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.
5 Watt Class-A Audio Amplifier Circuit diagram
Parts:
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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