Amplifier

High-Fidelity Audio Power Amplifier (Class AB)

📌 Project Overview

This project involves the complete design, simulation, and PCB layout of a Class AB audio power amplifier designed specifically for high-frequency speakers (tweeters). The system is built around the Texas Instruments LM3886 power amplifier IC and features active filtering using the OPA2134 SoundPlus™ audio operational amplifiers.

Key Features:

• Output Power: 60 W into an 8 Ω load.
• Amplifier Class: Class AB.
• Active Crossover (Filters):
  • High-Pass Filter (HPF) cutoff frequency: ~400 Hz.
  • Low-Pass Filter (LPF) cutoff frequency: ~20 kHz.
• Input: Standard 3.5mm audio jack (PC/Laptop sound card level).
• Volume Control: Integrated variable gain stage in the preamplifier.
• Protection: Built-in overvoltage, undervoltage, and SPiKe thermal protection (via LM3886).

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📐 Design & Specifications

Power Calculations

To achieve the target 60 W continuous output power for an 8 Ω speaker:

• Target speaker current: I_rms ≈ 2.74 A
• Target speaker voltage: V_rms ≈ 21.91 V
• Voltage gain required (for a 1 V input): G ≈ 21.9 V/V
• Power Supply: Symmetrical ±35 V DC.
• Calculated maximum output power: P_out ≈ 72 W (providing safe headroom for the 60 W requirement).

Component Selection

1. Power Stage (LM3886): Chosen for its high continuous average output power (up to 68 W), ultra-low distortion (0.03% THD+N), and robust SPiKe protection circuitry.
2. Pre-amp & Active Filters (OPA2134): Selected for its true FET-input, extremely low distortion (0.00008%), and excellent dynamic behavior tailored for high-end audio applications.

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💻 LTspice Simulation

Before moving to physical design, the entire circuit was simulated in LTspice to verify stability, frequency response, and power efficiency.

1. Frequency Filters (Bode Analysis)

• High-Pass Filter (Sallen-Key): 2nd-order active filter. Simulated cutoff frequency (f_c) ≈ 399.6 Hz with a stopband roll-off of -40 dB/decade.
• Low-Pass Filter: 2nd-order active filter. Simulated cutoff frequency (f_c) ≈ 20.8 kHz with a stopband roll-off of -40 dB/decade.

2. Transient & Power Analysis

• Simulated using a 1 kHz sine wave input.
• Input voltage: 1 V | Output voltage: ~32 V (peak).
• Verified the required 60 W average power on the 8 Ω load.
• Calculated Efficiency: η ≈ 71% (at maximum power output).

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🖨️ PCB Design (KiCAD)

The schematic was transferred to KiCAD for PCB layout. The physical layout was designed with strict analog audio guidelines:

• Signal Flow: Left-to-right signal path to prevent interference.
• Trace Widths: Calculated and adjusted for high-current power and output lines.
• Grounding: Large copper ground planes implemented to minimize noise and improve shielding.
• Thermal Management: Dedicated space and mounting holes provided for the LM3886 heatsink.
• I/O: Strategic placement of the 3.5mm jack, power terminals, and output connectors at the board edges.
• Form Factor: 2-layer FR-4 board, 50.8 x 48.26 mm.

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🛠️ Bill of Materials (BOM)

Key components used in this design (SMD 0805 for passives to maintain a compact footprint):

• ICs: 1x LM3886, 2x OPA2134
• Capacitors (SMD): 1 nF, 2.2 nF, 100 nF, 120 nF, 220 nF, 1 µF
• Resistors (SMD): 100 Ω, 366.9 Ω, 500 Ω, 1.8 kΩ, 2 kΩ, 3.9 kΩ, 6.8 kΩ
• Connectors: 3.5mm Audio Jack, Molex SL Screw Terminals
• Controls: 10 kΩ THT Potentiometer for volume control

📦 Manufacturing

The PCB was cost-analyzed and prepared for fabrication using JLCPCB specifications:

• Material: FR4-TG135 (1.6mm thickness)
• Copper Weight: 1 oz
• Surface Finish: HASL

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👨‍💻 Author

Davyd Tkalich
Electrical and Computer Engineering | Hardware Design Enthusiast
LinkedIn | Edmonton, AB, Canada