commonly used as a low-side switch or signal amplifier driven by a microcontroller digital output pin via a current-limiting base resistor.
1	6.35 mm mono input jack 1/4 inch mono The socket where the guitar cable plugs into the pedal.
1	6.35 mm mono output jack 1/4 inch mono The socket that sends the final guitar signal to the amplifier.
1	Film coupling capacitor 100 nF film A non-polar capacitor that passes guitar audio into the transistor while blocking DC bias voltage.
1	Metal-film resistor 1 MΩ A resistor that supplies the upper half of the transistor bias voltage.
1	Metal-film resistor 1 MΩ A resistor that supplies the lower half of the transistor bias voltage.
1	Metal-film resistor 4.7 kΩ A resistor that sets the class-A transistor collector current and gain.
1	Metal-film resistor 1.5 kΩ A resistor that stabilizes the transistor current and reduces distortion.
1	Electrolytic bypass capacitor 47 µF, 16 V A polarized capacitor that raises the audio gain while the emitter resistor keeps the DC operating point stable.
1	Film coupling capacitor 100 nF film A non-polar capacitor that removes the transistor DC voltage from the boosted output signal.
1	Audio taper potentiometer 100 kΩ audio taper A knob that adjusts how much boosted signal is sent to the external-effect loop.
1	2.1 mm center-negative DC pedal jack 2.1 mm centre-negative Panel power socket for a regulated 9–18 V center-negative pedal supply.
1	Resettable polyfuse 150 mA hold, 300 mA trip A small resettable fuse that limits damage if the pedal supply is shorted.
1	Reverse-polarity P-channel MOSFET protection stage 30 V P-MOSFET ideal-diode circuit A small protection circuit that passes correctly wired pedal power with very little voltage loss and blocks reversed power.
1	9–18 V to regulated 18 V boost-buck converter module 18 V regulated, 150 mA minimum A regulated converter module that provides the same clean 18 V analog rail from either 9 V or 18 V pedal power.
1	Electrolytic supply capacitor 100 µF, 25 V A polarized capacitor that smooths the protected pedal power near the input.
1	3PDT latching footswitch stompbox 3PDT A nine-lug mechanical footswitch that directly selects bypass or the pedal circuit and switches the status LED.
1	Linear potentiometer 2 kΩ linear Gain Front-panel Gain control that adjusts the amount of transistor boost.
1	SPST Clean/Vintage toggle switch SPST on-off A two-position front-panel switch that selects clean response or a higher-gain vintage character.
1	Panel status LED red 3 mm LED A small light that shows when the active boost circuit is selected.
1	Metal-film resistor 8.2 kΩ, 1/4 W A resistor that limits current through the status LED.
1	JFET unity-gain output buffer module J201 source-follower module A preassembled JFET buffer module that gives the output a low impedance without adding gain.
1	Metal-film resistor 1 kΩ A small series resistor that works with the input capacitor to keep radio interference out of the boost stage.
1	C0G ceramic RF capacitor 220 pF, 50 V A small capacitor that drains radio-frequency noise from the guitar input before amplification.
1	6.35 mm mono SEND jack 1/4 inch mono A panel-mounted quarter-inch jack that sends the boosted guitar signal to an external effect pedal.
1	6.35 mm mono switched RETURN jack 1/4 inch mono, normally closed tip contact A panel-mounted quarter-inch jack that accepts audio back from an external effect and passes the dry boosted signal through when empty.

Assemble it in 6 steps

1. Mount the controls and four audio jacks

Fit the INPUT, OUTPUT, SEND, and switched RETURN quarter-inch jacks, DC jack, 3PDT footswitch, red LED, Gain control, Volume control, and Clean/Vintage toggle into the stompbox enclosure. Label SEND as “to external effect input” and RETURN as “from external effect output.”

Use an isolated plastic jack or insulating washers if the enclosure is metal and your grounding layout requires the jack sleeves not to touch the case.
A switched RETURN jack has a third signal lug: identify its normally-closed tip lug with the jack datasheet or a continuity meter before soldering.
Do not confuse SEND and RETURN: SEND feeds an external effect, while RETURN receives its output.
Do not let metal jack bodies touch uninsulated circuit-board pads; that can short the audio path to the enclosure.

2. Build the protected power section

On the PCB, wire the DC jack centre contact to ground and its outer sleeve through the 150 mA resettable fuse, reverse-polarity MOSFET stage, and bulk capacitor. Feed the protected supply into the regulated 18 V converter for the audio circuit.

Use capacitors rated 25 V or higher on every point that can see the protected input or the 18 V rail.
Keep converter wires and the bulk capacitor close together at one edge of the PCB, away from the INPUT jack and transistor base wiring.
Standard pedal power is centre-negative: the centre pin is ground and the outer sleeve is positive. Reversing these connections can damage the power section.
Use only a regulated 9–18 V pedal supply rated at least 150 mA; do not use an unregulated wall adapter.

3. Wire the boost and character control

Wire the active footswitch input through the 1 kΩ RF resistor and 100 nF input capacitor to the 2N3904 base. Add the two 1 MΩ bias resistors from 18 V to the base and from the base to ground. Wire the 4.7 kΩ collector resistor from 18 V to the collector. Wire the emitter through the 1.5 kΩ resistor and Gain control to ground. Connect the 47 µF capacitor through the Clean/Vintage switch so Vintage bypasses part of the emitter resistance and adds gentle saturation.

For a typical 2N3904 TO-92 package, viewed with the flat face toward you and leads downward, the leads are usually Emitter, Base, Collector from left to right; confirm the purchased part’s datasheet before soldering.
Keep the input jack, 1 kΩ resistor, RF capacitor, input capacitor, and base-bias parts physically close together.
The striped lead of the 47 µF capacitor goes to ground. A reversed electrolytic capacitor can overheat or fail.
Do not route the converter switching wiring beside the transistor base or input-jack trace, or audible whine may result.

4. Wire the external SEND and RETURN loop

Connect the boost output capacitor to the SEND jack TIP. Connect SEND TIP to the normally-closed TIP_NC lug of the RETURN jack; this passes the boosted signal onward when RETURN is empty. Connect the RETURN jack TIP to Volume control LUG 3. Connect both SEND and RETURN sleeves to ground. To use an external effect, run a patch lead from SEND to that effect’s input and a second patch lead from the effect’s output to RETURN.

With no plug in RETURN, the normal contact connects SEND to the Volume control path automatically.
An external effect keeps its own power supply; these SEND and RETURN jacks carry audio only.
Use a normally-closed switched mono RETURN jack. An ordinary mono jack will leave the pedal silent whenever no return cable is plugged in.
Turn down the external effect and the pedal Volume before connecting a new effect to avoid a sudden loud sound.

5. Finish the output, bypass, and status light

Wire the Volume control wiper to the JFET buffer input and wire the buffer output to the active-output contact on the footswitch. Wire one pole pair of the 3PDT so the guitar input goes either to the active boost input or directly to the output jack. Wire the second pole pair so the output jack receives either the buffered active signal or the direct bypass signal. Wire the LED long lead through the 8.2 kΩ resistor to protected positive supply, and use the third switch pole to connect its short lead to ground only when the effect is on.

Before closing the enclosure, use a continuity tester to confirm bypass connects INPUT tip directly to OUTPUT tip while the active signal is disconnected.
At 18 V, the 8.2 kΩ resistor keeps LED current modest; if the LED is too dim, use 6.8 kΩ but do not omit the resistor.
The LED long lead is positive and the short lead is negative; reversed LED wiring will not light.
Keep all jack sleeve, power-return, and circuit-ground connections solid; a missing ground causes hum or silence.

6. Inspect and test without an external effect first

With power unplugged, inspect every solder joint, confirm the electrolytic capacitor polarity, and check that no bare wires can touch the enclosure. Start with Gain and Volume low. Connect a regulated pedal supply, guitar, and amplifier, leaving SEND and RETURN empty; the switched RETURN jack should keep the active signal path working. Then test the loop using two patch leads and an externally powered effect.

Check the transistor collector DC voltage during prototype bring-up; it should sit near the middle of the 18 V rail for clean headroom.
If the loop loses sound only when RETURN is empty, recheck the normally-closed lug on the RETURN jack.
Never connect the pedal’s 9 V or 18 V rails to the unrelated ESP32 board shown in the workspace; it is not part of this analog pedal.
If you smell heating, see smoke, or the fuse trips, unplug power immediately and inspect the DC jack polarity and power wiring.

Review all connections

1. Connections between "dc_in" and "ESP32"

Function	dc_in	ESP32
`ground`	CENTER	GND
`power`	SLEEVE → Resettable polyfuse IN	EXT

2. Connections between "fuse1" and "ESP32"

Function	fuse1	ESP32
`power`	OUT → Reverse-polarity P-channel MOSFET protection stage VIN	EXT

3. Connections between "d_reverse" and "ESP32"

Function	d_reverse	ESP32
`ground`	GND	GND
`power`	VPROT	18V

4. Connections between "c_power_bulk" and "ESP32"

Function	c_power_bulk	ESP32
`ground`	-	GND
`power`	+	18V

5. Connections between "power_converter" and "ESP32"

Function	power_converter	ESP32
`power`	VOUT	18V
`ground`	GND	GND
`power`	VIN	18V

6. Connections between "jack_input" and "ESP32"

Function	jack_input	ESP32
`analog`	TIP → 3PDT latching footswitch IN_COMMON	EXT
`ground`	SLEEVE	GND

7. Connections between "switch_bypass" and "ESP32"

Function	switch_bypass	ESP32
`analog`	IN_EFFECT → Metal-film resistor A	EXT
`analog`	IN_BYPASS → 6.35 mm mono output jack TIP	EXT
`analog`	OUT_COMMON → 6.35 mm mono output jack TIP	EXT
`ground`	LED_COMMON	GND

8. Connections between "r_input_rf" and "ESP32"

Function	r_input_rf	ESP32
`analog`	B → Film coupling capacitor IN	EXT

9. Connections between "c_input_rf" and "ESP32"

Function	c_input_rf	ESP32
`analog`	A → Film coupling capacitor IN	EXT
`ground`	B	GND

10. Connections between "c_input" and "ESP32"

Function	c_input	ESP32
`analog`	OUT → 2N3904 NPN Transistor Base (B)	EXT

11. Connections between "r_bias_top" and "ESP32"

Function	r_bias_top	ESP32
`power`	A	18V
`analog`	B → 2N3904 NPN Transistor Base (B)	EXT

12. Connections between "r_bias_bottom" and "ESP32"

Function	r_bias_bottom	ESP32
`analog`	A → 2N3904 NPN Transistor Base (B)	EXT
`ground`	B	GND

13. Connections between "r_collector" and "ESP32"

Function	r_collector	ESP32
`power`	A	18V
`analog`	B → 2N3904 NPN Transistor Collector (C)	EXT

14. Connections between "q1" and "ESP32"

Function	q1	ESP32
`ground`	Emitter (E) → Metal-film resistor A	EXT
`data`	Collector (C) → Film coupling capacitor IN	EXT

15. Connections between "r_emitter" and "ESP32"

Function	r_emitter	ESP32
`ground`	B → Linear potentiometer LUG 1	EXT

16. Connections between "pot_gain" and "ESP32"

Function	pot_gain	ESP32
`ground`	WIPER 2	GND
`ground`	LUG 3	GND

17. Connections between "c_emitter" and "ESP32"

Function	c_emitter	ESP32
`analog`	+ → SPST Clean/Vintage toggle switch COMMON	EXT
`ground`	-	GND

18. Connections between "switch_voice" and "ESP32"

Function	switch_voice	ESP32
`analog`	VINTAGE → 2N3904 NPN Transistor Emitter (E)	EXT

19. Connections between "pot_level" and "ESP32"

Function	pot_level	ESP32
`ground`	LUG 1	GND
`analog`	WIPER 2 → JFET unity-gain output buffer module IN	EXT

20. Connections between "buffer1" and "ESP32"

Function	buffer1	ESP32
`analog`	OUT → 3PDT latching footswitch OUT_EFFECT	EXT
`power`	V+	18V
`ground`	GND	GND

21. Connections between "jack_output" and "ESP32"

Function	jack_output	ESP32
`ground`	SLEEVE	GND

22. Connections between "r_led" and "ESP32"

Function	r_led	ESP32
`power`	B → Panel status LED ANODE	EXT
`power`	A	18V

23. Connections between "led_status" and "ESP32"

Function	led_status	ESP32
`ground`	CATHODE → 3PDT latching footswitch LED_ON	EXT

24. Connections between "c_output" and "ESP32"

Function	c_output	ESP32
`analog`	OUT → 6.35 mm mono SEND jack TIP	EXT

25. Connections between "jack_send" and "ESP32"

Function	jack_send	ESP32
`ground`	SLEEVE	GND

26. Connections between "jack_return" and "ESP32"

Function	jack_return	ESP32
`analog`	TIP_NC → 6.35 mm mono SEND jack TIP	EXT
`analog`	TIP → Audio taper potentiometer LUG 3	EXT
`ground`	SLEEVE	GND

Deploy the firmware

main.cppOpen in Schematik

// This project is a fully analog guitar boost pedal with external SEND/RETURN jacks.
// No microcontroller is fitted; no firmware controls the audio path.
void setup() {}
void loop() {}