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From: Jon Kirwan on 5 Feb 2010 04:36 On Fri, 5 Feb 2010 00:33:53 -0500, "Paul E. Schoen" <paul(a)peschoen.com> wrote: > >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:iqomm597nf576piiskbpq3m14uuiheont3(a)4ax.com... >> >> Hmm. Totally new thoughts. So that's what the term >> "regulation" means. It's about the transformer design? And >> here I was off and away on the capacitive-filtered ripple >> side. Well, that's still useful to have gone back to, >> anyway. >> >> I'm not buying the 0.7V diode drop, yet. At peak currents >> near 10 times larger than average load currents, I have to >> imagine more than 0.7V drop with anything silicon and not >> schottky. Do they use schottky's? (Leakage comes to mind.) > >Silicon diodes are the norm except for high power, high efficiency, high >frequency, and low voltage. But they do have forward drops of 0.7 to 0.6 >volts at normal operating temperatures, and when drawing minimal current, >as is the case at the waveform peak under no load conditions. Even with a >capacitor, the diode current drops to near zero at the voltage peak. A >different result is expected if there is inductance, of course. I _do_ understand that the diode current drops to zero at peak. That's clear from the derivative term (cos() function.) The current into the capacitor is based upon dV/dt and if that is zero, so is the current. I think I'm gathering your point better now, though. I was talking about _under_ high current load, but you were ahead of me and addressing yourself to the much lower currents that take place _after_ the peak spike current occurs but _before_ it reaches the near-zero current at cos(pi/2), where the voltage continues to rise but the rate of change is slow and so only a little current is required to keep pace. Thanks for that clue. It makes sense and I missed thinking about it when I last spoke. I see it, now. >There is a separate regulation spec for the DC output. It is typically much >worse than the regulation of the transformer, as the capacitors quickly >discharge between peaks and can be charged up only as quickly as the >transformer and diodes allow during the conduction cycle. So we use big >capacitors and linear regulators, or resort to a switching supply. The two rails (prior to any linear reg) rise up and down (the ripple) in phase with each other, if I'm imagining it right. Which means that the (+) rail and the (-) rail will move in concert. Which makes me suspect that it may be okay for the output stage. What's your take on that? How much ripple on the rails is okay? >But if you are lucky enough to have three phase power, you can design a DC >supply with no capacitors and get something like 6% regulation (and >ripple). This is SOP for really high power DC, like 10kVA. That's way out of my league. >> Okay. So the 25V was specifying the peak, not the bottom >> side. And that is unloaded, basically. Which brings up the >> question of what exactly does 15% regulation _actually_ mean. >> What is the definition of "full load?" Since the peak diode >> currents can be quite a lot more than the average load >> current from my calculations, that seems to place quite a >> burden on the transformer ratings. > >Transformers are rated at RMS current, which is pretty much all that >matters for heating effect, and it is mostly related to the resistance of >the copper and the allowable rise in temperature in the core. Efficiency >aside, what matters is the temperature the insulation can withstand before >deteriorating, and usually that is at least 130C, or 100C above ambient. >The smaller the tranny, the better it sheds heat (surface area/volume), so >regulation and efficiency of smaller ones tend to be poorer. Still, my question seems to remain about the peak currents. If the peak diode current spike takes place say 30 degrees prior to the peak voltage and is 10X the average load current's needs, one cannot ignore that peak current having to come from a secondary that has ohmic resistance as well as the possible volt-second problems at 50Hz and 60Hz. Or? >Full load is just the maximum RMS current at which the transformer is >rated. This may be further complicated by duty cycle ratings, which can be >continuous or intermittent. Generally intermittent duty is 50% duty cycle, >with ON times not greater than 30 minutes, at least for larger transformers >with more thermal mass. At 50% duty cycle the output rating is 1.4 times >the true continuous rating. And then the allowable duty cycle is the >inverse of the square of the overload. For the circuit breaker test sets I >design, we specify output up to 10x the continuous rating, at which the >duty cycle is only 1%. But the ON time is limited to about 100 mSec, which >is more than enough to trip a circuit breaker instantaneously, and then you >should wait 10 seconds before doing it again. But here's the question, again. The currents via the bridge diodes are in no way very similar to sine waves. In fact, they almost suddenly rise up to follow the dV/dt requirement and then decay out somewhat later on and then very lightly (as you reminded me above) have any current requirement up to and perhaps shortly after pi/2. This is a complex waveform and not easily turned into rms. For example, the average current might be 1A and the RMS might actually be closer to 4A. Or with an average of 1A the RMS might instead be closer to 3A or 5A depending upon the capacitors and the phase and duration of the spike. That's got to give caution when thinking about these things. It would be very easy, without going carefully into the waveform itself, to make a misjudgment about it. And that still ignores _peak_ which might very well be 12A or 15A or more, despite the 3A rms or 5A rms figure. What drop will that represent? Seems a lot of factors to balance. >I designed a "Programmable Overload Device", or POD, which takes into >account the current and the time, as well as the actual temperature using a >thermistor, to enforce reasonable duty cycles. Fuses, circuit breakers, and >Motor Overloads do a similar function, but don't fully take into account >all the factors. The intelligence for this is buried in the PIC code, and >is rather involved and yet imperfect. If I could accurately model the >heating and cooling effects of current in a transformer, it would be ideal. >Now that's where theory can really help. :) >> So could you go further here? In other words, let's say I >> know that the average load current will be 1.4A, but that the >> peak diode current given the bridge/capacitor design will be >> 15A. The transformer is a 25.2Vrms CT unit. The DC rails >> are at -15 and +15, with 2200uF caps on each side to ground, >> and the ripple on them is about 3.8V peak to peak (+/-1.9V >> around 15V.) >> >> What's the VA rating here? And "regulation" number are you >> looking for in the transformer and how does it relate back to >> VA and other terms that might be used? > >It's really easier (and perhaps even more appropriate) to use a tool such >as LTSpice for this purpose. You could look at all the variables over time, >quantized to steps small enough to minimize error, and finally arrive at a >steady state solution where you may be able to describe such complex >entities as RMS current with an equation, but all you will have done is >spend a lot of time doing what LTSpice does so well and so quickly. So I >cobbed together a simple power supply simulation, which in this case models >part of a power supply that I have been using on my Ortmasters, with a >Signal 241-6-16 transformer. The ASCII file is at the end of this post. Thanks. >I'm using a voltage doubler circuit on each leg of the 16VCT transformer, >as I need to get at least 17 VDC for 15VDC linear regulators for the analog >portion of the circuit. I figure no more than 20 mA. So for simulation >puposes I use a 1k resistor as the load. The transformer is 32 VA, or 2A at >16V, and I estimate 15% regulation which is a 2.4 V drop at 16V or open >circuit 18.4 VRMS. I'm using a voltage source with 26 volts peak and 1.1 >ohms internal resistance. The capacitors are 220uF, and MURS120 diodes. As >a result, I get 22.35 VDC outputs, and the transformer current is 104 mA >RMS, with peaks of about 360 mA. > >Just for fun, I changed the output loads to 10 ohms, and I found that the >current is only 345 mA RMS, and the transformer current is 611 mA RMS, with >peaks of about 1 amp. The capacitively coupled design is inherently >current-limited, which can be a good thing. Okay. Well, I'll take a look. Are you using one of the two non-linear inductors used to model core saturation? >>>If you put a capacitor on the output, it eventually charges to the peak >>>voltage. This is the high limit that must be considered for design. It >>>may >>>not be exact, and probably will be a bit lower, because a power >>>transformer >>>is usually designed to operate in partial saturation, so the output will >>>not increase linearly above its design rating. >> >> Ah. Core saturation is __intended__ as part of the design? I >> haven't done that one before. What guidance can you give on >> that aspect? > >Maximum use of the iron occurs near the maximum flux density. It results in >increased current which actually occurs at 90 degrees to the applied >voltage, so the distortion is not in the form of a flattening of the >voltage waveform but rather like crossover distortion. But it does result >in a somewhat non-linear effect, as it interacts with the resistance of the >windings. See the following for more information: >http://openbookproject.net/electricCircuits/AC/AC_9.html Will do. >and more about regulation: >http://www.allaboutcircuits.com/vol_2/chpt_9/6.html > >It is most pronounced in ferroresonant transformers: >http://www.ustpower.com/Support/Voltage_Regulator_Comparison/Ferroresonant_Transformer_CVT/Constant_Voltage_Transformer_Operation.aspx Something new, again. Thanks. >>>Under load, the output will drop, caused by the effects of primary and >>>secondary coil resistance as well as magnetic effects. These will cause >>>heating over a period of time, and the coil resistance will increase, >>>adding to the effect until a point of equilibrium is reached based on the >>>ambient conditions and removal of heat via conduction, convection, and >>>radiation. >> >> Now that, I understand and worry about. > >That's why most designs are made with a generous safety factor so you do >not need to worry about these effects. They can be predicted approximately >and that is good enough. And I'm not designing to save every nickel. Which helps. I can see that the design issues become very peculiar when every shaved penny is desired while still preserving design function. Of course, there is more money available for the required extra design time then, too, I suppose. >> Hehe. I want to _learn_ to design to specified criteria, >> have a comprehensive view of the theoretical concepts >> involved, and that means I need to only pick the first one. >> The 'quickly' is unimportant -- one to two years is good >> enough. The 'cheaply' is equally unimportant. If it costs >> me 10 times as much in terms of parts and time as it would >> just buying something commercial, buying a commercial >> solution will teach me exactly zero about what I need to >> learn to design what my daughter needs. And there is NOTHING >> on the market to get there, either. No one else has my >> problem. Or few do. > >It might be worthwhile to discuss those details here to dig up some ideas. My daughter enjoys turning the volume to maximum on an amplifier and then curling up into a ball nearby or else running away from it and not coming back to change it downwards. We come in running and turn things down many times a day. (She turns them up again, sometime later on.) What I'd like is to limit the maximum volume for her -- but given any fixed output amplifier that is pretty easy to achieve, as I can insert dummy resistances in series with the speakers to get that (proper wattages, of course.) The other issue is that the volume needs to mute after some adjustable set time (on order of minutes), but only after the last time the volume control is touched. And any slight motion of it should restore the volume as last set. She knows how to work a knob and will quickly learn the behavior. I'm considering the use of an optical quadrature knob for this. Separately, I need to modify a microwave oven control. She destroys them, fairly routinely. So I have a nice collection of the transformers! But I also know that the magnetron and transformer and waveguide parts are fairly standardized, it seems. The controls are very custom, but that merely means I can design my own and fit that to the standard interface available for the magnetron section. Later project that will _also_ incorporate a temperature sensor system I'm working on right now that allows me to measure, in situ, in the oven chamber -- uses phosphor thermometry to get the job done and works well in this application. But that is more than a year out, right now. >> This is a "give a person a fish and they eat for a day, teach >> a person to fish and they eat for the rest of their lives" >> thing. > >I've heard it said that, "teach a man to fish, and he'll spend all day in a >boat drinking beer!" :) hehe. It's also said that for every saying there is an equal and opposite saying. One of those fundamental forces, you know? >> The digressions are great! I am NOT in a rush to build, >> though. I'm wanting to engage the math and learn what can be >> achieved by deducing from parsimonous theory. Then test a >> few things on the bench, ask questions, learn some more. Etc. >> So theory _and_ practical approaches are important. Not one, >> or the other, but both!! >> >> Pendulum motion is well understood. One might either have a >> practical knowledge about it and some tables and just go with >> that. Probably, lots of folks making pendulum clocks stop >> there and go no further and are none the worse for that. It >> is similarly very easy to develop the infinite series that >> describes it (or use the sqrt(L/g) proportionality as a first >> order approximation or for small starting angles) from the >> simple differentials involved and to take an entirely >> theoretical approach, as well. >> >> But I'm interested in more than that. Theory by itself lacks >> reality. Reality by itself lacks meaning sans theory. The >> two go together like hand in glove, though. Building even >> the most simple ones using a peg-in-hole method leads to the >> discovery of still more interesting effects, if you know some >> theory. For example, the rocking of the pin itself in the >> larger hole has a measurable impact of perhaps as much as 2 >> or 3 percent. It's useful to know that and understand it. >> Once that mechanism is itself understood, one can then dig >> even deeper to find more subtle (and possibly useful) effects >> to continue improvements. A practitioner lacking even the >> basic theory might accidentally happen upon some idea, of >> course. And a theoretician lacking practical reality to >> interfere might accidentally imagine some realistic effect to >> pursue, too. But it really takes a marriage of both to make >> quick work of progress forward, I think. >> >> Since theory is primary, I like to pursue that part of it >> earlier and move to experience once I have the mental tools >> required to make sense of the data that results. Without >> theory, data is pure noise. Without the theory of a sphere, >> even the gentle curvature at the horizon "seen" my a mountain >> climber is just so much useless noise to them. But _with_ >> that theory, the data _means_ much. > >I think I had problems in the EE program at Johns Hopkins because it was >too theoretical for my mindset, and I had fundamental problems with >advanced calculus. Which I find little other than a relaxing joy. >I aced the lab courses and helped others because I had >already designed and built many circuits. But, looking back, I see where >having a stronger grasp of theory would have helped. I still design >circuits with a highly empirical approach, using rule of thumb and >experience to choose components. Now that SPICE is freely available I find >it fascinating to try different values and placements and configurations >"just to see what happens". And I learn by looking at the time domain >simulation plots and determining what may have caused certain glitches or >oscillations that I did not foresee. What I'm going to say is NOT a judgment in any way. In fact, many of those I've enjoyed working with have had problems mastering math and theory and yet are extremely creative and they find the right crutches to help them where they are weaker and get the right job done, rightly. One I'm thinking about right now would use Excel at every turn, almost. But he had an instinct about _what_ to put into Excel and what questions to ask and what data to collect. I trusted his judgment and only very rarely grilled him to see if I could find holes. And his work habits were like mine -- work all hours, night and day, when people were depending upon us. I could call him up with some problem I was wrestling with some Sunday afternoon and he'd jump over to the lab, dropping everything else, and simply go, go, and go until we'd clobbered the problem together. We had each others' backs and I enjoyed that a lot. An interesting place where theory has served me well, personally, regarding one field of instrumentation. I was simply playing with some different ideas and mathematical derivations where I had no idea where they'd take me and landed upon a method that has so far been unexplored in the literature. It's self-calibrating for offset and gain because of the _shapes_ that reside in the manifold I was playing in and that means this can be made _cheaply_, as in a factor of 10 less than beforehand. I worked with one of the two physicists writing the seminal papers in the field (back in the 1950's) and I know that he didn't know about it. It's only possible to "see" it from a mathematical vision and it is incredible to see it perform in practice, auto- calibrating offset and gain. That can be applied as often as required to control for temperature and time drift, as well. It was a stunning insight which I am sure I could NOT have had with LTspice games. Not possible to hack into this insight. The odds against it are astronomically high. >My talents are more in the realm of imagination and thinking outside the >box. And sometimes it has gotten me into trouble. But I have also sometimes >been able to make a lot of progress in a short period of time. I think some >aspects of design are more of an art than a science, and I look for a sort >of elegance in the finished design of a circuit, even in the placement of >components on the schematic, and also in their placement on a PCB. I have some small imaginations from time to time (like the above one discovered while playing), but I enjoy drilling down into details. My wife is the other way round and has a stunningly brilliant way of combining things in new ways every single day and yet no willingness to painstakingly dig into them. Her insights aren't merely interesting, either. They are on-target. She is the one who saw this economic disaster heading our way and forced us to sell our properties "right away" in early 2005. We actually _made_ a fair amount of money in the last 5 years because she forced us to buy and sell oil at the right times and Euro-denominated securites, and so on. She is like a spider in a web who feels _real_ vibrations that mean something important and knows which to ignore, too. (I am still mystified by it. And I've known her from before I have any memory -- she is two years older and actually was asked to babysit me when I was little. So that long, at least.) Everyone has their strengths and we all need each other. There is no one right way to be and we each have to find our own paths. But it is really nice when you aer lucky enough to surround yourself with people with different penchants and skills, each having each others' backs and filling in where it counts without disingenuousness. We really do need each other in healthy complementary ways. None of us are whole, by ourselves. And thanks so very much by the way, Jon
From: Jon Kirwan on 5 Feb 2010 05:36 On Fri, 5 Feb 2010 14:47:16 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Thu, 04 Feb 2010 17:16:59 -0800, I wrote: >> >>> <snip> >>> Since theory is primary, I like to pursue that part of it >>> earlier and move to experience once I have the mental tools >>> required to make sense of the data that results. >>> <snip> >> >> Okay. On second thought... enough theory. I think it's time >> for practice. I already have triple output power supplies, >> but using them wouldn't be true to the actual amplifier >> situation. And any testing of distortions needs to cope with >> that reality. >> >> So I'm moving forward on the power supply rails. I need to >> scarf around and see what I have available. I'll post what I >> find, the resulting design and thinking, photos perhaps, and >> the results of testing with static loads. Once that is done, >> I'd like some advice about the next step, though. But until >> then, I'll just focus on getting that part put to bed. That >> much I can do right now. >> >> I've decided that your kick in the butt, Paul, was what I >> needed. I have enough in mind to move out of the thinking >> stage and into trying some different alternatives. I'll get >> going. >> >> Thanks, >> Jon > >There's this saying "Practice without theory is blind and theory >without practice is lame". I hope I made clear my recognition that _both_ are vital. If science processes hadn't caught wind of this centuries ago, we'd still be in the dark ages I suspect. >You've made it clear that you want to >thoroughly understand the hows and whys of amplifier design from >mathematical models. I have no quarrel with that approach and I >also use it myself within the limits of my own capability - *up >to a point*. But there comes a point at which striving for >absolute precision solely from theory results in diminishing >returns. Yes. As a neophyte, it's hard to know where that boundary is at, though. It's like the old software law. The first software attempt does far too little and there are many complaints and suggestions and flaws in it. The second attempt does way too much and is overburdened and complex almost to the point of uselessness. It's only by the time you've done the third one that you get it close to right. I'm trying to get past the first attempt and do too much for my second. Kind of a shortcut to getting to that sweet third one, I suppose. >Take the case of the pendulum you brought up earlier. The basic >theory is well established, but to predict the behaviour of a >practical pendulum with 100% precision will require taking into >account the effects of so many factors that it may well be >impossible. E.g., the aerodynamics of the pendulum's shape >including minute irreguarities on its surface, the exact strength >and orientation of the earth's magnetic field at the location and >its effect on traces of magnetic materials in the alloy, friction >with suspended particles in the air in addition to the air >itself, friction at the point of suspension and elasticity of the >suspension, etc., etc. Even if all these influencing factors are >included in the equation, the physical values to be entered can >never be measured with 100% accuracy. Theory let's you "see." Start with the basic pendulum theory with pendulums where angle=sin(angle), nearly. It allows you to observe such pendulum behavior with some understanding. The swings are not just random rockings back and forth or something that appears vaguely "regular," but now instead they are something to expect and recognize and "see." For a time, it's all you need. Then someone develops a better clock. Suddenly, you uncover the fact that various pendulums are NOT nearly as well predicted as they should be. They vary by 3% or so, which keeping the swing angles in good control is far beyond the tolerances of the theory. But without the prior theory, and better clocks, you wouldn't even notice. And now "seeing" you might start a new exploration to discover some new principles to add. And in doing so, you may now have a chance to discover yet another anomaly that resides still deeper yet. I think that was where I was headed, earlier. Countering by bringing up concepts of perfection (100%) really misses my point -- that theory is primary to developing meaning and understanding results. The fact that we do not have 100% knowledge or the ability to make infinitely precise measurements does NOT undercut it, at all. >Take the case of the forward drop of the diode in the power >supply that you've been discussing with Paul. This what I did >before personal computers and simulation progs became widely >available: I drew a curve of the diode's V-I characteristics on >graph paper up to the expected peak current. Then I drew a >straight line, approximately following the dynamic curve, from >the peak point down to the voltage axis. I took that voltage as a >constant forward drop and the slope of the line as a constant >series resistor. I then added that resistance to other source >resistances like the transformer winding resistance and either >use it to calculate the rectified and filtered voltage or, more >often, to determine it from a graph such as that in RDH. It also >comes in useful for finding the peak and rms currents. I don't >know if anyone else uses that method or how well it agrees with >theory, but it agrees pretty well with practical measurements. That is immediately obvious and makes good sense. The Shockley equation can be readily used in derivative form to predict the instantaneous resistance slope at any current you want. And the even simpler V=Vd+R*I equation often used for diodes (for current values near some nominal I value) works pretty well within those boundaries. The R can either be measured as you suggest or else, if the model parameters are available, easily derived and predicted with some expectation of closeness should it then _be_ measured. Your approach makes excellent sense to me. >I don't do this every time I design a power supply. I just make a >mental estimate based partly on theory and partly on past >experience. In short, there's a point at which it makes more >sense to make informed assumptions and approximations even before >doing physical construction. Which requires experience and practice. Before that is well acquired, it's good theory that best guides (if available.) I think I am ready regarding the power supply, though. And that is probably a good step. Except that I'm still questioning whether or not the rails can have ripple while the output stage is tied to them or if it really is necessary to bother with a crafted linear regulator, as well. (If I use a regulator, again it will have to be discrete parts and BJTs, but I think I can do that much, too, with four or five BJTs per side.) I guess another part of why I'm thinking through these things this closely, besides the fact that I don't have a lot of experience to rely upon and guide me, is simply that I'd like to "measure twice, cut once." Jon
From: Bob Masta on 5 Feb 2010 07:36 On Thu, 04 Feb 2010 12:07:16 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >There's another question that comes to mind regarding the >output stage. A lot of talk seems to revolve around >"crossover distortion." Seems almost very first thing folks >talk about when discussing class of operation if not also at >other times. > >Seems to me that in a three-rail power supply situation >without an output capacitor involved, the crossover takes >place near the midpoint (ground) voltage between the rails, >at a time when current into the speaker load is also near >zero. (I'm neglecting any thoughts about inductance in the >speaker and physical coupling into the air, for now.) In >other words, where power at the speaker is near zero. Is it >really that important to consider? >I was looking at that terrible large scale gain plot for the >quasicomplementary output stage on the web site recently >mentioned in the thread (the lower curve in Figure 4 on this >link): > >http://www.embedded.com/design/206801065?printable=true > >(It's not that terrible of a plot, as the variation is from >.96 to .98 with the "normal" middle at .97.) > >What's experience say here? Is it really so terrible as to >worry too much about something that takes place near zero >voltage, anyway? I'm just questioning the concern, for now. >I have no understanding about it, at all. Just wondering. > Crossover distortion is a more-or-less fixed amplitude, so at low signal levels it becomes a large percentage of THD. Our ears are sensitive to the relative amplitudes of components, so a hypothetical fixed amplitude distortion component might be totally inaudible when it is a low percentage of a large fundamental, yet be obnoxious as a larger percentage of a small fundamental. (This is the same essential problem as quantization distortion in digital circuits... you don't hear it on the peaks, only on the quiet parts.) Unless you have some application that doesn't involve soft passages in the signal (like a siren, or possibly a PA or musical instrument amp) you need to consider the crossover distortion. However, there is still a lower limit to absolute detection threshold, regardless of what percentage a component might be. If the system levels (program, amp, speakers, listening position) are set up such some signal component is below 0 dB SPL, most people aren't going to hear it even if it is the entire signal! Best regards, Bob Masta DAQARTA v5.00 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!
From: Jon Kirwan on 5 Feb 2010 14:32 On Fri, 05 Feb 2010 12:36:02 GMT, N0Spam(a)daqarta.com (Bob Masta) wrote: >On Thu, 04 Feb 2010 12:07:16 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>There's another question that comes to mind regarding the >>output stage. A lot of talk seems to revolve around >>"crossover distortion." Seems almost very first thing folks >>talk about when discussing class of operation if not also at >>other times. >> >>Seems to me that in a three-rail power supply situation >>without an output capacitor involved, the crossover takes >>place near the midpoint (ground) voltage between the rails, >>at a time when current into the speaker load is also near >>zero. (I'm neglecting any thoughts about inductance in the >>speaker and physical coupling into the air, for now.) In >>other words, where power at the speaker is near zero. Is it >>really that important to consider? > >>I was looking at that terrible large scale gain plot for the >>quasicomplementary output stage on the web site recently >>mentioned in the thread (the lower curve in Figure 4 on this >>link): >> >>http://www.embedded.com/design/206801065?printable=true >> >>(It's not that terrible of a plot, as the variation is from >>.96 to .98 with the "normal" middle at .97.) >> >>What's experience say here? Is it really so terrible as to >>worry too much about something that takes place near zero >>voltage, anyway? I'm just questioning the concern, for now. >>I have no understanding about it, at all. Just wondering. > >Crossover distortion is a more-or-less fixed >amplitude, so at low signal levels it becomes a >large percentage of THD. Our ears are sensitive >to the relative amplitudes of components, so a >hypothetical fixed amplitude distortion component >might be totally inaudible when it is a low >percentage of a large fundamental, yet be >obnoxious as a larger percentage of a small >fundamental. (This is the same essential problem >as quantization distortion in digital circuits... >you don't hear it on the peaks, only on the quiet >parts.) Okay. This is consistent with another way I was looking at this. Since the crossover takes place near where power is also close to zero, it's effect is fairly constant. If the amplifier's swing is large scale (nearer the limits it was designed to provide), it's not likely to be noticed buried within that. If the amplifier's swing were small (nearer that zero volt area) then the crossover distortion is quite a bit more noticeable. Makes sense, I think. >Unless you have some application that doesn't >involve soft passages in the signal (like a siren, >or possibly a PA or musical instrument amp) you >need to consider the crossover distortion. Best I consider it, then. I'm looking to learn, not specialize in bullhorns. Interesting to me that you say that crossover distortion might not be such an issue for a musical instrument amplifier, though. I take it you must mean for stage work where the power is going to be set pretty high, generally? >However, there is still a lower limit to absolute >detection threshold, regardless of what percentage >a component might be. If the system levels >(program, amp, speakers, listening position) are >set up such some signal component is below 0 dB >SPL, most people aren't going to hear it even if >it is the entire signal! Hehe. >Best regards, > >Bob Masta Thanks, Jon
From: Jon Kirwan on 5 Feb 2010 20:06
On Fri, 05 Feb 2010 16:46:22 -0800, I wrote: ><snip> >I tracked down a very nice transformer in my box which may be >okay. It has two secondaries and was intended for 60Hz use. >It weighs in at 2.8 lbs (1.25 kg.) > > Primary: > 115VAC, 5.0 Ohms, 16 gauge > Secondaries: (Tested using 120.5VAC RMS on primary) > 16VAC RMS CT, 0.05 Ohms, 14 gauge > 30.4VAC RMS CT, 2.6 ohms, 22 gauge > >The 16VAC RMS outer winding across a 56 ohm resistor yields >15.88VAC RMS. (I don't have a large wattage resistor with >lower values of resistance, so that needs to suffice.) Half >of the 30.4VAC RMS winding (CT to one side) yields 14.75VAC >RMS loaded with the same 56 ohm resistor. Across the entire >30.4VAC windings it is 28.9VAC RMS. (The poor thing is just >a 5W, so I didn't measure for longer than a few seconds.) > >The 30.4VAC secondary looks reasonable, I think, for the two >amplifier rails and ground. The 16VAC might make another >supply for some other reason or, perhaps, provide another >pair of rails to use for a 2 ohm speaker? > >I hadn't thought about that aspect, but as you earlier >pointed out the 25.2VAC CT standard transformer might be a >little light for a 10W amplifier... unless I spec'd a 4 ohm >speaker, I suppose. Then it might be fine. > >Anyway, it looks like it may be a reasonable choice as >something I have available and ready from the junk box. Second thoughts. The 30.4VAC RMS CT secondary shows 2.6 ohms and is 22 gauge. That's 1.3 ohms per half. I believe from calculation that the peak diode current _might_ be 8-10 times the load current in the ideal case (0 ohms.) Taking into account the winding resistance, I may need to think more closely about using this transformer in this application. The winding resistance will limit the current and thus the energy per unit time that can be transferred to the caps and that will very likely lower the achievable rail voltage on the other side of the bridge since the bridge itself simply won't ever see the idealized peak voltage even right up to the moment of peak where the dv/dt goes to zero. By the time that happens, the cycle will already be on a decline again while the resistance continues to limit inflow of charge. Cripes. Darn it. Back to monster caps to get a slight decent rail voltage there. Jon |