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  • tecnico0104 commented on balsuryana's instructable DIY Smartphone Notification Lamp3 months ago
    DIY Smartphone Notification Lamp

    Not all audio amplifiers need a decoupling capacitor at it's output. I will mention three designs: first: using a split power supply (v+ and v- voltages relative to GND) if the output use complementary transistors one of the speaker lead is attached to the two emitter junction of the output transistors and the other speaker's lead is attached to GND, At rest, with no input signal, the two emitter junction is at 0 V relative to GND. As each half amplifier is designed for class B operation (or better class AB operation for less crossover distortion) one transistor manages the positive semicycle of the signal an the other transistor manages the negative semicycle. Even though you are feeding DC voltages to the load (the speaker) it sees an acts as if it is (and REALLY IS) AC. This design ...see more »Not all audio amplifiers need a decoupling capacitor at it's output. I will mention three designs: first: using a split power supply (v+ and v- voltages relative to GND) if the output use complementary transistors one of the speaker lead is attached to the two emitter junction of the output transistors and the other speaker's lead is attached to GND, At rest, with no input signal, the two emitter junction is at 0 V relative to GND. As each half amplifier is designed for class B operation (or better class AB operation for less crossover distortion) one transistor manages the positive semicycle of the signal an the other transistor manages the negative semicycle. Even though you are feeding DC voltages to the load (the speaker) it sees an acts as if it is (and REALLY IS) AC. This design is used in high power audio amps, not probable in smartphones. The second design, uses a similar thinking as the first, but with a trick:, as the amplifier is powered by a single power supply ( v+ and GND) as you mention earlier, with no input signal, at the output you will have v/2. The trick here is that if you build and active voltage divider with an output at exactly v/2 you can attach one speaker lead here and the other speaker lead to the amp's output, as the two speaker terminals are at V/2, you got 0 V betwen speaker terminals at no signal input. At last but not the least, the for the third design, you can use a BTL (Brigde thru load) design. This design uses two amplifiers to drive one speaker, each speaker lead is connected at each one of the amplifier's output. The input signal is feed to both amplifiers, but one is in-phase and the other is 180 degrees out of phase. With no input signal both amplifier outputs are at v/2 but the speaker see 0 V thru it's terminals; with input signal one amplifier output swings toward GND and the other amplifier output swings toward V+, these swings alternate between amplifiers, so the speaker sees effetively, AC. This design, though, has a drawback to be used in smartphones, Can you tell which one it is?

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  • tecnico0104 commented on balsuryana's instructable DIY Smartphone Notification Lamp3 months ago
    DIY Smartphone Notification Lamp

    Point 1 is relative: Example: a CRT anode voltage can be as high as 30 kV (kilovolt) but only few microamperes, if you short this voltage to earth nothing happen at the high voltage side of the flyback, instead you blown fuses at the low voltage side. By the way, fuses blown by amperage not voltage (indeed the voltge drop trhu a fuse before blow can be less than .1 volt). Point 2 its far less than 99.9 % off cases an audio output is after an capacitor.Point 2.1 You oversigth that when you apply an audio voltage to the earphones, the amplifier see an impedance, which can be tenfold the DC resistance of the earphones, obviusly, if you apply a DC voltage to an earphone you blown itPoint 3 is wrong, 2000 mA is 2 Amp.In solid state design you must take into account current first, voltage sec...see more »Point 1 is relative: Example: a CRT anode voltage can be as high as 30 kV (kilovolt) but only few microamperes, if you short this voltage to earth nothing happen at the high voltage side of the flyback, instead you blown fuses at the low voltage side. By the way, fuses blown by amperage not voltage (indeed the voltge drop trhu a fuse before blow can be less than .1 volt). Point 2 its far less than 99.9 % off cases an audio output is after an capacitor.Point 2.1 You oversigth that when you apply an audio voltage to the earphones, the amplifier see an impedance, which can be tenfold the DC resistance of the earphones, obviusly, if you apply a DC voltage to an earphone you blown itPoint 3 is wrong, 2000 mA is 2 Amp.In solid state design you must take into account current first, voltage second.

    Point 1 is relative: Example: a CRT anode voltage can be as high as 30 kV (kilovolt) but only few microamperes, if you short this voltage to earth nothing happen at the high voltage side of the flyback, instead you blown fuses at the low voltage side. By the way, fuses blown by amperage not voltage (indeed the voltge drop trhu a fuse before blow can be less than .1 volt). Point 2 its far less than 99.9 % off cases an audio output is after an capacitor.Point 2.1 You oversigth that when you apply an audio voltage to the earphones, the amplifier see an impedance, which can be tenfold the DC resistance of the earphones, obviusly, if you apply a DC voltage to an earphone you blown itPoint 3 is wrong, 2000 mA is 2 Amp.In solid state design you must take into account current first, voltage sec...see more »Point 1 is relative: Example: a CRT anode voltage can be as high as 30 kV (kilovolt) but only few microamperes, if you short this voltage to earth nothing happen at the high voltage side of the flyback, instead you blown fuses at the low voltage side. By the way, fuses blown by amperage not voltage (indeed the voltge drop trhu a fuse before blow can be less than .1 volt). Point 2 its far less than 99.9 % off cases an audio output is after an capacitor.Point 2.1 You oversigth that when you apply an audio voltage to the earphones, the amplifier see an impedance, which can be tenfold the DC resistance of the earphones, obviusly, if you apply a DC voltage to an earphone you blown itPoint 3 is wrong, 2000 mA is 2 Amp.In solid state design you must take into account current first, voltage second.

    I'd like to do some precisions: first it's not voltage who would fried either the phone's output or LED, it's current. Second, I doubt it's the battery full batery voltage at the audio output even at full power. Try a smartphone's earphone at an audio system and feed an audio signal 3.2 V amplitude and you'll see what happen to the earphones. Third: taking as example a red LED (one of the less electrically exigent LED) for a good illumination level it need an voltage of about 1.8 Volts at 20 mA. My question will be if the smartphone's output its capable of supply such 20 mA?

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