Mini-box 2W Amplifier
Electronic Schematics > Audio > Mini-box 2W Amplifier


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

Circuit diagram:

2 Watt Amplifier


P1 = 10K


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 = 10µF

63V Electrolytic Capacitors

C3 = 100µF

25V Electrolytic Capacitor

C4,C7 = 470µF

25V Electrolytic Capacitors

C5 = 47pF

63V Ceramic Capacitor

C6 = 220nF

63V Polyester Capacitor

C8 = 1000µF

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




SPST switch




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.

Title: Mini-box 2W Amplifier
electronic circuit
Published on: 2005-02-01
Reads: 1678
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