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Judgment of system noise sources

First, how to measure the amplifier noise floor? Anyone who has used a digital multimeter knows that if the watch is set to millivolt and the test leads are vacant without anything, the number on the header has begun to jump. The number of beatings is between 3 and 5 millivolts, and as many as several dozen millivolts. Is the accuracy of this multimeter so low that it is impossible to measure millivolt-level parameters? Not too. As long as the two pens are docked together, the header immediately returns to zero. So how did the numbers that beat just now come from? Because there are stray electromagnetic interference signals scattered around the space, and the resistance of the multimeter is relatively large, the interference signal can be received very sensitively. Once the pens are docked, these interfering electromagnetic signals are short-circuited by the test leads. When the multimeter displays the stable zero-return numbers, it shows how large the minimum signal range it can measure. Looking back at the amplifier, if you open the input terminal and turn the volume up to the maximum, the noise you hear at this time, just like the theory of the multimeter, has quite a lot of space electromagnetic interference noise. Then you find a signal plug without a connection and connect the two ends of the plug into a short circuit. What do you hear at this time? May not hear anything or very little noise. This is the noise floor of the amplifier. Well, you may also ask, isn't it possible to beats headphones on sale turn the volume down to a minimum? Yes, sometimes this may be the case, for example, the volume potentiometer is at the front of the circuit. However, some circuit volume potentiometers are not at the forefront, and there may be other circuit components or wiring in front of it. You turn it down to the minimum, and the reflected noise is just the noise of the circuit after the potentiometer. Therefore, the correct measurement method should be a short input terminal, and the noise at the maximum volume is the noise floor of the amplifier.

Second, maybe you will ask, that can not reduce the input impedance of the amplifier to zero, just like the tester of the multimeter, the interference signal is not there? Yes! Can you think about it, the signal transmitted in front of it is not there? Therefore, the input impedance of the back amplifier can not be beats earbuds absent. If it is low, the received space interference noise is smaller. Therefore, the volume potentiometer used to 20k is a lot of people's practices. Of course, the input impedance of the amplifier circuit behind the potentiometer is higher or better, which is good for capturing weak signals.

Third, but why, after the amplifier is connected to the CD player, even if the volume is turned to the maximum, the noise will be very small? Because the internal resistance of the signal output end of the CD player is very low, the spatial electromagnetic spurious signal sensed on the signal line is short-circuited by the output circuit inside the CD player. If the noise that you hear at this moment is shorter than the amplifier input, then this is the noise floor of the CD player.

Fourth, please note that the noise floor of the CD player is not as simple as the above. Because most CD players, especially those importing even the low-end CD players, have a mute circuit at the front of the signal output terminal. When the CD player stops playing, when the song is changed, the song pauses for a long time. When the memory jumps, the mute circuit starts working. The small noise heard at this time does not mean that the background noise of the CD player is small. The same is true when listening to headphones with a CD headphone jack. In addition to the above several times, the noise heard by the CD player when playing the sound has two aspects. First, the CD machine's chip grade level and the noise level determined by the circuit design level, this is the noise floor of the CD machine; the other side That is the noise on the recording. There is no way to do this.

Fifth, the headset has high sensitivity, of course, you can cheap beats by dre hear more noise. But how much of this noise is heard by amplifiers? According to the above, you can make your beats by dre cheap own judgment.

To add, it is the source of uncertain noise connected to the system equipment. Sometimes, the noise of each single unit of the system is normal. Once the system is interconnected, there will be more noise. Typically, this noise is a pulsating component of 50 or 100 Hz. The main reason is the potential difference between the various locations of the system. This potential difference is sometimes the impact of the circuit pulsating current on the location, and sometimes the location of transformer leakage magnetic interference. In this way, when the ground potentials in different systems are connected, a ground loop current will be formed and noise will be generated.