Mini-box 2W Amplifier

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

Bass-boost switch

Circuit diagram:

2 Watt Amplifier


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__________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

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, iPod and CD players, computers and similar devices fitted with line or headphone output. Of course, in most cases you will 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 behavior and to avoid hard to find components. The amplifier(s) can be conveniently supplied by a 12V wall plug-in adapter.
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 into 8 Ohm, 2.5 Watt into 4 Ohm, 3.5 Watt into 2 Ohm (1KHz sinewave)
100mV input for 1.5W output @ 8 Ohm
Frequency response:
30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz & 10KHz:
<0.2% @ 8 Ohm 1W, <0.3% @ 4 Ohm 2W, <0.5% @ 2 Ohm 2W