Category: audio amplifiers

How does a stereo convert 110 volt electric current into the positive and negative current that is sent to power the speakers? — JF

A stereo contains a power supply that converts 110-volt alternating current into lower-voltage direct current. This direct current is ultimately when powers the speakers. The stereo’s power supply first lowers the voltage with the help of a transformer. Alternating current from the power line flows back and forth through a coil of wire in this transformer, the primary coil, and causes that coil to become magnetic. Since the coil’s magnetism reverses 120 times a second (60 full cycles of reversal each second), along with the alternating current, it produces an electric field—changing magnetic fields always produce electric fields. This electric field pushes current through a second coil of wire in the transformer, the secondary coil, and transfers power to that current. There are fewer turns of wire in the secondary coil than in the primary coil, so charges flowing in the secondary coil never reach the full 120 volts of the primary coil. Instead, more current flows in the secondary coil than in the primary coil, but that secondary current involves less energy per charge—less voltage. In this manner, power is transferred from a modest current of high voltage charges in the primary coil to a large current of low voltage charges in the secondary coil.

Having used the transformer to produce lower voltage alternating current, the power supply than converts this alternating current into direct current with the help of four diodes and some capacitors. Diodes are one-way devices for electric current and, with four of them, it’s possible to arrange it so that the alternating current leaving the transformer always flows in the same direction through the circuit beyond the diodes. The diodes act as switches, always directing the current in the same direction around the rest of the circuit. The capacitors are added to this circuit to store separated electric charge for the times while the alternating current is reversing and the diodes receive no current from the transformer. The capacitors store separated charge while there is plenty of it coming from the transformer and provide current while the alternating current is reversing. Overall, the stereo’s power supply is a steady source of direct current.

What happens when a speaker blows?

A speaker produces sound by using magnetic forces to push or pull a thin surface—the speaker cone—toward or away from the listener. As the cone moves forward, it compresses the air in front of it and as the cone moves backward, it rarefies the air in front of it. These compressions and rarefactions are what produce sound. But if you try to drive the cone into motions that are too extreme by turning up the volume of an amplifier too high, the cone will reach the limits of its motion. At that point, the cone may tear away from the electromagnetic coil that pushes it back and forth or it may tear away from the supports at its outer edge. The electromagnetic coil may also burn up because of overheating. All of these failures are lumped together as “blowing a speaker.”

How does a car horn work? — CP

While some modern car horns are actually specialized computer audio systems, the old-fashioned electromagnetic car horns are still common. An electromagnetic horn uses an electromagnet to attract a steel diaphragm and turns that electromagnet on and off rhythmically so that the diaphragm vibrates. In fact, it uses the diaphragm’s position to control the power to the electromagnet. Whenever the diaphragm is in its resting position or even farther from the electromagnet, a switch closes to deliver electric current to the electromagnet. The electromagnet then attracts the diaphragm’s center. But when the diaphragm moves closer to the electromagnet, as the result of this attraction, the switch opens and current stops flowing to the electromagnet. Because of this arrangement, the diaphragm moves in and out and turns the electromagnet off and on as it does. The diaphragm’s tone is determined by the natural resonances of its surface.

What is a Zobel network in an audio amplifier and how does it work? Is it an effective device or not? — CV, Cape Town, South Africa

My understanding is that a Zobel network consists of a resistor in series with a capacitor and that the capacitor is normally connected to ground. When you attach the free end of this network to a wire carrying an audio signal, the network acts like a frequency-dependent load. At very low frequencies, the capacitor has plenty of time to charge through the resistor and the network has little effect on the audio signal—it acts as though it weren’t there. At very high frequencies, the capacitor has no time to charge through the resistor and behaves like a wire. As a result, the network acts as though it were just the resistor connecting the audio signal wire to ground. So the impedance of the Zobel network varies from infinite at low frequencies to become equal to the resistance of the resistor at high frequencies. The crossover between these two behaviors is related to the RC time constant. I think that Zobel networks are used in audio amplifiers to dampen out high frequency oscillations that might occur in the absence of loads at high frequencies.

How does a transistor amplify an input signal in an audio amplifier? — AR, Pierrefonds, Quebec

The answer depends a little on which type of transistor is used, so I’ll consider only an audio amplifier based on MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). One of these three-electrode devices allows a tiny electric charge on its gate electrode to control a substantial current flowing between its source and drain electrodes. In a typical amplifier, the current flowing in the input circuit is allowed to deposit or remove electric charge from the gate electrode(s) of one or more MOSFETs. This action dramatically changes how much current flows in a second circuit. This second circuit is ultimately responsible for the current that passes out of the amplifier and through the speakers that reproduce sound. As the current in the input circuit fluctuates to represent a particular musical passage, the charges on the gates of the MOSFETs also fluctuate and the MOSFETs vary the current through the output circuit and the speakers. Because MOSFETs are so sensitive to even a tiny amount of charge, it doesn’t take much current in the input circuit to cause large changes in the current of the output circuit.

What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. — DG, Houston, TX

In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a tape recorder’s microphone rises during one moment of a song, an electric current in the recorder will rise and a region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall, each value of air pressure is converted to a particular value of the physical quantity.

The problem with analog recording is that when the sound is recreated, any defect in the physical quantity representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces won’t be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by recording the information as bits. The physical quantity such as magnetization is representing bits (which take only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor changes in the physical quantity representing these bits won’t change the bits. Thus imperfections in the recording or playback process won’t affect the sound quality.

What is white noise? – AT

Acoustic “white noise” is a collection of random sounds that together have the same volume at every frequency or pitch. It’s defined more accurately as having the same amount of power in each unit of its bandwidth, so that the acoustic power between 20 and 21 cycles per second is the same as the acoustic power between 500 and 501 cycles per second.

Is it possible to sense when a person touches a car, even if the car is painted? – AW

Yes. I wouldn’t try to detect mechanical contact, because you’d have trouble differentiating between forces exerted on the car by a hand and those exerted on it by sound waves. But you can tell whether a conducting object (such as a person) is near the car by looking at the car’s electric properties. If you were to send electric charge on and off the car rapidly with a source of high-frequency alternating current, you would find that the amount of charge that flowed on or off the car during each cycle would change as the person’s hand approached the car. That’s because the charges on the car would push or pull on charges in the person’s hand and the charges in the person’s hand would move. In effect, the person’s hand would make the car “larger” and it would draw more charge from your current source. Even if the person didn’t touch the car, the nearness of the hand and car would change the way current flowed on and off the car. Such a change would be easy to detect with laboratory equipment and could probably be made by cheap consumer equipment, too. The only complications would be in not detecting everything—passing cars for example—and in not damaging the device with static discharges. Still, I think all of that could be done.

Is there a touch sensor that can sense when you touch the body of a car? – AW

The same touch sensors that are used in “touch” lamps or some elevator buttons could be used to sense when you touch a car. A car is essentially insulated from the ground by its rubber wheels, so that when you touch it there is a tendency for electric charge to be transferred between the earth and the car through you. That’s why you may receive a shock when you touch a car on a cold winter day. Many electronic devices are capable of detecting this charge transfer (in fact, many of them would be damaged by such sudden and large charge transfers). So building a car touch sensor would be easy. Whether there is a commercial product that does this is another matter, and I am not sure of the answer.

I recently visited an audio store where I saw electrostatic speakers. These speakers have no moving parts like conventional speakers and are more expensive. How do they produce sound? — BC, Ottawa, Canada

Electrostatic speakers uses the forces between electric charges (so called “electrostatic forces”) to move a thin metal diaphragm back and forth rapidly. The motions of this diaphragm compress and rarefy the air in front of it, producing sound. On each side of the diaphragm is a rigid metallic grill that can hold electric charges. When the speaker is silent, the diaphragm has a large positive electric charge on it and both the metal grills have large negative charges on them (it could be the other way around, depending the speaker’s exact design). The diaphragm is then attracted equally toward both grills and the electrostatic forces cancel perfectly. The diaphragm doesn’t undergo any acceleration. To make the speaker produce sound, the electric charges on the two grills are changed so that the electrostatic forces on the diaphragm don’t cancel. Instead, the diaphragm is pulled strongly toward whichever grill has more negative charge on it (or less positive charge). The charges on the grills fluctuate as the music plays and the diaphragm accelerates back and forth between the grills. It pushes on the air as it does and produces sound. You’ll notice that the diaphragm is a moving part, so the claim that the speaker has “no moving parts” is misleading. The speaker cone of a conventional speaker only moves back and forth, too, so it has an equal claim to having “no moving parts.” The relative expense of an electrostatic speaker comes from the requirement of careful construction and the need for a high voltage adapter to match an amplifier to the speaker.