Mini-box 2W Amplifier

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

Circuit Diagram:

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 Resistor

C1, C2 - 10uF 63V Electrolytic Capacitors
C3 - 100uF 25V Electrolytic Capacitor
C4, C7 - 470uF 25V Electrolytic Capacitors
C5 - 47pF Ceramic Capacitor
C6 - 220nF 63V Polyester Capacitor
C8 - 1000uF 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 PNP Transistor

SW1 - SPST Switch

SPKR - 3 to 5 Watts 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 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 10-15mA.

To measure this current, wire a DC current meter temporarily in series with the collector of Q3.

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Techical data:
Output power:
1.5Watts RMS into 8ohm, 2.5Watts into 4ohm, 3.5Watts into 2ohm (1kHz sine wave)
Sensitivity:
100mV input for 1.5W output @ 8ohm
Frequency response:
30Hz to 20kHz -1dB
Total harmonic distortion @ 1kHz & 10kHz
lower than 0.2% @ 8 Ohm 1W,
lower than 0.3% @ 4 Ohm 2W,
lower than 0.5% @ 2 Ohm 2W.