Questions and answers about the power amplifier

by:Gisen     2020-06-05
Q: Are the power amplifier specifications expressed by 'RMS (root mean square power)' or 'Peak (peak power)'? A: Please avoid this misunderstanding ... There is no such thing as 'RMS power' at all. This is indeed a frequently used term, it is usually understood as 'cONtinuous' continuous power or 'long term' long-term power, but technically speaking, this is not the case. So don't understand RMS as continuous power. You can actually measure the RMS voltage (effective value voltage) or current value of any part of the waveform, in the short or long term. RMS is a technical term used to accurately express AC voltage or current. It helps to solve measurement problems and express AC. In contrast, DC is very easy to measure, measuring a battery with a voltage of 9V, 9V, and measuring it again, it is still 9V, there is no change, except for some slight fluctuations or dips caused by loads or other effects. On the other hand, AC power is constantly changing rapidly. Your power cycle is 50 or 60 times per second, and the audio cycle is from 20 to 20,000 times per second. You can choose a lot of instantaneous voltages to measure, like 20, -2, 92, 12, -29V. But the AC signals they describe are not all meaningful or accurate, you need to select a sample within a period of time (so Choose samples that are either continuous or as many as possible) and take their average, and describe the meaning of their power. Therefore, RMS represents the root mean square value, how is it obtained? First of all, it is an alternating current signal. The above picture is an example. This is a simple sine wave (green curve) with zero as the starting point and +1 and -1 as the peak. For simplicity, we will only consider it as a voltage. The power is proportional to the square of the voltage. For example, a 1V load is twice the voltage of 0.5V, but it corresponds to four times the power, so we determined our first clue, the square of the voltage V2 is represented by a red line. Please note that its frequency has doubled, is always proportional, and peaks at +1 or 0. This curve is similar to the instantaneous power in the circuit. The actual power will depend on the current and voltage. Next, we need to take the average of the power, which is the value we require. In an analog measurement circuit, this can use an integration circuit. Some advanced AC RMS voltmeters allow you to choose the integration time. In the digital signal processor, it can do a simple number calculation. The approximate value of the average power (the approximate value is only the average power to be simulated) is represented by the purple curve. Please note that after the initial fluctuations, the V2 waveform in the center gradually becomes gentle and approaches 0.5. Now to go back to the voltage problem, we need to take the square root of the approximate value of the average power. What we find is the effective value of the RMS voltage, which is represented by the blue line. It can be noticed that when the value is 0.707, the curve gradually becomes flat. When you multiply the voltage value by the current value, you will find the power value. When you multiply the effective value of the voltage by the effective value of the current, what you get is not the effective value of the RMS power--you get the average value of the power. Therefore, what we often call 'RMS power' should actually be called 'continuous average power'. Amplifiers with these specifications in digital form are reliable amplifiers. In addition, please do not misunderstand the concept of 'peak' in the power amplifier specifications. Q: What do the power specifications 'FTC' and 'EIA' mean? Answer: These are the two most common methods of measuring power amplifiers. However, there was a standard called IHF measurement method many years ago, which only involved the maximum RMS voltage measured when the load was 8.4 ohm or 2 ohm when the amplifier clipped. The most important part of the measurement is to input a 1KHz sine wave signal with repetitive pulses within 20 milliseconds, and then drop 20dB (in other words, 1% power) for 480 milliseconds. Therefore, the specification measures the top of the head Space margin, rather than continuous power, has caused many crappy amplifier manufacturers to use this beautiful data as a boast of their product capital. It is gratifying that this measurement method of IHF has been discontinued today. The EIA standard is a more rigorous method, which requires the use of constant amplitude mid-band signals for measurement. Most power amplifier manufacturers choose 1kHz. The amplifier outputs a sine wave to the load indicated on the power amplifier index, such as 8Ω, 4Ω, or 2Ω, and the gain rotation reaches the power amplifier's clipping point. The clipping point is defined as the level at which the total harmonic distortion of the sine wave reaches a specified value, such as 0.05%, 0.1%, or 1%. The disadvantage of the EIA specification is that apart from the limited spectrum, it allows manufacturers to drive a single channel at a time under load, which can mask the disadvantage of weaker amplifier power, but not all manufacturers will drill this vulnerability. Loophole. FTC is a stricter standard and was adopted in May 1974. The Federal Trade Commission developed protection measurement methods for home entertainment equipment. The FTC test standard requires a sine wave to be cyclically tested within a specified frequency bandwidth, instead of using only a single mid-band EIA or IHF. Although the nominal frequency range of various power amplifiers may be different, the commonly used test range is 20Hz to 20kHz. In addition, all channels will drive the load at the same time, and the power amplifier with weak power supply capacity will easily 'leak out of the horse.' But the most demanding of the FTC standard is that the following conditions must be met in advance, including: all channels must be run for 60 minutes at 1/3 power test standard, all channels participate in the test, according to the nominal impedance test, 1KHz test The amplifier should not have overheating, automatic shutdown, current limitation or other problems. Now, to be honest, it is impossible for manufacturers to test each power amplifier according to the published parameters. Manufacturers have made the durability of the power amplifier as a part of the design process, so we can know whether some indicators of the power amplifier are qualified. But if you can't pass those harsh test requirements, you can't meet the FTC standard. This is why there are a few power amplifiers that can pass the EIA standard but not the FTC standard when driving a 2Ω load per channel, although they usually operate normally under a 2Ω load. Like the EIA standard, the FTC nominal maximum power depends on the distortion level corresponding to THD (Total Harmonic Distortion). The typical values ??of THD selected by the manufacturer are 0.025%, 0.03%, 0.05%, 0.1%, etc. However, the choice of 'THD (Total Harmonic Distortion Standard)' must be effective at 250 milliwatts. When the rated power level is reached, the crossover distortion becomes more pronounced. FTC is a rigorous and demanding measurement standard, but it also has a disadvantage because it cannot describe the performance of most power amplifiers when operating under a 2Ω load. Q: I have a 2400W power amplifier, but its manual says that it only has 12 amps (110V) at 120W. Are they kidding? A: No one is joking with you, this really makes sense. You need to let your amplifier run at full power to cope with peaks and large transient response. Fortunately, for your power amplifier and power supply system, these peak and transient changes will not last long. A power amplifier with good power reserve capacity can temporarily release power that exceeds the load of the AC power supply system, but if the average power is calculated for a period of time, even within a few seconds, the actual power shown will be far below the peak value . This is the so-called crest factor (also known as crest factor). Different types of music and other program materials make it as high as 20dB, which means that although the peak clipping point of 2400W is reached, the average power is only about 24W. Le compression can obtain a low crest factor, while running in a slight clipping state can also increase the average power a little. So even if a bass player plays loud and dynamic music for a long time, if the compressor is used, the sound processing will be very easy. The 1/8 times full power pink noise (equivalent to an effective crest factor of 9dB) used by most security agencies for testing is equivalent to the typical maximum music level and does not cause any distortion. Safety agencies have always been known for their rigor and conservatism. For users of general routine operations, the current consumption measurement level at 1 / 8th power is a fairly safe indicator.
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