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Low Frequency 20W Audio Amplifier

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Hi

Attached is a circuit of car siren which produces ear piercing sound when powered on.

I am interested in the amplifier section of the circuit which is simple and cheap. I need to amplify simple tones with frequencies less than 1kHz. I tried the circuit as shown attached but wasn't successful. Any suggestions to make the circuit work shall be appreciated.
 

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Hi,

I'd use either an class B or class AB linear amplifier like LM1875. Simple, cost effective.
Or - if efficiency is an issue - then use a class D amplifier.

Klaus
 

Don't expect quaility audio! The circuit just pulses current to the speaker but needs to be driven with pulses going from less than about 0.6V to peaks of more than 1.5V to operate. Too much low voltage or low period will damage the speaker.

Brian.
 

betwixt said:
Don't expect quaility audio! The circuit just pulses current to the speaker but needs to be driven with pulses going from less than about 0.6V to peaks of more than 1.5V to operate. Too much low voltage or low period will damage the speaker.

Brian.
Click to expand...

As I mentioned earlier, I need to amplify simple tunes (300-800Hz) generated by a 555 or 556. So the quality won't be a big issue, I think.
 

Just add a Mosfet on the 555's output, driving a speaker.
 

What Resistance DS (on) will be suitable?
Click to expand...

Your question isn't clear. If you mean Rds(ON), the resistance between a MOSFETs drain and source when fully conducting, it doesn't really matter. The lower the value the louder it will be but the difference in volume between one with 10 Ohms and one with 0.01 Ohms will be barely noticable.

Wire the speaker in the drain pin and ground the source pin or you will have difficulty driving it with a 555.

Brian.
 

Who says that the original crappy circuit produces an output 20W?
Assuming that the car battery is fully charged at 13.8V then the darlington transistors conduct with a voltage loss of about 1.5V leaving a peak-to-peak voltage of 12.3V across the speaker. 12.3V p-p is 4.35V RMS then the power in an 8 ohm speaker is 4.35V squared/8 ohms= 2.4W. Maybe you could say that the sound is a squarewave with an additional 2.4W of harmonics then the power is 4.8W, still not anywhere near 20W.
 

betwixt said:
Your question isn't clear. If you mean Rds(ON), the resistance between a MOSFETs drain and source when fully conducting, it doesn't really matter. The lower the value the louder it will be but the difference in volume between one with 10 Ohms and one with 0.01 Ohms will be barely noticable.

Wire the speaker in the drain pin and ground the source pin or you will have difficulty driving it with a 555.

Brian.
Click to expand...

Yes, I was referring to Rds(on) value in Ohms or mOhms.
Thanks for your input.

And sorry to CHEMELC for my un-clear question.

- - - Updated - - -

Audioguru said:
Who says that the original crappy circuit produces an output 20W?
Assuming that the car battery is fully charged at 13.8V then the darlington transistors conduct with a voltage loss of about 1.5V leaving a peak-to-peak voltage of 12.3V across the speaker. 12.3V p-p is 4.35V RMS then the power in an 8 ohm speaker is 4.35V squared/8 ohms= 2.4W. Maybe you could say that the sound is a squarewave with an additional 2.4W of harmonics then the power is 4.8W, still not anywhere near 20W.
Click to expand...

I was deceived by the rating on the speaker used it is "8 Ohms - 20W".
But with 2.4W or 4.8W, could the sound be excruciating loud?
 

It will be very loud!

Bear in mind the 20W rating on your speaker probably assumes two things:
1. The sound level isn't constant, for example as in music and,
2. The speaker cone travels in both directions because it is driven from AC.

The simple current switch can only provide current in one direction so half the potential sound pressure change is lost anyway.
The loudspeaker enclosure will make a huge difference to the volume, remember the cone displaces air as it moves and it is the pressure wave you hear. As the cone moves it produces air compression on one side and a pressure drop on the other so try to eliminate a path between them or the air will flow around the speaker instead of outwards.

Brian.
 

Some speakers produce 90dB with 1W at a distance of 1 metre. But the sound spreads out if the distance is increased then at 2m it is 84dB and at 4m it is 78dB. At 5m it is 72dB which is not loud.
At 5m, 2W makes 75dB, 4W makes 78dB and 8W makes 81dB which is fairly loud. 16W makes 84dB and 32W makes 87dB and 64W makes the original 90dB. Sometimes loud peaks in music produce 120dB, calculate how many thousands of Watts are needed.

Speakers and amplifiers made by cheap manufacturers multiply the actual maximum power of their products by the age of their grandmother.
 

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