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Efficient amplifier experiment

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Dr.EM

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I have read about digital amplifiers recently, and understand the concept. It seems like quite a neat idea. I designed this circuit off the top of my head just to experiment with the idea. It uses a frequency modulated 555 running at above 20khz, which drives a speaker via a driver then power transistor. I was amazed when this worked as intended straight off. The output is quite low; when you turn it up by raising the signal level at the input, distortion becomes evident very quickly, especially on low frequency signals. But with a 14v supply coming from batteries the output of higher frequency signals can be quite a respectable level.

As it is, i'm sure the circuit is quite pointless. Although the signal quality is ok, the passive filter i'm using is at 1khz, 12db, the woofer output from a 3 way crossover. Can anyone explain why the 3055 transistor gets fairly warm though (tiny heatsink, TO3 package)? I'm sure for the output level it gets as warm as any other amp. It must be only on or off? I even added inverters after the 555 to help square it, but it remains warm. Increasing the 100ohm base resistor lowers the temperature, but lowers ther output a bit too.
 

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Hi Dr. Em,
Doesn't pin 2 of the 555 connect to its pin 6?

The voltage drops of all the transistors add up and are a conspiracy against your idea:
1) The output high voltage of a 555 is about 1.2V less than its supply voltage.
2) The emitter high voltage of the little emitter follower transistor is about 1V less than its input high signal.
3) The emitter high voltage of the 2N3055 emitter follower transistor is up to 3V less than its input signal.
Therefore when the emitter of the 2N3055 is high, the transistor has up to 5.2V across it and 1.1A through it which is 5.7W for half the time.

Use the 555 to drive the gate of an N-channel Mosfet with its source grounded. Then the speaker between its drain and the 14V will burn-out with the entire 14V across it for half the time. :lol:
 
Yes, pin 2 and 6 are joined, oops, drew that quickly after I made it.

I see what your saying, the speaker i'm using might even be a 4ohm which probably doesn't help. Didn't know the 3055 had such a large voltage drop. I had noticed that the proper designs use Mosfets, I guessed the voltage drops would be why, I got to order some power Mosfets next time.

Not too worried about burning out my test speaker, its a 40w car thing anyhow, but why would it do that, because the square waves essentially give dc at the peak (though they will be rounded by the filter anyhow)? What you described would be a nice simple solution to a low power decent efficiency amp otherwise.
 
Hi Dr. EM,
Amplifiers don't give DC to a speaker. They have an output coupling capacitor or use a positive and a negative supply so the voltage swing at the speaker averages 0V.
DC on a speaker would drive its cone to the end of its travel where it would poorly reproduce sound, and the high current would burn it since with sound, it receives a low average power.
The 2N3055 transistor has a max current rating of 15A. But its min gain at only 10A is only 5 (2A base current), and its worst saturation voltage is 3V with a whopping 3.3A of base current. It is also pretty slow, 20kHz is hard for it. :lol:
 
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