3 - 5 Watt Class-A Audio Amplifier

In the old valve days, most commercial audio amplifiers suited for compact integrated mono or stereo record players used a one-valve amplifier topology. The circuit was usually implemented by means of a multiple type valve, e.g. a triode pentode ECL86.Common features for those amplifiers were: Class A operation, output power in the 3 - 5W range, input sensitivity of about 600mV for full output power, THD of about 3% @ 3W and 1KHz.Best types showed THD figures of 1.8% @ 3W and 0.8% @ 2W.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.









Parts:

P1_____________47K Log. Potentiometer (Dual-gang for stereo)

R1____________100K 1/4W Resistor
R2_____________12K 1/4W Resistor (See Notes)
R3_____________47K 1/4W Resistor
R4______________8K2 1/4W Resistor
R5______________1K5 1/4W Resistor (Optional, see Notes)
R6______________2K7 1/4W Resistor
R7,R9_________100R 1/4W Resistors
R8____________560R 1/2W Resistor (See Notes)
R10_____________1R 1/2W Resistor

C1,C2__________10µF 63V Electrolytic Capacitors
C3_____________47µF 25V Electrolytic Capacitor
C4____________100µF 35V Electrolytic Capacitor
C5____________150nF 63V Polyester Capacitor (Optional, see Notes)
C6,C7_________220µF 25V Electrolytic Capacitors
C8___________1000µF 25V Electrolytic Capacitor

Q1___________BC560C 45V 100mA Low noise High gain PNP Transistor
Q2,Q3________BD439 60V 4A NPN Transistors

SPKR___________One or more speakers wired in series or in parallel
Total resulting impedance: 8 Ohm
Minimum power handling: 5W



Technical data:

(measured on 8 Ohm resistive load unless otherwise specified)

Sensitivity:
230mV input for 1.5W output
380mV input for 3.5W output
560mV input for 5.6W output

Sensitivity with bass-boost:
400mV input for 1.5W output
630mV input for 3.5W output
850mV input for 5.6W output

Sensitivity with 8 Ohm nominal, loudspeaker cabinet load:
210mV input for 1.5W output
325mV input for 3.5W output
477mV input for 6.9W output

Frequency response:
100Hz to 20KHz 0dB; -3dB @ 40Hz

Frequency response with bass-boost:
+5dB @ 100Hz; +3.9dB @ 200Hz; +2.5dB @ 400Hz; -1dB @ 10KHz and 20KHz

Total harmonic distortion @ 1KHz:
0.3% @ 0.5W; 0.45% @ 1W; 1% @ 5.6W

Unconditionally stable on capacitive loads



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.