30 Watt Audio Power Amplifier Schematic
Including Preamp, Tone Controls, Reg dc Power Supply, 18 Watt into 8 Ohm - 30W into 4 Ohm loads
This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality. The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with 0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.
To obtain such a performance and to ensure overall stability of this very simple circuitry, a suitable regulated dc power supply is mandatory. This is not a snag because it also helps in keeping noise and hum of the preamp to very low levels and guarantees a predictable output power into different load impedance. Finally, as the amplifier requires only a single rail supply, a very good dc voltage regulator capable of supplying more than 2 Amps @ 40V can be implemented with a few parts also.
Circuit diagram:
Power Amplifier Parts:
R1 = 2K2 1/4W Resistor
R2 = 27K 1/4W Resistor
R3 = 2K2 1/2W Trimmers Cermet
R4 = 2K2 1/2W Trimmers Cermet
R5 = 100R 1/4W Resistor
R6 = 1K 1/4W Resistor
R7 = 330R 1/4W Resistors
R8 = 330R 1/4W Resistors
C1 = 22µF 25V Electrolytic Capacitor
C2 = 47pF 63V Polystyrene or Ceramic Capacitor
C3 = 100µF 50V Electrolytic Capacitors
C4 = 100µF 50V Electrolytic Capacitors
C5 = 2200µF 50V Electrolytic Capacitor
Q1 = BC550C 45V 100mA Low noise High gain NPN Transistor
Q2 = IRF530 100V 14A N-Channel Hexfet Transistor (or MTP12N10)
Q3 = IRF9530 100V 12A P-Channel Hexfet Transistor (or MTP12P10)
Setting up the Power Amplifier:
1. The setup of this amplifier
must be done carefully and with no haste:
2. Connect the Power Supply
Unit (previously tested separately) to the Power Amplifier but not the Preamp:
the input of the Power Amplifier must be left open.
3. Rotate the cursor of R4
fully towards Q1 Collector.
4. Set the cursor of R3 to
about the middle of its travel.
5. Connect a suitable
loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
6. Connect a Multimeter, set
to measure about 50V fsd, across the positive end of C5 and the negative
ground.
7. Switch on the supply and
rotate R3 very slowly in order to read about 23V on the Multimeter display.
8. Switch off the supply,
disconnect the Multimeter and reconnect it, set to measure at least 1Amp fsd,
in series to the positive supply (the possible use of a second Multimeter in
this place will be very welcomed).
9. Switch on the supply and
rotate R4 very slowly until a reading of about 120mA is displayed.
10.
Check again the voltage at the positive end of C5 and readjust R3
if necessary.
11.
If R3 was readjusted, R4 will surely require some readjustment.
12.
Wait about 15 minutes, watch if the current is varying and
readjust if necessary.
13.
Please note that R3 and R4 are very sensitive: very small
movements will cause rather high voltage or current variations, so be careful.
14.
Those lucky enough to reach an oscilloscope and a 1KHz sine wave
generator, can drive the amplifier to the maximum output power and adjust R3 in
order to obtain a symmetrical clipping of the sine wave displayed.
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