250W Amplifier: High Power and Quality Sound Performance

The 250W amplifier is a high-quality and high-power device that has successfully passed field tests for a long time. This amplifier can be used with two 8 ohm speakers or one 4 ohm speaker in parallel. It exhibits excellent performance with low harmonic distortion and high sound quality.

Gain Adjustment and Technical Details

Gain refers to the sensitivity to the input signal and is adjusted using a voltage divider consisting of two resistors. In addition, a sample is taken from the output and input signals and directed to the non-inverting input of the TL071 at the output of the amplifier. Gain is provided by the 33K resistor connected in series with the 1K resistor.

If the sound level in your amplifier is too low without using a pre-amplifier, you can increase the 33K resistor value up to 100K. However, if you are using a pre-amplifier, you do not need to make any changes to the 33K resistor value.

Note: Since the sound output of devices such as phones and computers is usually high enough, there is no need to use a pre-amplifier in most cases.

Protection Circuit

This amplifier design includes a transistor protection circuit that protects the amplifier against speaker failures. The 2N5551 and 2N5401 transistors stop the power transistors in case of excessive current draw. However, this protection system is not fast, so it is recommended that you do not test the output by short-circuiting it.

Quiescent Current Bias Setting

You should make the bias setting according to the voltage information specified in the diagram. When making the setting, short-circuit the audio input and do not connect a speaker to the amplifier output.

Output Transistors

Sanken 2SC3858 and 2SA1494 are used as output transistors. However, classic transistors such as 2SC5200 and 2SA1943 can also be preferred instead of these transistors. Alternatively, MJL21194 and MJL21193 transistors can also be used. With a good heat sink, there will be no problems with these transistors. This way, the PCB size was kept small and the cost was reduced. In addition, the original of Sanken transistors is hard to find and the price is high.

Power Supply Transformer and Speaker Selection

The maximum voltage of the transformer recommended for the amplifier should be 2x46VAC and the minimum power should be 300W. In high power tests, two 8 ohm 12 inch 300W speakers were used in parallel. For full power usage, 4 ohm RMS 400W...500W speakers are recommended.

Fake Opamp Problem

In the previous "200W 500W Amplifier Strong Bass Clean Sound" project, I stated that the circuit did not work with the TL071 opamp and that I used the LM741 instead. The same problem was encountered in this project. The same problem occurred in my second order and the opamp turned out to be fake. It is important to be careful not only for the TL071 but also for other integrated circuits with the same logo.

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