Build Your Own: Mini-Box 2W Amplifier Circuit Diagram

Designed for self-powered 8, 4 & 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 Trimmer Cermet
C1,C2 = 10uF 63V Electrolytic Capacitors
C3 = 100uF 25V Electrolytic Capacitor
C4,C7 = 470uF 25V Electrolytic Capacitors
C5 = 47pF 63V 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 4A PNP Transistor
SW1 SPST switch
SPKR 3-5 Watt Loudspeaker, 8, 4 or 2 Ohm impedance

Device purpose:

This amplifier was developed to be self-contained within a small loudspeaker enclosure. It can be powered by Walkman, Mini-Disc, and CD players, as well as computers and similar devices equipped with line or headphone outputs. Notably, a stereo configuration will necessitate the construction of two separate enclosures. The circuit’s design deliberately avoids the utilization of integrated circuits to achieve optimal harmonic distortion characteristics and to ensure the availability of components. The amplifier(s) are conveniently supplied via a 12V wall plug-in transformer. Activating SW1 enables a bass-boost function, simultaneously requiring an increase in the volume control setting to compensate for power loss at higher frequencies. During operation, R9 should be carefully adjusted to produce minimal audible signal crossover distortion, consistent with a minimal measured quiescent current consumption; an appropriate compromise would be to set the quiescent current at approximately 10-15 mA. To measure this current, a DC current meter can be temporarily connected 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

Integrated Circuits:

BC560C

The BC560C is a general-purpose NPN transistor widely used in amateur radio and low-power electronic circuits. This transistor exhibits low noise and high gain characteristics, making it suitable for amplification applications. It operates with a collector-emitter voltage of 45V and a continuous collector current of 100mA.

BC337

The BC337 is a general-purpose NPN transistor frequently employed in low-power circuits and audio amplifiers. It provides good stability and reliability, making it a dependable choice for a range of applications. This transistor can deliver an 800mA continuous collector current at a collector-emitter voltage of 45V.

TIP31A

The TIP31A is a high-current NPN transistor designed for driving inductive loads such as motors and relays. It offers robust current handling capabilities and features a continuous collector current rating of 4A at a collector-emitter voltage of 60V. It’s commonly found in switching power supplies and motor control circuits.

TIP32A

The TIP32A is a high-current PNP transistor designed for driving inductive loads similar to the TIP31A but with a PNP configuration. It can deliver a continuous collector current of 4A at a collector-emitter voltage of 60V. This component is often utilized in switching power supplies and motor control circuits, offering a complementary option to the TIP31A.

circuit from http://www.redcircuits.com/