- Aug 14, 2017 -
How to judge the noise source of the amplifier system?
How to measure the background noise of the amplifier? With the digital Multimeter's friends know that if the table in the MV file, the table pen is not connected to anything, the table head number has started to jump. The number of Beats is 3, 5 mv, and more dozens of MV. Is the accuracy of this multimeter so low that it cannot measure the parameters of the MV level? Not at all. As long as the two-sheet docking together, the table head immediately back to 0 stability. So what's the number of the beating? Because there is scattered around the space of stray electromagnetic interference signal, and the internal resistance of the multimeter is relatively large, to interference signals can be very sensitive to accept the reason. Once the table pen docking, the interference of the electromagnetic signal through the table pen short, then the multimeter display is stable back to 0 numbers, only to show that it can measure the minimum signal range. In turn, you see the amplifier, if you empty the input terminals and the volume to the maximum, then hear the noise, and just like the multimeter, there are quite a lot of space electromagnetic interference noise. Then you find a no wiring of the signal plug, the plug to the two ends of the short circuit, then what is heard? There may be nothing to hear or a very small noise. This is the background noise of the amplifier. Well, you might also ask, is it OK to turn the volume to a minimum? Yes, sometimes, for example, the volume potentiometer is at the front of the circuit. However, there is also the circuit volume potentiometer is not at the front, it may have other circuit components or wiring, you put it to the minimum, reflected the noise is only the potentiometer after the noise of the circuit. Therefore, the correct measurement method should be the input end of the short connection, the loudest noise is the amplifier's bottom noise.
Second, perhaps you will ask, that can put the amplifier's input impedance to 0, just like the multimeter's table pen docking, interference signal is not it? Is! But have you thought about it, and there's no signal coming in front of you? Therefore, the input impedance of the back amplifier can not be not without, low, the reception of space interference noise is smaller. So, the volume potentiometer used to 20k is a lot of people's practice. Of course, the potentiometer behind the amplifier circuit input impedance high one points or better, which is good for capturing weak signals.
Three, however, why the amplifier after the CD machine, even if the volume of the largest, noise will be very small? Because the internal resistance of the CD signal output is very low, the space electromagnetic stray signal on the signal line is shorted out by the output circuit inside the CD player. If the noise you hear is larger than the input of the amplifier, the big part is the background noise of the CD player.
Please note that the background noise of the CD machine is not as simple as it says. Because most CD player, especially the import of even low-grade CD machine, it in the signal output terminals in front of a point, there is a mute circuit, CD player stop playing time, the change of the song, the pause between the song long time Memory jump, the mute circuit is beginning to work. At this time the noise is small, not equal to the CD machine background noise is small. The same is true for CD headphone jacks when listening to headphones. Except for a few hours, CD player to hear when the noise there are two aspects, one is the CD chip grade level and circuit design level of noise, this is the CD machine bottom noise; another aspect is the noise on the recording, there is no way to do it.
Five, headphone machine sensitivity is high, of course, can hear more noise. But how much of this noise is the amplifier? According to the above, you can judge for yourselves.
Add, is the system equipment connection of the source of uncertainty noise. Sometimes, the system of each machine itself noise is very normal, once the system interconnection, there is a larger noise generated. Typically, this noise is a 50 or 100Hz pulsating component. The main reason is the potential difference between the various locations of the system. This potential difference is sometimes the effect of the circuit ripple on the location, sometimes the transformer leakage magnetic interference site potential. In this way, when the ground potentials in different systems are connected, the current of a ground loop is formed and the noise is generated.