Description

This solid-state push-pull single-ended Class A circuit is capable of providing a sound comparable to those valve amplifiers, delivering more output power (6.9W measured across a 8 Ohm loudspeaker cabinet load), less THD, higher input sensitivity and better linearity. Voltage and current required for this circuit are 24V and 700mA respectively, compared to 250V HT rail and 1A @ 6.3V filament heating for valve-operated amplifiers. The only penalty for the transistor operated circuit is the necessity of using a rather large Heatsink for Q2 and Q3 (compared to the maximum power delivered).In any case, the amount of heat generated by this circuit can be comparable to that of a one-valve amplifier. An optional bass-boost facility can be added, by means of R5 and C5.

Circuit diagram:How to build 5 Watt Class-A Audio Amplifier Circuit - circuit diagramParts:
  • P1 = 47K
  • R1 = 100K
  • R2 = 12K
  • R3 = 47K
  • R4 = 8.2K
  • R5 = 1.5K
  • R6 = 2.7K
  • R7 = 100R
  • R8 = 100R
  • R9 = 560R-1/2W
  • R10 = 1R-1/2W
  • Q1 = BC560
  • Q2 = BD439
  • Q3 = BD439
  • C1 = 10uF-63V
  • C2 = 10uF-63V
  • C3 = 47uF-25V
  • C4 = 100uF-35V
  • C5 = 150nF-63V
  • C6 = 220uF-25V
  • C7 = 220uF-25V
  • C8 = 1000uF-25V
  • SPKR = 5W-8R Speaker
Notes:
  • If necessary, R2 can be adjusted to obtain 13V across C8 positive lead and negative ground.
  • Total current drawing of the circuit, best measured by inserting the probes of an Avo-meter across the positive output of the power supply and the positive rail input of the amplifier, must be 700mA. Adjust R8 to obtain this value if necessary.
  • Q2 and Q3 must be mounted on a finned Heatsink of 120x50x25mm. Minimum dimensions.
  • Add R5 and C5 if the bass-boost facility is required.
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