Discussing audio amplifier design -- BJT, discrete
in [Basics]
|
Prev: Watch replacemnt (button) batteries?
Next: Low Power Satellite Based Laser / Imaging System Could Easily Track Activity Disturbing Cheap Reflecting Fibers
From: Jon Kirwan on 4 Feb 2010 15:07 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. Jon
From: Jon Kirwan on 4 Feb 2010 16:59 On Thu, 04 Feb 2010 05:34:52 -0800, I wrote: >But it is complicated slightly by the fact >that the transformer supplies the entire current draw (if it >can) for a short part of the cycle _after_ the peak, as its >slope is less than the droop slope of the cap. I overstated this. The capacitor does supply _some_ along with the transformer windings during this short phase, as the voltage on the cap is also declining with it. Jon
From: Jon Kirwan on 5 Feb 2010 00:07 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
From: Paul E. Schoen on 5 Feb 2010 00:33 "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. 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. 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. > 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. 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. 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. 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. >>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 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 >>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. > 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. > 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!" :) > 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. 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. 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. Paul ------------------------------------------------------------------ Version 4 SHEET 1 880 680 WIRE 48 112 -96 112 WIRE 48 144 48 112 WIRE 112 144 48 144 WIRE 224 144 176 144 WIRE 288 144 224 144 WIRE 384 144 352 144 WIRE 512 144 384 144 WIRE 528 144 512 144 WIRE 528 160 528 144 WIRE 224 176 224 144 WIRE 384 176 384 144 WIRE -96 208 -96 112 WIRE 48 256 48 224 WIRE 224 256 224 240 WIRE 224 256 48 256 WIRE 256 256 224 256 WIRE 384 256 384 240 WIRE 384 256 256 256 WIRE 416 256 384 256 WIRE 528 256 528 240 WIRE 528 256 416 256 WIRE 560 256 528 256 WIRE 608 256 560 256 WIRE 48 288 48 256 WIRE 608 288 608 256 WIRE 416 304 416 256 WIRE 560 336 560 256 WIRE 256 352 256 256 WIRE -96 400 -96 288 WIRE 48 400 48 368 WIRE 48 400 -96 400 WIRE 48 480 48 400 WIRE 96 480 48 480 WIRE 256 480 256 416 WIRE 256 480 160 480 WIRE 304 480 256 480 WIRE 416 480 416 368 WIRE 416 480 368 480 WIRE 528 480 416 480 WIRE 560 480 560 416 WIRE 560 480 528 480 FLAG 608 288 0 FLAG 512 144 V+ FLAG 528 480 V- SYMBOL ind2 64 240 R180 WINDOW 0 36 80 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName L1 SYMATTR Value 1 SYMATTR Type ind SYMATTR SpiceLine Rser=.01 SYMBOL ind2 64 384 R180 WINDOW 0 36 80 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName L2 SYMATTR Value 1 SYMATTR Type ind SYMATTR SpiceLine Rser=.01 SYMBOL polcap 176 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 220� SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=2.51 Rser=0.025 Lser=0 SYMBOL diode 240 240 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value MURS120 SYMBOL diode 288 160 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value MURS120 SYMBOL polcap 368 176 R0 SYMATTR InstName C2 SYMATTR Value 220� SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=2.51 Rser=0.025 Lser=0 SYMBOL voltage -96 192 R0 WINDOW 3 -8 233 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 -7 260 Left 0 SYMATTR Value SINE(0 26 60 0 0 0 100) SYMATTR SpiceLine Rser=1.1 SYMATTR InstName V1 SYMBOL res 512 144 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL polcap 160 464 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 220� SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=2.51 Rser=0.025 Lser=0 SYMBOL polcap 400 304 R0 SYMATTR InstName C4 SYMATTR Value 220� SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=2.51 Rser=0.025 Lser=0 SYMBOL diode 368 496 M270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value MURS120 SYMBOL diode 240 416 M180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D4 SYMATTR Value MURS120 SYMBOL res 544 320 R0 SYMATTR InstName R2 SYMATTR Value 1k TEXT -104 480 Left 0 !K1 L1 L2 1 TEXT -104 512 Left 0 !.tran 1
From: pimpom on 5 Feb 2010 04:17
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". 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. 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. 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. 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. |