Therefore, the transformation ratio of inter line transformer is 120:20, 120:10 and 120:2.8 respectively, and the power is the same as the rated power of loudspeaker.
2、 Constant resistance power amplifier
The output current of the constant resistance power amplifier is large and the voltage is low. It is a high-quality power amplifier, such as home use, KTV entertainment places, vehicle mounted power amplifier, which is suitable for direct cooperation with standard impedance speakers. The impedance of the horn is usually 4-8 Ω. The sound quality is good, and the transmission limit is within 100 meters. Most of these amplifiers are dual channel, and now there are a large number of more channel surround amplifiers.
The total load impedance of the constant resistance power amplifier must match the output impedance of the power amplifier. The total power of the load can be greater than the output power of the power amplifier.
Impedance matching is a common working state in power amplifier technology, which reflects the power transmission relationship between input circuit and output circuit. When the circuit realizes impedance matching, the maximum power transmission will be obtained. Conversely, when the circuit impedance mismatch, not only can not get the maximum power transmission, but also may cause damage to electronic components. Impedance matching is common between amplifier circuits at all levels, between amplifiers and loads, etc. For example, impedance matching must be achieved between the output circuit of the power amplifier and the loudspeaker. In case of mismatching, the output power of the power amplifier and the loudspeaker will not be sent to the loudspeaker. If the impedance of the loudspeaker is far less than the output impedance of the power amplifier, the power amplifier is in an overload state, and its final power amplifier tube is easy to be damaged. On the contrary, if the impedance of the loudspeaker is too much higher than the output impedance of the power amplifier, it will cause the output voltage to rise, which is also not conducive to the work of the power amplifier, and the sound will also produce distortion. Therefore, the closer the output impedance of the power amplifier circuit is to the impedance of the loudspeaker, the better. It is better that the load impedance is equal to the output impedance of the power amplifier. At this time, the maximum output power is also the best state.
In physics and electricity in middle school, we once talked about such a problem: when an electric appliance with resistance R is connected to a battery pack with electromotive force E and internal resistance R, under what conditions will the power output of the power supply be the largest? When the external resistance is equal to the internal resistance, the power output of the external circuit of the power supply is the largest, which is the power matching of the pure resistance circuit. If you change to AC circuit, you must also meet the condition of r=r in order to match the circuit. At this time, the output power is the maximum, which is also the best state.
If the fixed resistance power amplifier is used improperly, it may burn out the power amplifier or loudspeaker, which is mainly manifested in overload and light load (or no load), that is, the impedance and power mismatch. Constant resistance power amplifier requires constant impedance of load. When the input signal is constant, the output voltage changes greatly with the change of load.
The output impedance of constant resistance power amplifier often has 4 Ω, 8 Ω, 16 Ω, 25o Ω, etc., which can be divided into low impedance output and high impedance output. When the power amplifier is used alone in the classroom or venue, it usually adopts low resistance output; High impedance output or constant voltage output is generally used in school cable broadcasting lines. When low impedance output, it can be directly connected with loudspeaker; In case of high impedance output, the speaker shall be indirectly connected through the transformer between lines.
Matching principle of constant resistance power amplifier and loudspeaker:
1. The total power of the loudspeaker shall be equal to or slightly less than the rated output power of the power amplifier, and the total rated power of the loudspeaker must be greater than or at least equal to the output power of the power amplifier, so as to prevent the loudspeaker from being damaged due to overload. Generally speaking, the "total load power" refers to the sum of the power obtained by each speaker, rather than the sum of the rated power of each speaker.
2. After the loudspeaker is connected, the total impedance of the load line should be equal to the output impedance of the power amplifier. If the conditions are limited, the difference should not exceed 10%. At this time, the total impedance of the load line should be slightly greater than the output impedance of the power amplifier, but not less than its output impedance.
3. The power of each loudspeaker shall not exceed its rated power, generally 75% - 80% of the rated power. Although the sound is lighter, it has less distortion and long service life.
The matching between the constant resistance power amplifier with low impedance output and the loudspeaker
The matching between the constant resistance power amplifier with high impedance output and the loudspeaker
In the case of high impedance output, line to line transformers are generally used for "impedance matching". The secondary impedance of the transformer between lines is the same as that of the loudspeaker, and the primary impedance is determined according to the actual needs of the line; The power of the inter line transformer is the same as the rated power of the loudspeaker.
When the total input impedance of the loudspeaker is not equal to the output impedance of the power amplifier, it is necessary to add an inter line transformer. The relationship between the output impedance r of the power amplifier and the primary input impedance r of the inter line transformer is obtained from p=u2 / R: R in = P out r out / P in.
High impedance output is usually connected to speakers in parallel.
Example: the school has a 100W power amplifier with an output impedance of 250 Ω; There are three 25w16 Ω speakers and two 12.5w8 Ω speakers. How to choose the line to line transformer?
Solution:
P total = 3 × 25+2 × 12.5 = 100(W)
R25 = 100 × 250 / 25 = 1000(Ω)
R12.5 = 100 × 250 /1 2.5 = 2000(Ω)
Therefore, the power amplifier and loudspeaker should be connected as shown in the figure below.
If the power amplifier with constant resistance is connected to the speaker with constant voltage, the result is mainly that the volume is low. If the power amplifier with constant voltage is connected to the speaker with constant resistance, the result is a serious mismatch, which is very easy to damage the power amplifier or speaker due to overload. Generally, the long-distance broadcasting system uses constant voltage output, and the short-distance sound system uses constant resistance output.
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