Discussing audio amplifier design -- BJT, discrete
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From: Jon Kirwan on 2 Feb 2010 16:31 On Tue, 02 Feb 2010 13:01:18 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >Assume: > X = T^3 * Isat * e^(q*Eg/(k*Tnom)) > Y = Tnom^3 * Ic * e^(q*Eg/(k*T)) Sorry, should be consistent in terms with: X = T^3 * Isat * e^(q*Eg/(k*Tn)) Y = Tn^3 * Ic * e^(q*Eg/(k*T)) Jon
From: Jon Kirwan on 2 Feb 2010 16:52 On Tue, 02 Feb 2010 13:01:18 -0800, I wrote: >Because of that, I searched around and found out that there >is a correction structure to fix the quasi crossover problem. >It appears to use something called a Baxandall diode, though >for now I haven't learned the details of how it does what it >does. Here's some articles I found on the web by a single author on audio amplifier design: http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205207238&printable=true http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205601405&printable=true http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205801115&printable=true http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205917273&printable=true http://www.planetanalog.com/article/printableArticle.jhtml?articleID=206103226&printable=true The last one of the above links __mentions__ the Baxandall diode. In looking at those, there is this one also listed at the bottom of the last article above. I haven't read this one yet, but include it just the same: http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205202120&printable=true I need to read all of these, I suppose. Jon
From: Paul E. Schoen on 2 Feb 2010 21:04 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:eg7hm51jl8ogq5k6f6v1k00mvmhmuqrej3(a)4ax.com... > On Tue, 02 Feb 2010 13:01:18 -0800, I wrote: > >>Because of that, I searched around and found out that there >>is a correction structure to fix the quasi crossover problem. >>It appears to use something called a Baxandall diode, though >>for now I haven't learned the details of how it does what it >>does. > > Here's some articles I found on the web by a single author on > audio amplifier design: > > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205207238&printable=true > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205601405&printable=true > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205801115&printable=true > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205917273&printable=true > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=206103226&printable=true > > The last one of the above links __mentions__ the Baxandall > diode. > > In looking at those, there is this one also listed at the > bottom of the last article above. I haven't read this one > yet, but include it just the same: > > http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205202120&printable=true > > I need to read all of these, I suppose. This stuff is a bit too intense for my taste, but a Dogpile search of Baxandall Diode turned up this manual for an amplifier that uses tubes and transistors, and has a Baxandall diode in the output stage. Its stated purpose was to "improve symmetry" and appears to add an additional diode drop for the PNP-NPN pair to match the NPN-NPN darlington on the top side. http://www.wimdehaan.nl/downloads/technicalmanualnishiki41.pdf This article states that a Baxandall diode made little change in linearity: http://www.embedded.com/design/206801065?printable=true There is also a Baxandall Tone Control circuit which is discussed here: http://digitalcommons.calpoly.edu/eesp/14/ For my own purposes, distortion of anything less than about 1% is probably not worth paying for or striving to achieve. For audio, my ears are not all that good and I would probably welcome distortion in the form of non-linear frequency response to compensate for degraded sensitivity at the high end. And there is also the argument that any sound that is naturally produced will have some significant distortion that is actually part of the listener's experience. Whatever the acoustics in a given auditorium may be, they contribute to the waveshape as it is received by the listener's ears, and it varies depending on where one is seated. Sometimes added distortion, such as an echo, may enhance the enjoyment of the music, and coloration due to an imperfect amplifier might just as easily be perceived as pleasant rather than objectionable. It is in fact distortion that causes an audiophool to prefer the "warm" sound of a tube amplifier over a laboratory grade solid state amplifier. I am more impressed with amplifiers that are extremely efficient, such as PWM amps. And for some types of test equipment that I have designed, I had to deal with maintaining phase shift to better than 1 degree into a range of inductive, resistive, or capacitive loads, and with outputs of power line frequencies of 45-450 Hz, for voltage sources up to 300 VAC, and current sources up to 100 amperes, at 50 VA to 300 VA or higher. And they had to be able to withstand overloads and short circuits. Paul
From: Jon Kirwan on 2 Feb 2010 21:45 On Tue, 2 Feb 2010 21:04:53 -0500, "Paul E. Schoen" <paul(a)peschoen.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:eg7hm51jl8ogq5k6f6v1k00mvmhmuqrej3(a)4ax.com... >> On Tue, 02 Feb 2010 13:01:18 -0800, I wrote: >> >>>Because of that, I searched around and found out that there >>>is a correction structure to fix the quasi crossover problem. >>>It appears to use something called a Baxandall diode, though >>>for now I haven't learned the details of how it does what it >>>does. >> >> Here's some articles I found on the web by a single author on >> audio amplifier design: >> >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205207238&printable=true >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205601405&printable=true >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205801115&printable=true >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205917273&printable=true >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=206103226&printable=true >> >> The last one of the above links __mentions__ the Baxandall >> diode. >> >> In looking at those, there is this one also listed at the >> bottom of the last article above. I haven't read this one >> yet, but include it just the same: >> >> http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205202120&printable=true >> >> I need to read all of these, I suppose. > >This stuff is a bit too intense for my taste, It's gravy to me. If only I had the good sense to be able to tell if it is being comprehensively and accurately stated. >but a Dogpile search of >Baxandall Diode turned up this manual for an amplifier that uses tubes and >transistors, and has a Baxandall diode in the output stage. Its stated >purpose was to "improve symmetry" and appears to add an additional diode >drop for the PNP-NPN pair to match the NPN-NPN darlington on the top side. > >http://www.wimdehaan.nl/downloads/technicalmanualnishiki41.pdf I think that very same purpose is what I took it to mean, too. In a quasi-complementary output stage, the gain curve (less than 1 everywhere) shown over output would have to show an interesting tweak, midrange. In class-B especially, from one side it would look one way, from the other, somewhat different. Simply because the two quadrants just aren't the same structure. One uses two NPNs, the other an NPN and a PNP. That weirdness in gain has to translate to distortion of some kind. The fix, I'd read, is to use a diode on the complementary NPN/PNP side (and a resistor in parallel, I gather.) Supposedly, it flatten out the gain curve in just the right amount to balance things pretty well. It's an interesting point, if true, because the quasi-complementary output stage is attractive in that it can use the exact same NPN part number for both quadrants' output BJTs. I need to post up some different output structures in the wane hope someone will help me walk through an analysis of them. >This article states that a Baxandall diode made little change in linearity: >http://www.embedded.com/design/206801065?printable=true Thanks for the article. Same author!! If I read closely enough what he is saying, he is saying that the Baxandall diode adds little _in the case of class-A operation_. In the case of class-B, I think he argues it is worth having! He writes, "The choice of class A output topology is now simple. For best performance, use the CFP. Apart from greater basic linearity, the effects of output device temperature on Iq are servoed out by local feedback, as in class B. For utmost economy, use the quasi complementary with two NPN devices: these need only a low Vce(max) for a typical class A amp, so here is an opportunity to recoup some of the money spent on heatsinking. "The rules are different from class B; the simple quasi configuration will give first class results with moderate NFB, and adding a Baxandall diode to simulate a complementary emitter follower stage makes little difference to linearity." I think I represented the meaning of these two paragraphs, accurately. And if you look closely at Figure 4, you will see that there are two curves -- both are for quasi- complementary outputs. However, one of them is class-B -- the really nasty-looking one. That one cries out for fixing and is exactly what I was just talking about, above!!! So the Baxandall diode really seems to be useful in allowing one to _select_ class-B operation without having to pay much for it. Makes class-B lots more attractive with quasi- complementary outputs. Let me know if you think otherwise. >There is also a Baxandall Tone Control circuit which is discussed here: >http://digitalcommons.calpoly.edu/eesp/14/ I'd need to download that thing to read it, I guess. For now, I merely suspect that Baxandall writes about ideas from time to time and isn't known for one thing. >For my own purposes, distortion of anything less than about 1% is probably >not worth paying for or striving to achieve. For audio, my ears are not all >that good and I would probably welcome distortion in the form of non-linear >frequency response to compensate for degraded sensitivity at the high end. I don't have a number in mind because I'm _very_ ignorant about what I'd care about and what I wouldn't. I _do_ know one thing.... I really _hate_ the 10% THD computer speaker systems. That much I do know. There is a place in hell for people who pawn those things off as amplifiers with a nickel. >And there is also the argument that any sound that is naturally produced >will have some significant distortion that is actually part of the >listener's experience. hehe. I'm imagining Mister Magoo right now and what he'd consider "good." ;) With Magoo as the "listener" .... Alfred E Neuman's "What, me worry?" comes to mind regarding any amplifier system. Slightly more seriously, best of all would be that we somehow analyze each and every person's brain's responses to sound, in real time if possible, from the conscious interpretation back through to the cochlea and the transducers nearby, to the environment around it, and use a DSP to process the content first before driving a speaker system, at all. I expect to be dead before that happens, though. I'm good ignoring "listener's experience" and focusing on a more objective measure of some kind, letting the chips fall. >Whatever the acoustics in a given auditorium may be, >they contribute to the waveshape as it is received by the listener's ears, >and it varies depending on where one is seated. Sometimes added distortion, >such as an echo, may enhance the enjoyment of the music, and coloration due >to an imperfect amplifier might just as easily be perceived as pleasant >rather than objectionable. It is in fact distortion that causes an >audiophool to prefer the "warm" sound of a tube amplifier over a laboratory >grade solid state amplifier. Those arguments are "beyond my pay grade." I'll just retreat to something I can actually compute. >I am more impressed with amplifiers that are extremely efficient, such as >PWM amps. And for some types of test equipment that I have designed, I had >to deal with maintaining phase shift to better than 1 degree into a range >of inductive, resistive, or capacitive loads, and with outputs of power >line frequencies of 45-450 Hz, for voltage sources up to 300 VAC, and >current sources up to 100 amperes, at 50 VA to 300 VA or higher. And they >had to be able to withstand overloads and short circuits. I like efficiency as one goal. Especially as I'm getting to understand just how much power can be wasted without much value. This 10W thing, if we are talking about class-A and planning for 6db overhead (4X) as someone I read in one of those articles saying about it, might mean a 40W capability into 8 ohms, rails that are way out there and power waste that starts to look like a toaster. So I'm beginning to get my head turned 'round even at 10W!! Cripes, that spec is rapidly becoming something I'm beginning to respect a lot more and to realize that I might have landed on a number that is better for teaching than I'd first imagined it to be. Jon
From: David Eather on 3 Feb 2010 14:46
Jon Kirwan wrote: > On Sun, 31 Jan 2010 12:38:49 -0800 (PST), David Eather > <eather(a)tpg.com.au> wrote: > >> On Jan 30, 9:06 am, Jon Kirwan <j...(a)infinitefactors.org> wrote: >>> On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather >> <snip> >> >>>> Yes and No. All the published circuits are made by people who want to >>>> sell transistors, >>> A concern I care not the least about. My _real_ preference, >>> were I to impose it on the design, would be to use ONLY >>> PN2222A BJTs for all the active devices. One part. That's >>> it. Why? Because I've got thousands of them. ;) >>> >>> Literally. Something like 22,000 of the bastards. I give >>> them away like popcorn to students at schools. Got them >>> _very cheaply_. So if I were pushing something, I'd be >>> pushing a 10W PN2222A design, use signal splitting approach >>> probably (because it's the only way I think think of, right >>> now), and distribute the dissipation across lots and lots of >>> the things. >>> >>> What to go there? :) >> Signal Splitting? Can you sketch out what your thinking? > > Yeah, I think so. Something like this: > >> : | | >> : \ | >> : / R2 | >> : \ | >> : / | >> : | | >> : | |/c Q2 >> : +---------| >> : | |>e >> : | | >> : |/c Q3 | >> : -------| +----- >> : |>e | >> : | | >> : | |/c Q1 >> : +---------| >> : | |>e >> : | | >> : \ | >> : / R1 | >> : \ | >> : / | >> : | | > > The "signal splitter" here is Q3. It's also providing gain, > too, though. The emitter and collector move in opposite > directions and the signal "splits" at Q3. (The emitter > follows the base, the collector inverts the base.) > > If I read with any understanding about these things, properly > biasing Q3 is a pain, the Q3 gain varies with the load itself > as well as its bias, and compensation issues are complicated > a bit. > >> The wikipedia type circuit can use a few n2222 - I count a max of 5. >> Even if you could use only n2222 it would not be a good idea - making >> the circuit stable would be more difficult. > > Yes, ignorant as I am still of the details, I think that's > very true. The splitter has significant signal voltage on > its input and I've read that pole-splitting methods for > improving stability are harder to apply here. > >> On the good side the n2222 >> is a good choice for Q1,Q2 and as active replacements for R5,R6 and >> one other (optional) we haven't met yet. What makes it a good >> transistor is the large current gain / bandwidth product and the flat >> DC current gain over a wide range of viable bias currents. Both >> contribute to low distortion. >> >> http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3 >> graph) >> >> compared to say 2n3904 >> http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF >> >> where the flat portion of the DC gain curve is over a very limited >> range. > > Interesting point to consider. Something that had slipped by > me, so far. > >>>> not audio systems, power supplies or transformers. >>> Got it. >>> >>>> As a result the power supply is often assumed to be regulated, which >>>> is not true in this case, or the power supply is treated in a very >>>> perfunctory manner that is not at all compatible with good design. >>>> In this case you have the voltage you need for the 10 watts, plus >>>> voltage drop for the driver circuitry and output stage , plus ripple >>>> voltage, plus whatever is required for transformer regulation and >>>> mains regulation. When you add it all up you might find that a chosen >>>> transistor/component is actually not at all suitable for the job. Back >>>> to the drawing board. Change this change that recheck everything again >>>> etc. >>> In this case, though, there is nothing particularly >>> remarkable about the rails. Taken across the entire span, >>> even, doesn't exceed the maximum Vce of a great many BJTs. So >>> no real worry there. But I see some of where problems may >>> arise. Luckily, at this level I can side-step worrying about >>> that part and get back to learning about amplifier design, >>> yes? >> I come up with a figure of 50 volts rail to rail no load voltage - >> after picking out a common transformer with 15% regulation. A 30 volt CT transformer with 15% regulation and 7% mains over-voltage, less voltage drop for the diode bride would give rails of +/- 25. A dual 12.6 volt transformer would give a minimum (worst case with transformer at full load and mains 7% under voltage) of 16.something volts meaning big filter caps if you were serious at getting 10 watts. One of the reasons to go PSU first I think. (Also I live in a tiny jerk-water town where no one knows what a custom made transformer is let alone where you can get one wound) > > Okay. This is going to force me to sit down with paper and > work through. I was stupidly imagining +/-18V max, or 36V > rail to rail. I haven't considered the details of the output > section yet, driving a load from rails that run up and down > on capacitors that charge and discharge at 1A-level currents > into the load, and perhaps I need to spend some more time > there before moving on. > > There are so many ways to cut this. Start at the input and > that's one focus that may work okay. Start at the output > stage and that provides important power supply information, > though. So maybe I should start at that end? > >>>> If you do the power supply first you have the figures needed for your >>>> worst case already. It saves time and makes a better result (no >>>> tendency to comprimise to save all the calculations already done). >>> Well, does this mean we should hack out the power supply >>> first? I'm perfectly fine with that and can get back to you >>> with a suggested circuit and parts list if you want to start >>> there. We could settle that part before going anywhere else >>> and I'd be happy with that approach, too, because to be >>> honest I don't imagine it to put a horrible delay into >>> getting back to amplifier design. So I'm good either way. >> I'm looking at some of your other posts and I don't think you need a >> maths lesson from me. If you want to do a power supply great. Its a >> small one so nothing much too it. If you don't want, I'm OK too. > > I still haven't been down the path on my own, yet. So I > don't have strong opinions about this. It's like going to > Disneyland for the first time. Which land should I go to, > first? Later, after being there a few times, I may look at > the flow of people and decide that "Adventureland" is the > best first start. But first time out? Who knows? I'm open > to guidance. Everything is new. > >>>>>>>> I should also ask if you have a multi meter, oscilloscope (not necessary >>>>>>>> but useful)and how is your soldering? But it would be wise to keep this >>>>>>>> whole thing as a paper exercise before you commit to anything. >>>>>>> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS >>>>>>> handheld, two oscilloscopes (TEK 2245 with voltmeter option >>>>>>> and an HP 54645D), three triple-output power supplies with >>>>>>> two of them GPIB drivable, the usual not-too-expensive signal >>>>>>> generator, and a fair bunch of other stuff on the shelves. >>>>>>> Lots of probes, clips, and so on. For soldering, I'm limited >>>>>>> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and >>>>>>> somewhat wider spade tips in the 1.5mm area. I have tubs and >>>>>>> jars of various types of fluxes, as well, and wire wrap tools >>>>>>> and wire wrap wire, as well. I also have a room set aside >>>>>>> for this kind of stuff, when I get time to play. >>>>>> OK. Next serious project, I'm coming around to your place! >>>>> You come to the west coast of the US and I'll have a room for >>>>> you! >>>>>> Your gear is >>>>>> better than mine. I had to ask, rather than just assume just in case my >>>>>> assumptions got you building something you didn't want to, and got you >>>>>> splattered all over the place from the mains, or suggesting you choose >>>>>> the miller cap by watching the phase shift of the feedback circuit - I >>>>>> don't read a lot of the posts so I didn't know what you could do. >>>>> To be honest, I can do a few things but I'm really not very >>>>> practiced. My oscilloscope knowledge is lacking in some >>>>> areas -- which becomes all too painfully obvious to me when I >>>>> watch a pro using my equipment. And I'm still learning to >>>>> solder better. It's one of a few hobbies. >>>>>>> Jon >>>>>> Have a look at >>>>>> http://en.wikipedia.org/wiki/Electronic_amplifier >>>>> Done. >>>>>> The bits on class A might be interesting as it says 25% efficiency and >>>>>> 50% obtainable with inductive output coupling (i.e. with a transformer) >>>>>> which is what I said, not what blow hard Phil said. >>>>> What I first see there is the amplifier sketch at the top of >>>>> the page >>>> I wasn't going to prompt, but it is close to the sort of thing, I >>>> think, you should be aiming for . As someone has already noted (I >>>> would attribute you if I wasn't on GG, I'm sorry) it has been drawn up >>>> for a single supply, rather than a more common (for this size / >>>> configuration) split supply. >>> I had assumed we'd be using a split supply. > >> I think that's very much the preferred way. > > I feel more comfortable assuming it, too. > >>> I had assumed a speaker would be hooked up via a cap to the >>> output, so DC currents into a speaker coil would be removed >>> from any concern. But I was also holding in the back of my >>> mind the idea of tweaking out DC bias via the speaker and >>> removing the coupling cap as an experiment to try. And if >>> so, I'd pretty much want the ground as a "third rail." >> Exactly right! There are two common ways to reduce/remove any offset >>from the output. Neither is shown on the wikipedia circuit. If you >> have another split rail circuit it will certainly have one method - >> both methods involved use the diff amp. > > Thanks. > >>> (Playing just a bit upon the Chicago parlance about the once >>> dangerous rail in their transit system.) >>> >>>> (I don't really care too much about arguing about >>>>> efficiencies right now -- I'm more concerned about learning.) >>>>> The input stage shown is a voltage-in, current-out bog >>>>> standard diff-pair. First thing I remember about is that R4 >>>>> shouldn't be there >>>> Correct. Theory says it does nothing. I practice the theory but have >>>> the occasional heretical belief about that. >>> Actually, I think I've read that theory says it is _better_ >>> to be removed. The reason seemed pretty basic, as it's >>> easier to get close to a balanced current split; and this, I >>> gather, lowers 2nd harmonic distortions produced in the pair >>> -- notable more on the high frequency end I suppose because >>> gain used for linearizing feedback up there is diminishing >>> and can't compensate it. >>> >>> In other words, it's not neutral. It's considered to be >>> better if I gathered the details. Then even better, the >>> current mirror enforces the whole deal and you've got about >>> the best to be had. >>> >>> Of course, mostly just being a reader means I have no idea >>> which end is up. So I might have all this wrong. >> No. Thats all correct. I'll show a different circuit latter > > Okay. I'll enjoy the moment when it happens. > >>>> and better still both R3 and R4 should be >>>>> replaced with a current mirror. >>>> This would provide more differential gain. >>> _and_ improve distortion because the currents are forced to >>> be balanced in the pair, yes? >> yes. > > Okay. So I am picking up details not too poorly, so far. (Seriously, I think you are doing amazing) > >>>>> R5 should be a replaced with >>>>> a BJT, as well. >>>> In the right configuration it would reduce the common mode signal gain >>>> of things like mains hum and supply ripple (you mentioned power supply >>>> isolation before). >>> Yes, that's how I thought about it. >> >>>> Also, from another (what do you call it branch? thread?) you were >>>> discussing boot-strapping R6. This is not done so much as amplifiers >>>> get bigger but a BJT configured in the same way as the replacement for >>>> R5 is very common. I'm leaving the details to you - perhaps there is a >>>> way to reduce component count without affecting performance. (I am >>>> hoping this is what you wanted "nutting it out for yourself") >>> Yes! I don't want things handed on a platter. But I also >>> don't want to have to rediscover all of the ideas by making >>> all of the mistakes, either. This is the kind of "pointer" >>> towards something that I like a lot. It gives me a place to >>> think about something, but leaves me some reason to have to >>> do so and that helps me own it better. >>> >>> One general truth about learning is that you don't present >>> someone with a problem so out of their depth that they have >>> no chance at it. Doing that means they fail, they feel like >>> a failure, and it causes a student to just want to go away. >>> They lose motivation, usually, in cases like that. On the >>> other hand, providing no difficulty at all merely means >>> repetition of what they already know and they grow bored from >>> that, too. Finding the sweet spot where a student is faced >>> with interesting problems that are not already known, but >>> perhaps within reach of grasping at with some effort, is the >>> key. Then it can be fun, educational, and motivate. >>> >>> That's what you just did for me. >>> >>>> I assume the input impedance of that example >>>>> is basically the parallel resistance of R1 and R2, but if we >>>> Yes. >>> Okay. >> There is the parallel resistance of R5 x Beta Q1 as well, but this is >> normally so high it won't affect the result. And if R5 is replaced >> with an active device it can become essentially infinite. > > Okay. I've got that detail from other discussions, too. So > yes, understood. Also, I mentioned replacing R5, I think. In > replacing R5 with active parts, I'm thinking of two BJTs in a > usual form that seems to work pretty well over supply > variations. Two BJT's make a current sink with a nice sharp knee giving best CMRR. I'll point out a small "optimization" (price/component reduction but with a small degradation in performance - given the over abundance of 2n2222 in your area it probably doesn't count). You could use a single BJT current sink with a voltage reference (2 diodes, low voltage zenner or LED) tied to the negative rail. You could then use the same reference voltage for a 1 BJT current sink in place of R6. Saving a couple of components. > >>>>> use split supplies I'd imagine replacing the two of them with >>>>> a single resistor to the center-ground point. >>>> Yes, but you should probably think of a whole passive network to >>>> filter out low and high frequency - (think what happens if you amp is >>>> operated near a source of RF) >>> Well, every trace picks up like little antennae. All kinds >>> of trace voltages appearing here and there. Not good. >>> >>> So. Can you make an audio amplifier that can withstand a >>> microwave oven environment and deliver good performance while >>> irradiated with 1kW banging around in there? ;) >> If you can do that the military wants you to EMP harden all there >> electronics. The input is a little different because some user always >> want to stick a bloody gret big long wire onto it. > > :) > > I actually _do_ work on low-mass, direct-contact temperature > measuring devices designed to work within a microwave > environment. (But no electronics or metals inside.) > > But you brought up the microwave environment, so I hope you > don't mind the teasing about it. > No. :) >>>>> There's no >>>>> miller cap on Q3, >>>> Depending on transistors layout etc it might not be needed, but more >>>> often it is the size that is the question. >>> I was thinking it helped locally linearize the VAS section >>> and that such would be "good" most anywhere. But I am just >>> taking things without having worked through them on my own. >>> So... >> It sets the bandwidth of the VAS stage so you can use negative >> feedback without the whole thing turning into smoke. Do you know of >> control theory / bode diagrams. There is a minuscule amount needed >> for this app. > > I am familiar with _some_ closed loop control theory, > sufficient to get me by with PID controls (using _and_ > writing code for them.) Bode diagrams are something I have > not used, though I've seen them. My math is adequate, I > suspect. But I will have to read up on them, I suppose. > > For Laplace analysis, I'm familiar with complex numbers, > poles and zeros, partial fraction extractions, and so on. > Just inexperienced in the "short cuts" that many use to get > (and think about) answers. Bode diagrams are really simple and (for this type of thing) will get you where you want to go. > >>>>> I'd probably replace the two diodes with >>>>> one of those BJT and a few resistor constructions I can't >>>>> remember the name of (which allows me to adjust the drop.) >>>> Vbe multiplier... >>> Okay. Thanks. >>> >>>>> The feedback ... well, I need to think about that a little >>>>> more. There's no degen resistors in the emitters of Q4 and >>>>> Q5. >>>> Why would/should you use them? Jon, Just in case the question misled you - I was asking only about things that needed changing. So I asked a question about the degen Rs on Q4 and Q5, even though you would/should use them and for a few good of reasons of which I think thermal stabilization is the more important. >>> I'm still thinking about that. In general, I was thinking >>> about them because of the "little re" that is kT/q based in >>> each BJT, and varies on Ie. Since Ie is varying around, I >>> was thinking about something fixed there to overwhelm it and >>> "make it knowable" for the design, I suppose. Maybe that's >>> all wet, given your query. I'll toss the idea off the side, >>> for now. >> Try working through the thermal stabilization. Just make a stab at the >> transistor junction temperatures - it will be pretty hot (unless you >> can afford mega bucks for heatsinking) > > I need to understand the output configuration a little better > before I do that. > > Including thinking more closely about swinging one end of an > output cap around so that 1Amp rms can pass through it at > 20Hz. I = C dv/dt, but V=V0*sin(w*t), so I=C*w*V0*cos(w*t). > Assuming max current at the max slew rate for a sine at phase > angle zero, the w*t is some 2*PI*N thing, so cos(w*t) goes to > 1. That makes I=w*C*V0. But w=2*pi*20, or about 126 or so. > So I=126*C*V0. So with I=1A, C=1/(126*V0). With V0=15V, I > get about 530uF for the output cap. That's an amp peak only > at the right phase, too. It'll be less elsewhere. To make > that an amp rms, the cap would need to be still bigger. > > Peak current via the cap will take place right about the time > when the two BJTs's emitters are at their midpoint. One of > the BJTs will be supplying that. Not only that, but also > depending upon class mode of operation, supplying current to > the other one as well. How much is important to figuring out > the wattage. > > I need to sit down with paper, I suspect. But if you want to > provide some suggested thinking process here, I'd also be > very open to that, as well. I'll take a shot at it either > way, but it helps to see your thinking, too. If you can > afford the moment for me. If I had a split power supply I would *always* get rid of the output capacitor. It is not difficult to get the output DC to withing 50mv of gnd. A weird thing I have noticed, and I think you would have noticed it sooner, is that no one, even audio "golden ears" pay serious attention to the output cap. They just stick a plain old electrolytic of no particular type (some times it's a bipolar) in the output, make it bigger than needed for the LF -3db corner and call it "good". It would seem that some attention should be paid to "ripple" current at frequencies like 20khz etc, so some low esr caps would seem mandatory. That music has relatively less high frequency components is the only reason I can think of that this very lax approach might work. > >>>>> Um.. okay, I need to sit down and think. Mind is spinning, >>>>> but I've not set a finger to paper yet and there is lots to >>>>> think about in that one. I could be way, way off base. >>>> Not at all. >>> Thanks for that. I'm just glad to be able to talk to someone >>> about any of this, at all. So please accept my thanks for >>> the moments you are offering. >>> >>>> Is there a way you could post a schematic of where your thinking is >>>> and what you would like to discuss - there is no need for a complete >>>> circuit. >>> Yes. I can use ASCII here, for example. But before I go off >>> into the wild blue with this, do you want to focus on the >>> power supply first? Or just jump in on the amplifier? >> I don't mind. Earlier I put a stab at a no load worst case voltage, >> you can use that if you want to. Until you get to output stage power >> dissipation that is all you need. > > Maybe I'd like to focus on understanding different output > pair configurations, first. I frankly don't like the "haul > the output pair around with a collector on one side and a > resistor on the other with a rubber diode in between to keep > them biased up" approach. It's smacks of heavy-handedness > and I simply don't like the way it looks to me. Everything > tells me this works, but it is indelicate at the very least. Well it would be a Darlington or complementary pair (I can't remember the name sz...? )for the output transistors but I have no objections > > However, it is crucial that I understand it in detail before > deciding what I really think about it. For example, I might > want to replace the resistor with a current source. Sorry you lost me on which R But > without apprehending the output stage more fully, its time > domain behavior over a single cycle for example, I'm not > comfortable with hacking it here and there, ignorantly. > > Jon |