Discussing audio amplifier design -- BJT, discrete
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From: Jon Kirwan on 16 Feb 2010 23:30 On Tue, 16 Feb 2010 20:28:30 -0800, I wrote: >20V into 8 ohms, 15V sustained. I mean "20V instantaneous into 8 ohms, 15V sustained." Jon
From: Paul E. Schoen on 17 Feb 2010 16:20 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:kvlmn5dhciv4c51a0au32qi52rjnl67sro(a)4ax.com... > > Okay. I'm back to the power supply, again. (I'm convinced > that my junkbox unit will work fine -- I think it can hold > maybe 18V minimum under load on each rail. Which seems more > than enough headroom for 12.7V, plus output stage overhead. > > I take a little issue with your use of terms in this phrase, > "RMS power of even compressed samples of music is only about > 20% of the peak." Power is average and I don't think RMS > applies to power. Volts-to-power is a squared-phenomenon. So > are amps-to-power. RMS makes sense for those two. But power > is an average (integrated Joules divided by time.) > > So I believe I have to interpret your meaning as suggesting > that the short-term power required (also an average of some > ill-defined kind, I suppose) when playing music can be a > factor of 5 times more than its long-term average power. You > also mentioned a figure as low as 12.5%, which would suggest > a factor of 8 used as a margin instead of 5. > > But a requirement to support short-term power levels is > really just a compliance requirement on the power supply > rails, isn't it? > > So put another way, if I wanted a long-term average of 10W > output and I wanted the extra margins required to support the > worst case estimate of a factor of 8 for short-term power > bursts, then I'd need to design rails that support a voltage > compliance level substantially higher. The parts would need > to withstand it, too. And because of the much higher rail > voltages that need to be dropped most of the time, the output > BJTs would need to have just that much more capacity to > dissipate. > > Or put still another way, assuming that my output swing at > the output stage emitters cannot exceed a magnitude of 15V > and that everything is sized for dissipating 10W, does this > mean the amplifier is a 10W amplifier that can support a peak > of 14W=(15^2/(2*8))? (Which isn't so good, considering your > comments above regarding "music?") > > What is meant when one says, '10 watts?' > > This gets worse when I consider the class of operation, > doesn't it? I mean, class-B might be specified as 10W into 8 > ohms, but wouldn't that be 20W into 4 ohms? But if class-A, > it's pretty much 10W no matter what? > > I'm beginning to imagine amplifiers should be specified as to > their peak output voltage compliance into 8, 6, and 4 ohms; > instantaneous and sustained without damage to the unit. For > example, 35V into 8 ohms instantaneous, 15V sustained. Or > 80W instantaneous, 15W sustained. That way, someone might > have some knowledge about how well it might handle _their_ > music at, say, 15W average power. And could compare that > against another unit specified as 20V into 8 ohms, 15V > sustained. > > How does one know what they are buying? What a headache. Yes, as an extension of what (I think) Mark Twain said, there are lies, damn lies, statistics, and specifications. Then there is the matter of testing. An amplifier is a complex entity and its performance depends on the power supply, the load, its components, environmental conditions, and the nature of the signal being applied. So it may seem fair to level the playing field by testing with a pure sine wave at certain frequencies and determining that it maintains a certain level of maximum distortion without overheating or shutting down over an extended period of time in a controlled environment. But in real life there are many more factors involved, and the actual performance in an individual situation may vary widely. Power is indeed an average function, but the ability to provide power involves efficiency and a duty-cycle rated function of maximum temperature of components, and also the ability of the power supply to maintain a certain voltage level for long enough to "ride out" brief peaks in the signal of typical music. The power that can be supplied to various loads depends largely on impedance matching. But most solid state amplifiers are capable of supplying a certain amount of current, so if it is optimized for eight ohms, it may be able to provide even less continuous power at 4 ohms, but possibly more peak power. You have brought up some good points. But for most purposes, an amplifier rated conservatively at 10W continuous power should be plenty for home music listening. When pushed beyond its normal limits, much depends on how the amplifier handles overloads, and your personal threshold of annoyance when the inevitable distortion occurs. Paul
From: Jon Kirwan on 17 Feb 2010 17:10 On Wed, 17 Feb 2010 16:20:40 -0500, "Paul E. Schoen" <paul(a)peschoen.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:kvlmn5dhciv4c51a0au32qi52rjnl67sro(a)4ax.com... >> >> Okay. I'm back to the power supply, again. (I'm convinced >> that my junkbox unit will work fine -- I think it can hold >> maybe 18V minimum under load on each rail. Which seems more >> than enough headroom for 12.7V, plus output stage overhead. >> >> I take a little issue with your use of terms in this phrase, >> "RMS power of even compressed samples of music is only about >> 20% of the peak." Power is average and I don't think RMS >> applies to power. Volts-to-power is a squared-phenomenon. So >> are amps-to-power. RMS makes sense for those two. But power >> is an average (integrated Joules divided by time.) >> >> So I believe I have to interpret your meaning as suggesting >> that the short-term power required (also an average of some >> ill-defined kind, I suppose) when playing music can be a >> factor of 5 times more than its long-term average power. You >> also mentioned a figure as low as 12.5%, which would suggest >> a factor of 8 used as a margin instead of 5. >> >> But a requirement to support short-term power levels is >> really just a compliance requirement on the power supply >> rails, isn't it? >> >> So put another way, if I wanted a long-term average of 10W >> output and I wanted the extra margins required to support the >> worst case estimate of a factor of 8 for short-term power >> bursts, then I'd need to design rails that support a voltage >> compliance level substantially higher. The parts would need >> to withstand it, too. And because of the much higher rail >> voltages that need to be dropped most of the time, the output >> BJTs would need to have just that much more capacity to >> dissipate. >> >> Or put still another way, assuming that my output swing at >> the output stage emitters cannot exceed a magnitude of 15V >> and that everything is sized for dissipating 10W, does this >> mean the amplifier is a 10W amplifier that can support a peak >> of 14W=(15^2/(2*8))? (Which isn't so good, considering your >> comments above regarding "music?") >> >> What is meant when one says, '10 watts?' >> >> This gets worse when I consider the class of operation, >> doesn't it? I mean, class-B might be specified as 10W into 8 >> ohms, but wouldn't that be 20W into 4 ohms? But if class-A, >> it's pretty much 10W no matter what? >> >> I'm beginning to imagine amplifiers should be specified as to >> their peak output voltage compliance into 8, 6, and 4 ohms; >> instantaneous and sustained without damage to the unit. For >> example, 35V into 8 ohms instantaneous, 15V sustained. Or >> 80W instantaneous, 15W sustained. That way, someone might >> have some knowledge about how well it might handle _their_ >> music at, say, 15W average power. And could compare that >> against another unit specified as 20V into 8 ohms, 15V >> sustained. >> >> How does one know what they are buying? What a headache. > >Yes, as an extension of what (I think) Mark Twain said, there are lies, >damn lies, statistics, and specifications. Then there is the matter of >testing. An amplifier is a complex entity and its performance depends on >the power supply, the load, its components, environmental conditions, and >the nature of the signal being applied. So it may seem fair to level the >playing field by testing with a pure sine wave at certain frequencies and >determining that it maintains a certain level of maximum distortion without >overheating or shutting down over an extended period of time in a >controlled environment. > >But in real life there are many more factors involved, and the actual >performance in an individual situation may vary widely. Power is indeed an >average function, but the ability to provide power involves efficiency and >a duty-cycle rated function of maximum temperature of components, and also >the ability of the power supply to maintain a certain voltage level for >long enough to "ride out" brief peaks in the signal of typical music. > >The power that can be supplied to various loads depends largely on >impedance matching. But most solid state amplifiers are capable of >supplying a certain amount of current, so if it is optimized for eight >ohms, it may be able to provide even less continuous power at 4 ohms, but >possibly more peak power. > >You have brought up some good points. But for most purposes, an amplifier >rated conservatively at 10W continuous power should be plenty for home >music listening. When pushed beyond its normal limits, much depends on how >the amplifier handles overloads, and your personal threshold of annoyance >when the inevitable distortion occurs. > >Paul It sure has been an education, so far. Now I am beginning to understand the technical motivation for LOTS of rails and the ability to select between them (perhaps automatically) in those fancy-pants amplifier designs; dropping in (or out) stacked BJTs as needed. Though I am loathe to even attempt thinking more about them. ..... Now, I want axial leaded diodes for the bridge. From simulating a load of 8 ohms, 1kHz, average power of 10W, and my secondary winding resistance of 2.6 ohms, I'm finding that each diode suffers under a quarter watt of dissipation. So, any recommendations about diodes? Obviously, for a one-off, cost is not really an issue. How important is 'fast recovery'? (Outside of its impact on dissipation.) Seems that anything with 100V or better for reverse voltage standoff, 1/4 watt or better, should work. Leakage probably isn't that important (except against as it may add to dissipation.) I'm expecting to use caps on the order of perhaps 2.2mF 50V, to be secure about the rails. But I expect to want to play with that, once everything is working, to see just how bad I can make it while seeing what that means for the output. And then see if I can calculate a prediction that isn't too far from those results, on paper. Jon
From: pimpom on 18 Feb 2010 15:50 Jon Kirwan wrote: > On Wed, 17 Feb 2010 16:20:40 -0500, "Paul E. Schoen" > <paul(a)peschoen.com> wrote: > >> "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >> news:kvlmn5dhciv4c51a0au32qi52rjnl67sro(a)4ax.com... >>> >>> Okay. I'm back to the power supply, again. (I'm convinced >>> that my junkbox unit will work fine -- I think it can hold >>> maybe 18V minimum under load on each rail. Which seems more >>> than enough headroom for 12.7V, plus output stage overhead. >>> > .... .............<snip>............. > > Now, I want axial leaded diodes for the bridge. From > simulating a load of 8 ohms, 1kHz, average power of 10W, and > my secondary winding resistance of 2.6 ohms, I'm finding that > each diode suffers under a quarter watt of dissipation. So, > any recommendations about diodes? Obviously, for a one-off, > cost is not really an issue. How important is 'fast > recovery'? (Outside of its impact on dissipation.) Seems > that anything with 100V or better for reverse voltage > standoff, 1/4 watt or better, should work. Leakage probably > isn't that important (except against as it may add to > dissipation.) > Fast recovery is not important here since the diodes work at mains frequency. 10W sinusoidal into 8 ohms = 1.58A peak = 0.503A dc average for Class B. Add some mAs for the driver stages. That's slightly more than 0.25A each for diodes in full-wave rectification. The ubiquitous 1N4002 to 1N4007 rated for 1 Amp diodes will do fine. They differ only in the maximum reverse voltage ratings and cost almost the same. As a matter of convenience, I stock only the 1000-volt 1N4007. At less than 2 cents US each retail, I buy them in batches of hundreds at a time. > I'm expecting to use caps on the order of perhaps 2.2mF 50V, > to be secure about the rails. But I expect to want to play > with that, once everything is working, to see just how bad I > can make it while seeing what that means for the output. And > then see if I can calculate a prediction that isn't too far > from those results, on paper. > I have my own rule of thumb here for acceptable levels of ripple and load regulation. I divide the full supply dc voltage with the current at maximum output. This gives the equivalent dc load as seen by the power supply. In the sample design under consideration, that's roughly 30 ohms on each side of the split supply. Calculate the reactance of the filter capacitor at the pulsating dc frequency which is twice the mains frequency for full-wave. My rule of thumb is to get an R/Xc ratio of the order of 50 for a medium quality amp. Your choice of 2200uF agrees well with this.
From: Jon Kirwan on 18 Feb 2010 16:54
On Fri, 19 Feb 2010 02:20:29 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Wed, 17 Feb 2010 16:20:40 -0500, "Paul E. Schoen" >> <paul(a)peschoen.com> wrote: >> >>> "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >>> news:kvlmn5dhciv4c51a0au32qi52rjnl67sro(a)4ax.com... >>>> >>>> Okay. I'm back to the power supply, again. (I'm convinced >>>> that my junkbox unit will work fine -- I think it can hold >>>> maybe 18V minimum under load on each rail. Which seems more >>>> than enough headroom for 12.7V, plus output stage overhead. >>>> >> .... >............<snip>............. >> >> Now, I want axial leaded diodes for the bridge. From >> simulating a load of 8 ohms, 1kHz, average power of 10W, and >> my secondary winding resistance of 2.6 ohms, I'm finding that >> each diode suffers under a quarter watt of dissipation. So, >> any recommendations about diodes? Obviously, for a one-off, >> cost is not really an issue. How important is 'fast >> recovery'? (Outside of its impact on dissipation.) Seems >> that anything with 100V or better for reverse voltage >> standoff, 1/4 watt or better, should work. Leakage probably >> isn't that important (except against as it may add to >> dissipation.) > >Fast recovery is not important here since the diodes work at >mains frequency. Thanks. That had certainly crossed my mind as I was writing. I just wanted to be sure I hadn't missed something important. >10W sinusoidal into 8 ohms = 1.58A peak = 0.503A dc average for >Class B. Add some mAs for the driver stages. That's slightly more >than 0.25A each for diodes in full-wave rectification. The >ubiquitous 1N4002 to 1N4007 rated for 1 Amp diodes will do fine. >They differ only in the maximum reverse voltage ratings and cost >almost the same. As a matter of convenience, I stock only the >1000-volt 1N4007. At less than 2 cents US each retail, I buy them >in batches of hundreds at a time. I pull them out of CFL lamps before disposal. So they are "free" to me. I've quite a few, now. 1200V PIV, I think. Way overkill. But free. I'll use them. >> I'm expecting to use caps on the order of perhaps 2.2mF 50V, >> to be secure about the rails. But I expect to want to play >> with that, once everything is working, to see just how bad I >> can make it while seeing what that means for the output. And >> then see if I can calculate a prediction that isn't too far >> from those results, on paper. > >I have my own rule of thumb here for acceptable levels of ripple >and load regulation. I divide the full supply dc voltage with the >current at maximum output. This gives the equivalent dc load as >seen by the power supply. In the sample design under >consideration, that's roughly 30 ohms on each side of the split >supply. Calculate the reactance of the filter capacitor at the >pulsating dc frequency which is twice the mains frequency for >full-wave. My rule of thumb is to get an R/Xc ratio of the order >of 50 for a medium quality amp. Your choice of 2200uF agrees well >with this. Thanks for your thinking on this. I used more mathy stuff to get there, but I like your practical slice through all that. It is easy to follow. So I'm settled on those particulars, now. The only thing I don't have, right now, are the caps. Well, maybe. I just found a 2.2mF, 35V cap. So that gives me one. I've got all kinds of 200V caps, up to about 470uF. But still looking for one more 'something close' on the order of 35-50V. I'll keep looking through the junk box some more. Might turn up another one. If so, I then need to figure out all the mounting stuff for the hardware. I have the AC plugs and grommets and fuse holders. I can also pull a transorb out of the junk box (from those CFL lamps, again.) The transformer doesn't have any brackets or mounting holes in the laminated steel core so I will have to fashion one from a simple strap of metal, drilled out. Then to wire it all up and do the smoke test and verify the output, with and without a load on it. There and done, I'm ready to move on. Thanks, Jon |