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Questions and Answers on the amplifier (b)

In Electronic Infomation Category: Q | on April 15,2011

Q: amplifier specification is "RMS (root mean square power)" or "Peak (peak power)" to represent it?

A: Please avoid the fact that there is no misunderstanding ... "RMS power" of this assertion. This is indeed a frequently used term, it is usually understood as "cONtinuous" continuous power or a "long term" long-term power, but technically speaking, is not the case. So do not be understood as continuous RMS power. You can actually measure the waveform of any part of the RMS voltage (RMS voltage) or current value, in the long or short term.

RMS is used to represent accurately the AC voltage or current terminology, it helps to solve the measurement problem and BA1450S datasheet and the expression of AC. In contrast, DC is very easy to measure, and BA1450S price and measure the voltage of 9V batteries, 9V, and BA1450S suppliers and then measure again, still 9V, no change, except for some or other of the load caused by minor fluctuations or dips.

The other hand, the AC rapidly changing constantly, and your power acting as a 50 or 60 cycles per second, audio operation cycle from 20 to 20,000 times per second.

You can choose to measure many of the transient voltage, such as 20, -2, 92, 12,-29V., But they are incomplete as described in the AC signal is meaningful or accurate, you need to select a time period samples (the samples selected either continuous or as much as possible) and take their average, and to describe the meaning of its power.

Therefore, RMS stands for root mean square value, how it come from? First, it is the exchange of electrical signals, the above figure, for example, it is a simple sine wave (green curve) to zero as a starting point, +1 and -1 for the peak voltage it can also be a current, but for simplicity we will only it identified as voltage.

Power and voltage proportional to the square, for example, the load is a 0.5V voltage 1V twice, but it corresponds to four times the power, so we confirmed our first clue, the square of the voltage V2 with a red line .

Please note that the frequency doubling Zongshi Chengzheng Bi, Bing + 1 or 0 in the time peak. This curve is similar to the instantaneous power circuit. The size of the actual power will depend on current and voltage.

Next, we need to take power on average, this is our required value. In an analog measuring circuit, which can be integral circuit, a number of advanced AC RMS voltage meter lets you choose the integration time. In the digital signal processor, it can do a simple mathematical operations.

Approximate average power (the approximate average power was only analog) with purple curves. Please note that the initial fluctuations, the center of the V2 waveform gradually becomes flat and close to 0.5.

Now continue back to the voltage problem, we need to take the square root of average power approximations. We calculate the RMS voltage RMS, expressed by the blue line. May be noted that when the value for the 0.707 curve gradually flattened.

When you use the value of voltage multiplied by the current value, you will find the power of values. When you use RMS voltage multiplied by the RMS current, RMS power you seek is not a valid value - you seek the power of the average. So often we call the "RMS power" values ??should actually be called "continuous average power." The number of these specifications in the form of marked power amplifier power amplifier is reliable. Also, please do not misunderstand the specifications on the amp "peak" of the concept.

Q: Regarding the power of the specification "FTC" and the "EIA" mean?

A: The measurement of the power amplifier is the two most common methods. But years ago, a measurement known as IHF standard, which only involves clipping the amplifier when the load is 8.4 ohms or 2 ohms and the maximum measured RMS voltage.

Most important part of measurement is 20 ms, enter the nature of repetitive pulses with a 1KHz sine wave signal, and then decreased 20dB (in other words, 1% power) for 480 ms test, so the specification measure the head space margin, rather than continuous power, creating a lot of crappy amp manufacturers to this beautiful product data as boasting that his capital.

The good news is, IHF This measurement has been discontinued in today. The EIA is a more stringent standard method, which requires use of a constant amplitude in the frequency band signal measurements. Most amplifier manufacturers to choose 1kHz. Amplifier outputs a sine wave to the amplifier load marked on the index, such as 8, 4 or 2, rotating to amplifier gain until the clipping point.

Clipping point is defined as the total harmonic distortion sine wave reaches a specified level when the value, for example, 0.05%, 0.1% or 1%.

EIA standard drawback is that - apart from a limited spectrum, which is to allow manufacturers in the case of a load every time a single channel driver, which can disguise the shortcomings of amplifier power is weak, but not all vendors are drilling This vulnerability could take advantage.

FTC to be more stringent standards, in May 1974 was adopted, the U.S. Federal Trade Commission established for the protection of home entertainment equipment, methods of measurement. FTC testing standards need to use a sine wave within a specified frequency bandwidth of the test cycle, instead of using only a single band in the EIA or IHF. Although various indicators

nominal frequency range of amplifiers may be different, but often used by the test range 20Hz to 20kHz. In addition, all channels will drive the load at the same time, the power supply capacity will be very easy for power amplifier weak "leakage himself away."

But the most demanding standards of the FTC is that is must be pre-meet the following conditions, including: all channels to 1 / 3 power testing standards operate for 60 minutes, all channels are involved in testing, according to the nominal impedance, 1KHz test amplifier can not appear in the overheated during this period, automatically shutdown, current limit, or other problems.

Now, tell the truth, the manufacturer can not be in accordance with the published parameters to test each amplifier, power amplifier manufacturers have durability as a design will be part of the process, so we can know the power amplifier part of the index eligibility.

But if not through those rigorous testing requirements, you can not meet FTC standards. This is why there are a few drive 2 per channel amplifiers load can not pass through the EIA standard FTC standards, but the reason, although they usually 2 load may be normal operation. As with the EIA standards

, FTC nominal maximum power depends on the THD (total harmonic distortion) corresponds to the distortion level. THD chosen by the manufacturers typical value is 0.025%, 0.03%, 0.05%, 0.1% and so on.

However, "THD (total harmonic distortion standard)" option, you must come into effect until at 250 mW to reach the rated power level, the crossover distortion will become more apparent. FTC is a rigorous and demanding standards of measurement, but also has a drawback because it can not describe most of the power amplifier at 2 load run-time performance.

Q: I have a 2400W amp, but its manual says current when it 120W only 12 amps (110V). They are kidding?

A: No one in and you joke, it does have its reasons. You need to run your amp full power to meet peak and high transient response. Fortunately, for your power amplifier and power supply system, these peaks and transients will not last long.

Reserve power capacity has a good amplifier Keyi beyond short-term release of the AC power supply system load, but if the average power calculated over time, even in the brief few seconds, showing the actual power will be far well below the peak.

This is called the crest factor (also known as crest factor), different types of music and other program material so that it can be as high as 20dB, which means that, although to a 2400W peak clipping point, but only about 24W average power .

Compressed music get a low crest factor, while running slightly under clipping that can improve the average power. So even if a bass player for a long time playing the music loud and dynamic, if you use a compressor, the sound of the processing will be very easy.

Most of the security agencies tested using the 1 / 8 times the full power pink noise (and 9dB quite effective crest factor) is equivalent to the largest music typical level, without causing any distortion.

Security agencies has always been known for rigorous and conservative, conventional operation for the average user, the current consumption level in the 1/8th power measurement is a fairly safe indicators.

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