Log In    |    Register
Home / News > Basic circuit knowledge in headphones

News

Basic circuit knowledge in headphones

First, introduce some common terms that describe the nature of the headset:

1) Working point: If the circuit to be analyzed is divided into two two-end networks A and B, the volt-ampere characteristic curves of the two networks are drawn in the same coordinate system. The intersection of the two curves is called a working point. The current and voltage values ??corresponding to the operating point are both the output current and output voltage of A and the input current and input voltage of B.

2) Impedance matching: Calculating the output power of the actual power supply shows that when i = Io/2, the output power of the power supply is maximum. The corresponding load resistance at this time is when the load resistance and the internal resistance of the power supply are equal, beats by dre studio the output power of the power supply is the largest, which is impedance matching. In an actual circuit, there is not much time to pursue impedance matching because although the output power is the largest at impedance matching, half of the power is consumed in the internal resistance, and the efficiency is too low. In order to improve the energy Cheap Beats Headphones utilization efficiency, and also to avoid the rear end of the load causing a relatively large impact on the front end, the input impedance of the rear end is generally several orders of magnitude larger than the output impedance of the front end.

3) Sound source: From the point of view of the circuit, the sound source is an active two-end network. If it is assumed that the frequency of the sound signal is fixed, then the sound source is a linear active two-end network, which can be described by the voltage source equivalent model. In order not to let the output signal of the audio source be affected by the back-end load, the output impedance of the audio source is quite low, generally only a few ohms or even less than 1 ohm, and the volt-ampere characteristic curve of the audio source is close to the ideal voltage source.

4) Amplifier: Under the condition that the frequency of the audio signal is fixed, the amplifier can be regarded as a linear active four-terminal network. The actual amplifier can be seen as two power supplies with internal resistance and limited operating range, where the output voltage is proportional to the input voltage within a certain range. It should be noted that for a four-terminal network, the impedance seen from the input can be different from the impedance seen from the output. In order to improve the efficiency from the energy use, while reducing the impact on the sound source, the input impedance of the amplifier is quite high, there are generally more than ten thousand euros or even tens of kiloohms. Therefore, the volt-ampere characteristic of the amplifier input is close to the ideal current source.

The output impedance of the amplifier should also be as small as possible, but due to the need to adjust the volume, the output impedance of the amplifier is adjustable. Adjust the output impedance, you can change the headset volume. Let the input voltage be Uo and the amplification factor be A. Then the maximum output voltage is AUo. The volt-amp characteristic at the output of the amplifier is a series of straight lines passing through a fixed point on the Y-axis.

5) Headphone: Assuming that the frequency of the audio signal is fixed, the headphone can be considered as a linear passive two-terminal network, which is equivalent to a resistor. The volt-amp characteristic curve of a headphone is the same as the resistance and is a straight line passing through the origin. According to different sounding principles, the earphone can be divided into three types: dynamic coil, piezoelectric, and electrostatic. (Electrostatic headphones have not been contacted before and they are not discussed). The principle of the dynamic earphone is to place the live coil in the magnetic field, and the coil is forced in the magnetic field to drive the diaphragm to sound. The amount of force applied by a live coil in a magnetic field is proportional to the current flowing through the coil. The greater the current, the greater the force. The principle of the piezoelectric earphone is to apply a voltage on both sides of the piezoelectric material to cause a deformation of the piezoelectric material, thereby driving the diaphragm to sound. The degree of deformation of the piezoelectric material is proportional to the voltage across the two surfaces, and the greater the voltage, the greater the deformation.

Then you can take a look at a complete headset system.

The volt-ampere characteristic of the source is close to the voltage source, and the volt-ampere characteristic of the amplifier input is close to the current source. If the operating point is within the amplifier's linear operating range, the maximum voltage at the output of the amplifier is directly proportional to the beats by dre studio output voltage of the audio source. If the impedance of the amplifier output is fixed, the operating point of the headphone is as shown in the figure.

It can be seen from the figure that under the condition that the sound source and the headphone are not changed, the output impedance of the amplifier becomes smaller, the current and voltage at the operating point become larger, and the volume of the headphone accordingly becomes larger. In addition, under the condition that the sound source and the amplifier do not change, the higher the impedance of the earphone, the smaller the current at the operating point and the smaller the driving force obtained by the dynamic earphone. Therefore, if the sensitivity is equal, the higher the impedance of the earphone, the better.

Let's explain why the power marked by the Walkman is far less than the maximum rated power marked by the headphones. When the volume is large, it is still prone to break or even damage (especially low-resistance and high-sensitivity earplugs). The more standard walkman power representation method is: **mw+**mw(*.times.ohm), indicating that when the headphone impedance is *.times.ohm, the output power of the walkman is **mw per channel. However, the volt-ampere characteristic of the amplifier (output) is actually an area, and **mw+**mw (x ohm) is only a point on the graph. Knowing this point in the area covered by the volt-ampere characteristic of the amplifier (output) does not have any practical significance. Even if this is the maximum output power (ie, impedance matching) of the amplifier when the load is 16 ohms, the impedance of the amplifier can still be reduced, the operating point can continue to move up, and the corresponding output current, output voltage, and output power can continue. Increase. Therefore, even if the power of the portable tag is very small, it is still possible to damage beats by dr dre the headphones if the volume is turned upside down.

In addition, the phrase "promoting does not mean pushing good" can also be a reasonable explanation. Under the premise that the output impedance of the amplifier is fixed to Re, when the signal of the sound source changes, even though the current value of the operating point is relatively large, the magnitude of the change of the working point current is only 1/Re of the voltage change amplitude. If the sensitivity of the earphone is not high enough, the result will be: The volume of the earphone is large enough, but it is not sensitive to the change of the sound source signal. The specific manifestation in the sound is the loss of details, lack of control, and it will not be received (especially It is low frequency. The whole sound is a little fuzzy and it is imaginary.