Power Amplifier Circuit Using Two 2N3055

60 Watt Power Amplifier Circuit Using 2N3055 Transistors: Build Guide

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Discover how to build a 60-watt power amplifier circuit using two 2N3055 transistors. This step-by-step guide covers components, wiring, and tips for optimal audio performance.

60 Watt Power Amplifier Circuit

Circuit Working

This is a Class AB push-pull amplifier with a pre-amplifier stage, driver stage, and output stage:

  1. Power Supply:
    • Dual supply: ±45V (likely from a transformer, rectifier, and filter capacitors).
    • Filter capacitors (100µF) are placed across the supply rails to reduce ripple.
  2. Input Stage (Pre-Amplifier):
    • Q1 (2SA733): A PNP transistor configured as a common-emitter amplifier.
    • The audio input signal is fed through a 100nF coupling capacitor, with resistors (33kΩ, 100kΩ, 470Ω) setting the bias and gain.
    • The 100µF capacitor at the emitter stabilizes the DC bias.
  3. Driver Stage:
    • Q2 (BD139): An NPN transistor that amplifies the signal further and drives the output stage.
    • Q3 (BD140): A PNP transistor working with BD139 to provide the necessary drive current for the 2N3055 transistors.
    • Diodes (D3, D4: 1N4007) in the driver stage bias the output transistors to reduce crossover distortion.
  4. Output Stage (Push-Pull):
    • Q4, Q5 (2N3055): Two NPN power transistors in a Darlington-like push-pull configuration.
    • The 0R5 (0.5Ω) resistors at the emitters provide thermal stability and current sharing.
    • The output is coupled to the speaker (8Ω) through a 100nF capacitor for high-frequency stability (Zobel network).
  5. Feedback and Stability:
    • A feedback loop (22kΩ resistor) from the output to the input stage stabilizes the gain and reduces distortion.
    • The 1kΩ resistor and 100µF capacitor at the input stage set the low-frequency response.
  6. Output Power:
    • With ±45V rails, the peak output voltage is ~40V (accounting for losses).
    • Power = V²/R = (40²)/(8) ≈ 200W peak. RMS power is ~100W into an 8Ω speaker, which aligns with the 2N3055’s capability.

Component Specifications

  1. Transistors:
    • Q1: 2SA733 (PNP, 50V, 150mA, hFE ~200) – Pre-amplifier.
    • Q2: BD139 (NPN, 80V, 1.5A, 12.5W) – Driver.
    • Q3: BD140 (PNP, 80V, 1.5A, 12.5W) – Driver.
    • Q4, Q5: 2N3055 (NPN, 60V, 15A, 115W) – Output stage.
  2. Diodes:
    • D1, D2: Not specified, but likely 1N4007 (1A, 1000V) for polarity protection or biasing.
    • D3, D4: 1N4007 – Biasing diodes to reduce crossover distortion.
    • D5: 1N4007 – Protects BD140 from reverse voltage.
  3. Resistors:
    • R1: 100kΩ, 0.25W – Input bias.
    • R2: 33kΩ, 0.25W – Bias for Q1.
    • R3: 470Ω, 0.25W – Emitter resistor for Q1.
    • R4: 1kΩ, 0.25W – Sets input stage gain.
    • R5: 540Ω, 0.25W – Collector load for Q1.
    • R6: 2kΩ, 0.25W – Bias for BD139.
    • R7: 4k7Ω, 0.25W – Bias for BD140.
    • R8: 22kΩ, 0.25W – Feedback resistor.
    • R9: 220Ω, 0.5W – Bias for the output stage.
    • R10, R11: 0R5 (0.5Ω), 5W – Emitter resistors for 2N3055.
    • R12: 22Ω, 0.25W – Zobel network resistor.
  4. Capacitors:
    • C1: 100nF, 50V – Input coupling.
    • C2: 100µF, 25V – Emitter bypass for Q1.
    • C3: 22µF, 50V – Coupling between stages.
    • C4, C5: 100µF, 63V – Power supply decoupling.
    • C6: 100µF, 63V – Feedback capacitor.
    • C7: 100nF, 50V – Zobel network capacitor.
  5. Speaker:
    • 8Ω, 100W – Suitable for the output power.
  6. Power Supply (Inferred):
    • Transformer: 32-0-32V, 5A (to provide ±45V after rectification).
    • Rectifier: Bridge rectifier (e.g., 4x 1N5408, 3A, 1000V).
    • Filter Capacitors: 4700µF, 63V (x2 for +45V and -45V rails, not shown in the diagram but necessary).

Insulation Details for Efficiency and Safety

  1. Power Supply Insulation:
    • Transformer: Use a 32-0-32V, 5A transformer (similar to your earlier 12-0-12V design but scaled for higher voltage).
      • Windings: Primary (230V, ~800 turns, 28 SWG), Secondary (32-0-32V, ~110 turns per side, 14 SWG).
      • Insulation: 3 layers of polyester tape (0.05 mm each) between primary and secondary. Ensure 4 mm clearance for 230V isolation (IEC standards).
      • Varnish the transformer for moisture resistance.
    • Rectifier and Capacitors: Mount on a PCB with 2 mm trace spacing for ±45V rails to prevent arcing.
  2. Circuit Board Insulation:
    • Use an FR4 PCB (1.6 mm thickness) with 1.5 mm trace spacing for ±45V to avoid arcing.
    • Apply conformal coating to the PCB to protect against moisture and dust.
    • High-current traces (e.g., to the speaker) should be at least 3 mm wide to handle ~5A RMS.
  3. Component Insulation:
    • 2N3055 Transistors: Mount on a heat sink with mica insulators and thermal grease to isolate them electrically while ensuring thermal conductivity.
      • Heat sink rating: ~2°C/W (each 2N3055 dissipates ~30W at full load; max junction temp 150°C).
    • BD139/BD140: If heat-sinked, use mica insulators as well.
    • Use heat-shrink tubing on exposed high-voltage connections (e.g., rectifier diodes, if added).
  4. Safety Features:
    • Add a 1A slow-blow fuse on the transformer primary.
    • Include 5A fast-blow fuses on the ±45V rails.
    • Ground the transformer core and PCB ground plane to prevent floating potentials.
    • Add a thermal fuse (e.g., 100°C cut-off) near the 2N3055 transistors to prevent overheating.

Efficiency Optimization

  • Class AB Efficiency: The push-pull configuration with biasing diodes (D3, D4) achieves ~60–70% efficiency, minimizing crossover distortion.
  • Component Selection: 2N3055 can handle 15A and 115W, well-suited for ~100W output. BD139/BD140 drivers handle the drive current efficiently.
  • Low Ripple: Add 4700µF capacitors (not shown) to keep ripple below 100 mV for clean audio.
  • Heat Management: The heat sink for 2N3055 should dissipate ~60W total (30W each). Ensure proper ventilation.

Summary

  • Circuit: Class AB push-pull amplifier with 2N3055 output transistors, ±45V supply, ~100W into 8Ω.
  • Components: 2SA733 (pre-amp), BD139/BD140 (drivers), 2N3055 (output), 1N4007 diodes, resistors/capacitors as specified.
  • Insulation: Polyester tape for transformer, FR4 PCB with conformal coating, mica insulators for transistors, 4 mm clearance for 230V.
  • Safety: Fuses (1A primary, 5A rails), grounded core, thermal fuse.
  • Efficiency: ~60–70% with proper biasing and heat sinking.

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