Discussing audio amplifier design -- BJT, discrete
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From: Jon Kirwan on 22 Feb 2010 11:41 On Mon, 22 Feb 2010 22:56:22 +1000, David Eather <eather(a)tpg.com.au> wrote: >On 17/02/2010 2:28 PM, Jon Kirwan wrote: >> On Thu, 11 Feb 2010 17:51:27 +1000, David Eather >> <eather(a)tpg.com.au> wrote: >> >>> <snip> >> >> But a requirement to support short-term power levels is >> really just a compliance requirement on the power supply >> rails, isn't it? > >Yes. Okay. These concepts are slowly settling into my brain. >> 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 other ways around. The design will deliver ten watts maximum >(disregarding clipping) but the average power output will actually be >much lower - hence you can "skimp" a bit on the supply transformer and >heatsinks - which wrt overheating have very long time constants relative >the the peak output demands. Cripes! Really? So a 10W amplifier isn't designed to actually deliver a full 10W steadily into a load? That's the peak power capability? Cripes. Let me put this another way. I design a class-B output stage with rails capable of 10W compliance into 8 ohms (roughly 13V peak, so rails at maybe +/-17V or so?) With 10W into the 8 ohm load, let's say this means the upper power BJT is handling about 4-5W and the lower BJT is handling 4-5W, as well. Call it 10W total dissipation inside the amp while 10W are dissipated in the speaker. But I don't have to go find BJTs able to dissipate 4-5W, because the 10W spec is just a max-unsustained case and the real situation is more like 2W into the load, continuous? In short, I need to find a BJT that only needs to dissipate 1W for the high side and 1W for the low side? I could use two PN2222As in parallel to do that! I can _cheat_ like that and call it a 10W design? It doesn't actually _have_ to sustain 10W without burning up? Okay, now I'm depressed. I go buy a 50W amplifier, stick a sine wave signal generator on it and watch the thing toast itself, bursting in fire soon enough? >> 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?' > >A ten watt amp delivers a sine wave producing 10 watts of output power >into a specified load. Ideally this would be 10 watts for an infinite >period of time but for audio amps, due to the nature of the signal, an >"infinite period of time" in practical terms may be as short as a few >seconds. Yeah. A few seconds. So... now I can go back with a quasi-comp output stage and use a pair of those PN2222As for it, without heat sinking! Nice little TO92 packages, too. ;) >> 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? > >40W output. I**2 x R. The power supply voltage is >approximately constant. I was looking at some actual measurements taken by Mr. Self on an actual class-B amplifier when I wrote that. I didn't do a theory-based analysis. Just read off the figures when he was comparing a class-A with a class-B into different loads. Now I'll do that. I had then imagine it came from V^2/R and knowing that V^2 remains the same for a given amplifier and only the R changed from 8 to 4. Which makes sense then that it would double, not quadruple, the power output. From an I^2*R perspective, I get the same estimate because a smaller load does double the current, but the R divides in half, so the combination is still just twice, not four-times. Can you explain this 40W statement better for me? >> But if class-A, it's pretty much 10W no matter what? > > If class A, power is 5 watts out with 4 ohms. Current is held > constant. Again, looking at Self's chart (page 322 on his 5th edition) I see a slight degregation into 4 ohms, going from about 20W into 8 ohms to 15W into 4 ohms. I'm not entirely sure of 'theory' here, but I took this to suggest that at the higher currents the drive circuitry's compliance coupled with the likely somewhat lower gain caused by somewhat higher currents now needed accounted for the droop. But his chart certainly doesn't suggest 1/2 rated power. I guess I need to delve into this a bit more to make sure I understand. The class-B case seems easier for me to follow (assuming I'm right, above, which I of course may not be.) >> 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. > >Your argument here is reasonable but ..... it is also the beginning of >the PMPO fiasco - since no advertising department could agree on what >constitutes "music" they used what ever figures looked best - and that >led to the PFPO (peak fantasy power output) fiasco where you just put >anything you like on the box. > >For a short time some (better) manufactures used a figure they called >"headroom" which was the maximum possible instantaneous power output >when the power caps are fully charged divided by the long term power >output (10 watts in this case). It was always expressed in db - but was >confusing to the customer - so it disappeared. Okay. Well, I can say one thing. I've learned that there are output specs and there are output specs and what they actually mean is yet another question, usually unanswered. As a consumer, I've become a little better informed even if all that means is I'm a lot more suspicious than before. >> How does one know what they are buying? What a headache. > >Wait till you start talking about speakers! Hehe. Now I'm really scared. ;) Jon
From: Jon Kirwan on 22 Feb 2010 13:03 On Mon, 22 Feb 2010 08:41:39 -0800, I wrote: ><snip> >Again, looking at Self's chart (page 322 on his 5th edition) >I see a slight degregation into 4 ohms, going from about 20W >into 8 ohms to 15W into 4 ohms. I'm not entirely sure of >'theory' here, but I took this to suggest that at the higher >currents the drive circuitry's compliance coupled with the >likely somewhat lower gain caused by somewhat higher currents >now needed accounted for the droop. > >But his chart certainly doesn't suggest 1/2 rated power. ><snip> Another thought crossed my mind, too. If the amplifier he was testing used a Vbe multiplier to achieve class-A operation, that won't be enough when faced with 4 ohms. If so, it will degrade into class-AB operation. Not sure that that means, yet. Need to think more on that, as well. Jon
From: Jon Kirwan on 22 Feb 2010 13:47 On Mon, 22 Feb 2010 10:03:41 -0800, I wrote: >On Mon, 22 Feb 2010 08:41:39 -0800, I wrote: > >><snip> >>Again, looking at Self's chart (page 322 on his 5th edition) >>I see a slight degregation into 4 ohms, going from about 20W >>into 8 ohms to 15W into 4 ohms. I'm not entirely sure of >>'theory' here, but I took this to suggest that at the higher >>currents the drive circuitry's compliance coupled with the >>likely somewhat lower gain caused by somewhat higher currents >>now needed accounted for the droop. >> >>But his chart certainly doesn't suggest 1/2 rated power. >><snip> > >Another thought crossed my mind, too. If the amplifier he >was testing used a Vbe multiplier to achieve class-A >operation, that won't be enough when faced with 4 ohms. If >so, it will degrade into class-AB operation. Not sure that >that means, yet. > >Need to think more on that, as well. Sorry to keep responding to myself, but even more crosses my mind, including VAS loading. So I stopped letting things cross my mind and set up a spice simulation to see what it tells me. (I hate doing this, without applying theory, but I feel time is of the essence and like cheating.. for now.) Class-A appears to deliver the same thing as class-B, at least using a TIP3055 and TIP2955 output pair, and using an idealized voltage source between the bases to set the class of operation. With a 4-ohm load and the exact same drive voltage (using again a voltage source as the VAS output), I got 20.55 watts into 4 ohms with class-A operation and 10.31 watts into 8 ohms. (Which is not a 4X but 2X phenomenon.) In class-B, this was 19.66 watts into 4 ohms and 10.16 watts into 8. Again, 2X. (I think I might have been just slightly into class-AB with that last test, but I got it close.) So it maybe doesn't matter about class of operation. But is about the quiescent current flowing via the vbe multipler and what is available to _drive_ the output BJTs and perhaps also some estimation about output drive impedance of the VAS which hauls the output section around in real amplifiers that caused the table entry values I saw with Self's book. Jon
From: pimpom on 22 Feb 2010 16:22 Jon Kirwan wrote: > On Mon, 22 Feb 2010 02:58:40 +0530, "pimpom" > <pimpom(a)invalid.invalid> wrote: > >> Jon Kirwan wrote: >> >> ......<snipped for now>......... >> >> Jon, it's getting very close to 3 am here and I've got to >> sign >> off for now because I have to get up earlier than my usual >> 11-12 >> noon tomorrow. > > I see that you are +530 and I am -800, so we are either +1330 > or -1030 apart (using my time as 0000) from each other -- > roughly speaking at opposing corners of the day. > > (Not sure how to apply the chemical analogy of para- to this. > Para-dies?) > > 11AM your time is 11+10.5 or 9:30PM my time. Which suggests > you are dead to me from 1PM to 10PM, my time. ;) Yeah. Though, with your DST system, it's not easy to keep track of the exact difference. > >> Maybe I glanced through your calculation of Iav too cursorily >> and >> wrongly concluded that you made a mistake where you didn't. >> I'll >> give it a closer look and come back when I can. > > I very much appreciate any thoughts. So a lot of thanks go > to you for even taking a moment, at all. Whenever you feel > able and willing is nothing less than a fantastic boon to me. I just had a quick look again and it seems I noticed only the part where you got Iav as 2*Ipk/pi and missed the place where you later corrected it to Ipk/pi for the series push-pull topology. Sorry. I'd like to chime in on your discussion of commercial power ratings with David, but I'll have to leave that for later.
From: Jon Kirwan on 22 Feb 2010 16:33
On Tue, 23 Feb 2010 02:52:59 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Mon, 22 Feb 2010 02:58:40 +0530, "pimpom" >> <pimpom(a)invalid.invalid> wrote: >> >>> Jon Kirwan wrote: >>> >>> ......<snipped for now>......... >>> >>> Jon, it's getting very close to 3 am here and I've got to >>> sign >>> off for now because I have to get up earlier than my usual >>> 11-12 >>> noon tomorrow. >> >> I see that you are +530 and I am -800, so we are either +1330 >> or -1030 apart (using my time as 0000) from each other -- >> roughly speaking at opposing corners of the day. >> >> (Not sure how to apply the chemical analogy of para- to this. >> Para-dies?) >> >> 11AM your time is 11+10.5 or 9:30PM my time. Which suggests >> you are dead to me from 1PM to 10PM, my time. ;) > >Yeah. Though, with your DST system, it's not easy to keep track >of the exact difference. > >> >>> Maybe I glanced through your calculation of Iav too cursorily >>> and >>> wrongly concluded that you made a mistake where you didn't. >>> I'll >>> give it a closer look and come back when I can. >> >> I very much appreciate any thoughts. So a lot of thanks go >> to you for even taking a moment, at all. Whenever you feel >> able and willing is nothing less than a fantastic boon to me. > >I just had a quick look again and it seems I noticed only the >part where you got Iav as 2*Ipk/pi and missed the place where you >later corrected it to Ipk/pi for the series push-pull topology. >Sorry. No problem, at all. It means you are engaging yourself witgh what I write, even if sometimes that means a mistake is made in reading. And that means a great deal to me. Don't apologize. I am honored even by the fact that you imagined a problem there, because it means you scanned through it with your educated eye. >I'd like to chime in on your discussion of commercial power >ratings with David, but I'll have to leave that for later. Understood, and thanks in advance for any thoughts when they happen. Jon |