8R/6.25W Loudspeaker Driven by Power Amplifier

8R/625W Loudspeaker Driven by Power Amplifier


The circuit was constructed for the purpose of producing a power amplifier that will be used by loudspeakers with rating of 6.25W/8R.

Terminology Power Amplifier – a type of amplifier built and designed for the purpose of delivering and supplying sufficient and maximum high output power to directly drive loudspeakers or loads, within a given percent of distortionNE5534 – a high performance low noise operational amplifier having a combined excellent AC and DC characteristics and features such as capability of external compensation, capability of offset nulling, wide range of voltage supply, low harmonic distortion, with output voltage swing at peak to peak, high slew rate, 100 dB common mode rejection ratio, high gain of DC Voltage, unity gain bandwidth at 10 MHz, and noise voltage of equivalent inputBD242 – an epitaxial-base silicon PNP transistor used for power amplifier and high-speed switching applications with features such as 3 MHz minimum frequency at 10V & 500 mA, 5A rated collector current, and 40W at 25°C case temperature Circuit Explanation

The amplifier used in the circuit produces negative feedback wherein the original signal is opposed by the fraction of the output of the amplifier that was combined with the input. In effect, the load current will not depend so much from the impedance of the loudspeaker but rather from the input signal. The current in the inductor of the loudspeaker produces a voltage as it combines with the resistor R7. This is the driving voltage to the inverting input of operational amplifier IC1, which is responsible for producing the negative feedback. The circuit’s total voltage gain will depend on the relation of loudspeaker impedance to the value of R7.

The output connection of IC1 in the ground is unusual since the current comes from the transistor Q1 while Q2 is not driven by the IC1 but by the supply lines. The current is established by R2, R3, R5, & R6 and has a value from 50 mA to 100 mA in the idle state, as it operates in class A. In this class, the bias current is greater than the maximum output current which makes all output transistors constantly conducting current. It has the least distortion and is the most linear class of amplifiers.

The operation of transistors Q1 and Q2 should be matched in order to prevent the creation of large voltage and current deviation. The balancing of bias can be achieved by changing the resistors R2 and R3, to be able to produce the current emitter around 500 mA by Q1 and Q2. for the 8R and 6.25W type of loudspeaker, the total harmonic distortion is 0.01%, while being supplied with 18 V.

Part List

R5-6=1ohm 1W
R7=0.5ohm 1W

C1=470nF 100V MKT
C2=22pF ceramic
C3-5=100nF 100V MKT
C4=220nF 100V MKT

RV1=100Kohm trimmer
M1=8ohm >20W Loudspeaker


In a transmission or output stage, power amplifiers are widely used as the last stage amplifier that requires most attention to efficiency of power, depending on the classes used. Some power amplifiers are versatile enough to drive speakers in stereo mode with different impedance ratings by allowing two separate internal amplifier circuit to bridge for producing higher power sound to one channel. Other applications of power amplifiers may also be found in radio waves and satellite communication, deep sea telephone cables, biomedical engineering, creation of electron tubes, and hi-fi phonograph systems.

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