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From: David Eather on 24 Feb 2010 00:07 On 23/02/2010 4:03 AM, Jon Kirwan 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. I'm not sure that is right, pure Class A doesn't degrade - does it have a Vbe multiplier? Class AB can degrade into pure class B if the Vbe multiplier is not set properly. > > Jon
From: David Eather on 24 Feb 2010 00:09 On 23/02/2010 4:47 AM, Jon Kirwan wrote: > 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. No problem for me. > > 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 I don't mind at all but your post gets a little weird from here on. > Path: border1.nntp.dca.giganews.com!border2.nntp.dca.giganews.com!nntp.giganews.com!postnews.google.com!z35g2000yqd.googlegroups.com!not-for-mail > From: HC<hboothe(a)gte.net> > Newsgroups: sci.electronics.basics > Subject: Re: Stepper motor controller current limiting question > Date: Mon, 22 Feb 2010 14:50:10 -0800 (PST) > Organization: http://groups.google.com > Lines: 64 > Message-ID:<dc3deac8-526d-4905-a712-dc56208dc8e0(a)z35g2000yqd.googlegroups.com> > References:<6fbefe76-25c0-43b8-bfa5-4eef048414a7(a)o30g2000yqb.googlegroups.com> > <4b7beb28.1102823(a)news.eternal-september.org> > NNTP-Posting-Host: 64.31.193.99 > Mime-Version: 1.0 > Content-Type: text/plain; charset=ISO-8859-1 > Content-Transfer-Encoding: quoted-printable > X-Trace: posting.google.com 1266879010 20924 127.0.0.1 (22 Feb 2010 22:50:10 GMT) > X-Complaints-To: groups-abuse(a)google.com > NNTP-Posting-Date: Mon, 22 Feb 2010 22:50:10 +0000 (UTC) > Complaints-To: groups-abuse(a)google.com > Injection-Info: z35g2000yqd.googlegroups.com; posting-host=64.31.193.99; > posting-account=c36OOwkAAAAFaUePMpNASSMX39HewSQW > User-Agent: G2/1.0 > X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; > InfoPath.1; .NET CLR 1.1.4322),gzip(gfe),gzip(gfe) > Bytes: 3875 > X-Original-Bytes: 3832 > Xref: number.nntp.dca.giganews.com sci.electronics.basics:320498 > > On Feb 17, 7:19=A0am, N0S...(a)daqarta.com (Bob Masta) wrote: >> On Tue, 16 Feb 2010 15:57:04 -0800 (PST), HC >> >> >> >> >> >> <hboo...(a)gte.net> wrote: >>> Hello, all. =A0I'm trying to build my first stepper motor controller and >>> I have two questions. =A0In trying to learn about stepper motors and >>> controllers I read the document AN907 from Microchip among other >>> documents and sites:http://ww1.microchip.com/downloads/en/AppNotes/00907= > a.pdf >> >>> In that document the author writes about current limiting a stepper >>> motor that is purposefully driven at a voltage that is higher than its > 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: Jon Kirwan on 24 Feb 2010 03:06 On Wed, 24 Feb 2010 15:03:18 +1000, David Eather <eather(a)tpg.com.au> wrote: >I'm leaving all the sniping to you because your the one who knows what >you want to investigate further. "Sniping" in the US has a negative connotation, which I'm not sure you intended. I hope I'm not coming across in some negative way. If so, I do apologize and will try for better. I really do appreciate the time you've offered me. >Also, for today, I am using mostly using "perfect components" and >theoretical efficiencies etc to make my life easier - they will show the >point and I am talking about an amp spec'd for consumer audio. Okay. >On 23/02/2010 2:41 AM, Jon Kirwan wrote: >> 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. > >No, that's the power of a 12.7 volt (peak) sine wave into an 8 ohm load. >An instantaneous peak power figure would be (Vmax**2)/R or 20watts. Clearly understood. I just wasn't thinking well at the moment. I'm exactly with you on this. >> 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? > >Think of it this way: > >You build an amplifier that puts out a 10 watt sine wave into 8 ohms >100% of the time. For a power transformer you will need something like a >30 volt CT rated at 40VA for this design (this one is a realistic not >theoretical estimate). > >Use this amp in a consumer environment and the customer is happy about >everything except the cost. > >Can you lower the cost without damaging the output quality? > >Yes. As mentioned earlier the long term average output power of the >amplifier will be about 2 watts and any transformer will have a very >long thermal time constant compared to any other component in the >amplifier, so there is no danger of overheating during a peak in the >music output. 40VA x 0.2 = 8VA. An 8VA transformer is big enough (in >real life you would use a little bigger because of the increased I**2 x >R losses 10VA would probably be a good choice, (if you had more >information you could make a better choice but the result would be very >close to 10VA) > >A 10VA transformer costs a whole lot less than 40VA and all you have >done is removed an unnecessary over-specification of a component and >that will have zero effect to the consumer. > >You can do the same thing with the heatsink, but it is not so dramatic a >change, and it needs more care. On a small amp like this where the cost >of a heatsink is low I wouldn't bother - except as an exercise or if you >were making hundreds of them. You won't be able to proceed here until >you have a more finalized design. I think I follow all of this. I guess my earlier writing was about my own realizations and nothing else. I wrote more strongly then because I'm just "seeing" a little better, is all. >> 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. > >recheck everyone's figures Tentatively, I'm lumping his tabled results into effects I'm less aware of, for now. Context will become clearer, later. >> 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? > >20 Watts - you were right. My figure wrong. Thanks. >>>> 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.) > >You seem to be doing OK. Maybe we should brush up and compare notes on >the meanings of average and RMS but that's about it I think. > >A class A amp say at 10 watts into 8 ohms will have an output stage with >a constant current sink (or source) set at 1.59 amps. I know there are a number of structures, but I like to think in terms of two BJTs, one NPN and one PNP, in a push-pull arrangement to the rails -- for class-A -- with a Vbe multiplier set to cause both BJTs to have at least some emitter current at all times. I know that there is also a single-ended arrangement. But I never give that one more than a very slight nod. It's way, way too inefficient to care about. I'm wondering if that is what you are talking about here. If so, then you'd indeed set Iq to be 1.59 amps, either to the (+) or (-) rails, as I read you saying here. Because then the single-ended BJT can either source/sink nothing causing the -1.59 (or +1.59) amps to flow from speaker to rail or the single-ended BJT can source/sink 3.18 amps, only 1.59 of which gets wasted via Iq and the rest going to the speaker. However, I don't think much about that arrangement and I'm sure that Self wasn't talking about an amplifier designed that way. I'm pretty sure he was discussing a push-pull class-A amplifier. >If the speaker >load changes to 4 ohms the maximum current into and out of the speaker >is still 1.59 amps. In the single-ended case, _very_ generally, yes. But let me walk you through my single-ended thoughts. (I hate single ended designs, so I hope I don't have to think about them again!) Let's assume we have (+), (-), and ground rails. Let's arrange it so that the Iq current is a sink, as you stated. So it goes from (-) to the speaker pin. There is a BJT that goes from the speaker pin and up to (+). It's base is driven by the VAS and the emitter simply follows that. The speaker load goes to ground. That's what I'm imagining you are talking about. At 8 ohms and Iq = -1.59A, lets say that the emitter can rise up to the point where Iload = +1.59A. (So about +12.7V at the emitter, as discussed regularly.) This means the BJT's emitter must be sourcing 3.18 amps, enough to supply both the speaker and Iq. If the BJT effectively turns off and its emitter current goes to about 0, then Iq causes Iload = -1.59A. A 4 ohm load would still "see" no lower than -1.59A, since that is all that Iq can do. However, when the emitter rises again to it's +12.7V (driven by the same exact signal at its base, by assumption), then we will have 12.7V across 4 ohms and +3.18A into the speaker. The BJT will not only have to supply that, but also the 1.59A required by Iq. That's 4.77A total. So it will operate in class-AB, now. So I'm pretty sure I'm _not_ following you when you say the output current is the same. Which I take to mean there is something very wrong with the way I'm seeing this. >How's the power now? Best to wait for your knock on my head about this. I would have first preferred to talk about a push-pull class-A case, which seems fundamentally different. But I'm still learning and have to assume I am getting all of this wrong. >Self is talking about his practical results and if you dig around you >will see/find he believes in over-biasing the output stage current >source by 50% - 100% hence the apparent anomaly. ><snip> I think he was NOT talking about single-ended class-A designs, but instead push-pull class-A. I get your point, I think, about over-biasing -- it is something I already feel I'd want to do, too... though I'm not sure 50% is right and even then I don't think he spent much of any time at all talking about single-ended designs (for reasons I think I agree with.) I feel like I probably failed to get some point I should have and so I'll stop here and wait. Very much appreciated, Jon
From: Jon Kirwan on 24 Feb 2010 03:09 On Wed, 24 Feb 2010 15:07:23 +1000, David Eather <eather(a)tpg.com.au> wrote: >On 23/02/2010 4:03 AM, Jon Kirwan 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. > >I'm not sure that is right, pure Class A doesn't degrade In my mind it can. But I only think in terms of class-A push pull class-A, not single-ended. Maybe I should be more detailed when I write? > - does it have a Vbe multiplier? It can. It doesn't have to have one, though. I was just thinking about the case where there is one -- a push-pull that looks a lot like a class-B stage but with higher Vbias. >Class AB can degrade into pure class B if the Vbe >multiplier is not set properly. Right. Jon
From: David Eather on 24 Feb 2010 06:13
On 24/02/2010 3:03 PM, David Eather wrote: > Joh, > > I'm leaving all the sniping to you because your the one who knows what > you want to investigate further. > > Also, for today, I am using mostly using "perfect components" and > theoretical efficiencies etc to make my life easier - they will show the > point and I am talking about an amp spec'd for consumer audio. > > On 23/02/2010 2:41 AM, Jon Kirwan wrote: >> 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. > > No, that's the power of a 12.7 volt (peak) sine wave into an 8 ohm load. > An instantaneous peak power figure would be (Vmax**2)/R or 20watts. > >> >> 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? > > > Think of it this way: > > You build an amplifier that puts out a 10 watt sine wave into 8 ohms > 100% of the time. For a power transformer you will need something like a > 30 volt CT rated at 40VA for this design (this one is a realistic not > theoretical estimate). It's been a bad day 40VA is way wrong. It should be more like 12VA > > Use this amp in a consumer environment and the customer is happy about > everything except the cost. > > Can you lower the cost without damaging the output quality? > > Yes. As mentioned earlier the long term average output power of the > amplifier will be about 2 watts and any transformer will have a very > long thermal time constant compared to any other component in the > amplifier, so there is no danger of overheating during a peak in the > music output. 40VA x 0.2 = 8VA. This 8VA transformer should be more like 2.3VA. My apologies - brain is slowly dying. An 8VA transformer is big enough (in > real life you would use a little bigger because of the increased I**2 x > R losses 10VA would probably be a good choice, (if you had more > information you could make a better choice but the result would be very > close to 10VA) > > A 10VA transformer costs a whole lot less than 40VA and all you have > done is removed an unnecessary over-specification of a component and > that will have zero effect to the consumer. > > You can do the same thing with the heatsink, but it is not so dramatic a > change, and it needs more care. On a small amp like this where the cost > of a heatsink is low I wouldn't bother - except as an exercise or if you > were making hundreds of them. You won't be able to proceed here until > you have a more finalized design. > > > >> >> 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. > > recheck everyone's figures > >> >> 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? > > 20 Watts - you were right. My figure wrong. > >> >>>> 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.) > > You seem to be doing OK. Maybe we should brush up and compare notes on > the meanings of average and RMS but that's about it I think. > > A class A amp say at 10 watts into 8 ohms will have an output stage with > a constant current sink (or source) set at 1.59 amps. If the speaker > load changes to 4 ohms the maximum current into and out of the speaker > is still 1.59 amps. How's the power now? > > Self is talking about his practical results and if you dig around you > will see/find he believes in over-biasing the output stage current > source by 50% - 100% hence the apparent anomaly. > >> >>>> 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. ;) > > In Sound Lounges no one can hear you scream.... > >> >> Jon > |