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What is load of an electronics system


In electrical terms a load is something that dissipates power and does some work. The work done may take many forms, including generating heat as almost always happens as a side effect of work being done. Without a load no power can be transferred. A speaker is the load for a power amp. In order for current flow to occur a complete circuit must exist. In order for the circuit not to be a short-circuit (a decidedly bad thing) a load must be present to the power the amp. The power amp drives power through the circuit by way of increasing the voltage at its outputs and as a result the load (speaker) draws current and does work. In this case two major forms of work occur: The speaker moves and generates sound, and heat is produced. Any device you plug into an electrical outlet can be considered a load (toaster, light bulb, etc). Plug in too many devices drawing too much current and you will “load down” the power delivery system (another bad thing). In order to protect against this power delivery systems have fuses and circuit breakers to break the circuit when current flow gets too high. Many power amps employ current limiting devices in their output stages to limit current flow without interrupting the audio. It’s sort of a self regulating protection system (back in the old days the amp just blew up). An important thing to understand is that a load will DRAW from an available pool of power all of the current it needs to operate at the given voltage. This is somewhat simplified, but in principle remains fundamentally true for all electrical systems. A speaker’s impedance rating is an indication of what kind of load it presents to an amplifier. An appliance’s current or amperage rating is exactly the load it will place on the electrical system. The reason a speaker cannot be rated in exact terms of current usage is because the voltage and frequencies presented to it constantly change. Impedance is a way of approximating a speaker’s resistance to a varying voltage and frequency signal.Also related to us is acoustical loading. The efficiency of a loudspeaker depends to some extent on the acoustic load placed on it by the way it couples to a cabinet and the surrounding structures. A speaker placed in the throat of a horn, for example, will see a higher acoustic impedance than a speaker placed in a free space.





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  1. admin

    RFSIR.COM – RF DESIGN , DEVELOPMENT AND SALES

  2. Shams

    Editor’s note-I’m leaving this quote in it’s enrtiety, but take the manufacturer’s claims with a grain of salt.I believe the freq variation in many of these cheaper Class D amps is a result of where the Feedback is taken. I only recently learnt this myself after reading a manual produced by a manufacturer here is the section out of the manual.Start quote Class A and class B amplifiers do not have output filters and so the feedback network isalways taken off from the output node of the amplifier. Class D amplifiers have a “monkeyon their backs” in that they ALL have to have an output demodulator filter to get rid of thehigh frequency carrier. Class D amplifiers are simply Pulse Width Modulated powersupplies where the modulation is the audio signal. How good or bad they sound dependson how the whole design is implemented.Our class D amplifiers are of the self oscillating type which has proved to be simpler andbetter sounding than driven types (Where a fixed clock at some high frequency is used asthe carrier). Analogous to FM or AM radio where the signal is transmitted at some highfrequency, your radio picks up this signal (Which contains both the carrier and the audio)and then finally demodulates ie. filters out the carrier leaving just the audio.The absolute vast majority of sub woofer class D amplifiers sold today come from Asiaand they typically use designs from one or two different companies. They all make oneerror, their feedback is taken BEFORE the output filter. What does this mean? Quitesimply the deficiencies of the inductor and capacitor in the filter will contaminate the signaland the response will vary with frequency.The reactance (AC resistance) of the coil which after all is in series with the speaker iscalculated from this formula Xl = 2 x Pi x F x L.Xl = Reactance = AC resistancePi = 3.14F = Frequency at which we want to calculate XlL = Inductance of the coil.Simple example: F is 100KHz and L is 150uH (150 micto Henry) typical of these amps.Xl = 94.2 ohms at 100KHzXl = 0.0942 ohms at 100HzXl = 0.0188 ohms at 20Hz.The 94.2 ohms is great because at 100KHz we want it has high as possible for maximumrejection of the carrier. However we see in the narrow range from 20Hz to 100Hz is hasvaried by a ratio of 5 as to 1. We have not included the simple DC resistance of the coilwhich must be added to the 0.0942 and 0.0188 numbers. A typical coil in one of theseamplifiers uses a mean turn length of about 50mm (2”) per turn and may have between 70to 100 turns of wire. #15 wire (whether made from a single strand or many strands if finerwire) has a DC resistance of 3.18 ohms per 307 metres (1000’). So we will have about 70x 2” of wire = 140” = 11.66 feet call it 12 feet for round numbers. Well 12 feet have a DCRof 0.038 ohms and this does not include the second series coil used in everyone of theseFar Eastern made amplifiers. So giving the benefit of the doubt to the second coil lets giveit a DCR of only 0.002 ohms so our total DCR of wire in SERIES with the speaker is 0.04ohm. Well now what do we have here?Page 4MYTHS, MAGIC AND FACTSWhat we have is a big old amplifier with a 0.04 ohm “resistor” in series with the output.The DCR of the wire on the inductors is absolutely equal to a resistor with regards tostraight DCR. The output impedance of the amplifier is at best 0.04 ohms and this doesnot take into account the output stage’s resistance which typically is at least 0.01 to 0.02ohms. Rounding off let’s go for 0.05 ohms and this is at 20Hz. At 100Hz the value is 0.134ohms.Now I am not a big fan of the Damping Factor hoopla. What I object to is the claim of DFvalues for these “mega watt” and other class D offerings of 300, 500 and 1000 etc. At bestthe is 0.05 + 0.0188 (do not forget the AC impedance of the coil)= 0.0688 ohms. Divide 4 by 0.0688 and we have a DF of 58 with a 4 ohm load, 29 with a 2ohm load and just 14.5 with a 1 ohm load. at 4 ohm is 4/0.134 = 29.8, at2 ohm it is 14.9 and at 1 ohm 7.46 .mmm interesting is it not.As the saying goes, “make my day”. Please read the specifications of all of theseamplifiers and I bet each and everyone will claim massive DF numbers and distortion inthe “double 00’s”. The THD of an uncompensated output filter is quite high.These Far Eastern class D amplifiers typically use a switching frequency of 100KHz orless. OK for sub woofer application but useless for full range (20Hz-20KHz) operation.Unfortunately even at 100KHz one has to use a pretty aggressive filter to attenuate thecarrier. The inductors range from 80 to 200 micro Henry and the capacitors from 22mfd to220 mfd.Zed is not the first to use post filter feedback but it solves all of the above issues to a greatdegree. Here is what we incorporate. The distortion of the filter is reduced by the feedbackfactor and in our amplifiers is typically about 15 times. The DCR of our output inductors isan order of magnitude lower than the above, even with Draconia having the skinniest wirewe h use less turns as our inductors are in the 22-30uH range. We use switchingfrequencies close to half a megahertz (500KHz) and so our filter parts are of lower values.Now any DCR in the inductor is taken care of by the feedback network and so does notappear as a series resistive element. Our DF values are modest even with reflectedinductor DCR of about 0.002 ohms.Finally we have low THD as the imperfections of the filter components are almosteliminated by the feedback being post filter. The frequency response of our class Damplifiers is flat from less than 10Hz to 25KHz within 0.1dB. end quote






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