This circuit is inexpensive, easy and quite simple. This is general audio amplifier which can be used for computer, CD player or other devices which have headphone output.
The circuit was deliberately designed using no ICs and in a rather old-fashioned manner in order to obtain good harmonic distortion behaviour and to avoid hard to find components. The amplifier(s) can be conveniently supplied by a 12V wall plug-in transformer. Closing SW1 a bass-boost is provided but, at the same time, volume control must be increased to compensate for power loss at higher frequencies.
In use, R9 should be carefully adjusted to provide minimal audible signal cross-over distortion consistent with minimal measured quiescent current consumption; a good compromise is to set the quiescent current at about 10-15 mA.
To measure this current, wire a DC current meter temporarily in series with the collector of Q3.
|P1 = 10K||Log.Potentiometer|
|R1,R2 = 33K||1/4W Resistors|
|R3 = 33R||1/4W Resistor|
|R4 = 15K||1/4W Resistor|
|R5,R6 = 1K||1/4W Resistors|
|R7 = 680R||1/4W Resistor|
|R8 = 120R||1/2W Resistor|
|R9 = 100R||1/2W Trimmer Cermet|
|C1,C2 = 10΅F||63V Electrolytic Capacitors|
|C3 = 100΅F||25V Electrolytic Capacitor|
|C4,C7 = 470΅F||25V Electrolytic Capacitors|
|C5 = 47pF||63V Ceramic Capacitor|
|C6 = 220nF||63V Polyester Capacitor|
|C8 = 1000΅F||25V Electrolytic Capacitor|
|D1 = 1N4148||75V 150mA Diode|
|Q1 = BC560C||45V 100mA PNP Low noise High gain Transistor|
|Q2 = BC337||45V 800mA NPN Transistor|
|Q3 =TIP31A||60V 4A NPN Transistor|
|Q4 = TIP32A||60V 4A PNP Transistor|
|SPKR||3-5 Watt Loudspeaker, 8, 4 or 2 Ohm impedance|
- Output power: 1.5 Watt RMS @ 8 Ohm, 2.5 Watt @ 4 Ohm, 3.5 Watt @ 2 Ohm (1KHz sinewave)
- Sensitivity: 100mV input for 1.5W output @ 8 Ohm
- Frequency response: 30Hz to 20KHz -1dB
- Total harmonic distortion @ 1KHz & 10KHz: Below 0.2% @ 8 Ohm 1W, below 0.3% @ 4 Ohm 2W, below 0.5% @ 2 Ohm 2W.