Mini-box 2W Amplifier

Designed for self-powered 8, 4 & 2 Ohm loudspeakers
Bass-boost switch

Parts:
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
     
SW1     SPST switch
     
SPKR     3-5 Watt Loudspeaker, 8, 4 or 2 Ohm impedance



Device purpose:

    This amplifier was designed to be self-contained in a small loudspeaker box. It can be feed by Walkman, Mini-Disc and CD players, computers and similar devices having line or headphone output. Of course, in most cases you'll have to make two boxes to obtain stereo.
    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.

    


Technical data:

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.

from  redcircuits.com