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#1
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the impedance to be any good?
It has been my contention that power amps that double their power as
impedance halves are the product of manipulation of specifications for marketing, not how real-world audio technology actually works. Here's a real world data point for the controversy about power amps needing to be able to put out twice the power for half the impedance: I ran some bench tests on what may be one of the most overbuilt power amps in the history of audio - the Threshold SA/4e from the early 1990s. No visible effort was spared to build a robust 100 wpc power amp, as this article shows: http://www.thresholdlovers.com/artic...?lng=en&pg=394 The Threshold SA/4e weighs about 110 pounds, according to my calibrated biceps. You really want two people to tote it around, and it facilitates this by having two comfortable handles on each end. In contrast, in similar tests a Behringer A500 which weighs about 15 pounds puts out about the same amount of power at clipping at 8 ohms, but puts out about 144 watts at both 4 and 2 ohms. This is what I would expect from a typical contemporary mainstream amplifier. Among other features, the Threshold SA/4e amplifier has extremely soft clipping. While most power amps, even tubed power amps have a fairly well-defined clipping point on the bench, not so with the SA/4e. In the end I decided to measure its output power at about 0.1% THD @ 1KHz, as observed with a RTA. The actual measured power is as listed he http://cgi.audiogon.com/cgi-bin/auc....82027&auc&3&4& Namely 112 Watts per channel @ 8 ohms with both channels driven @ 1000 Hz, 196 Watts @ 4 ohms, 264 Watts @ 2 ohms. This is still very far from doubling power as load impedance halves, particularly at 2 ohms. |
#2
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"ScottW2" wrote in message
On Aug 18, 5:51 am, "Arny Krueger" wrote: It has been my contention that power amps that double their power as impedance halves are the product of manipulation of specifications for marketing, not how real-world audio technology actually works. Here's a real world data point for the controversy about power amps needing to be able to put out twice the power for half the impedance: I ran some bench tests on what may be one of the most overbuilt power amps in the history of audio - the Threshold SA/4e from the early 1990s. No visible effort was spared to build a robust 100 wpc power amp, as this article shows: http://www.thresholdlovers.com/artic...?lng=en&pg=394 The Threshold SA/4e weighs about 110 pounds, according to my calibrated biceps. You really want two people to tote it around, and it facilitates this by having two comfortable handles on each end. In contrast, in similar tests a Behringer A500 which weighs about 15 pounds puts out about the same amount of power at clipping at 8 ohms, but puts out about 144 watts at both 4 and 2 ohms. This is what I would expect from a typical contemporary mainstream amplifier. Among other features, the Threshold SA/4e amplifier has extremely soft clipping. While most power amps, even tubed power amps have a fairly well-defined clipping point on the bench, not so with the SA/4e. In the end I decided to measure its output power at about 0.1% THD @ 1KHz, as observed with a RTA. The actual measured power is as listed he http://cgi.audiogon.com/cgi-bin/auc....82027&auc&3&4& Namely 112 Watts per channel @ 8 ohms with both channels driven @ 1000 Hz, 196 Watts @ 4 ohms, 264 Watts @ 2 ohms. This is still very far from doubling power as load impedance halves, particularly at 2 ohms. This is going to largely depend upon the speakers the amp is matched with and if the amp is driven near clipping into it's load at whatever max spl the listener wants from their system. I think that the one of the major goals of the SA/4e was to produce an amp that was as independent as possible of the characteristics of its speaker load. Ideally, if you want an amp that can drive any speaker load you can come up with, an amp that doubles power from 8 to 4 and doubles again to 2 will probably be more versatile than one that can't. There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. I'd suggest most folks should select their speakers and then decide what amp requirements are appropriate. They should avoid speakers that put extreme demands on amplifiers, as bad design in the area of impedance curve suggests bad design elsewhere. |
#3
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"Arny Krueger" wrote: "ScottW2" wrote in message On Aug 18, 5:51 am, "Arny Krueger" wrote: It has been my contention that power amps that double their power as impedance halves are the product of manipulation of specifications for marketing, not how real-world audio technology actually works. Here's a real world data point for the controversy about power amps needing to be able to put out twice the power for half the impedance: I ran some bench tests on what may be one of the most overbuilt power amps in the history of audio - the Threshold SA/4e from the early 1990s. No visible effort was spared to build a robust 100 wpc power amp, as this article shows: http://www.thresholdlovers.com/artic...?lng=en&pg=394 The Threshold SA/4e weighs about 110 pounds, according to my calibrated biceps. You really want two people to tote it around, and it facilitates this by having two comfortable handles on each end. In contrast, in similar tests a Behringer A500 which weighs about 15 pounds puts out about the same amount of power at clipping at 8 ohms, but puts out about 144 watts at both 4 and 2 ohms. This is what I would expect from a typical contemporary mainstream amplifier. Among other features, the Threshold SA/4e amplifier has extremely soft clipping. While most power amps, even tubed power amps have a fairly well-defined clipping point on the bench, not so with the SA/4e. In the end I decided to measure its output power at about 0.1% THD @ 1KHz, as observed with a RTA. The actual measured power is as listed he http://cgi.audiogon.com/cgi-bin/auc....82027&auc&3&4& Namely 112 Watts per channel @ 8 ohms with both channels driven @ 1000 Hz, 196 Watts @ 4 ohms, 264 Watts @ 2 ohms. This is still very far from doubling power as load impedance halves, particularly at 2 ohms. This is going to largely depend upon the speakers the amp is matched with and if the amp is driven near clipping into it's load at whatever max spl the listener wants from their system. I think that the one of the major goals of the SA/4e was to produce an amp that was as independent as possible of the characteristics of its speaker load. Ideally, if you want an amp that can drive any speaker load you can come up with, an amp that doubles power from 8 to 4 and doubles again to 2 will probably be more versatile than one that can't. There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). It is important to understand, though, that you *must* limit the lower limit of impedance for which the amp is allowed to be a voltage source, or things will melt... Isaac |
#4
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
isw wrote:
Ideally, if you want an amp that can drive any speaker load you can come up with, an amp that doubles power from 8 to 4 and doubles again to 2 will probably be more versatile than one that can't. There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). Actually thats not true. Solid state amps are indeed near perfect voltage sources until the voltage drop across a near saturated bipolar transistor becomes non-negligible, or a FET nears its low limit of resistance. Then the effective series resistance will cut power at high current (low load impedance). Now of course this can be fix by installing a high quality fixed voltage limit of the output sage. In other words, very hard fix-voltage clipping. Then down to some low load resistance value, the power output really will be inversely proportional to load resistance. This is the opposite of "soft clipping" and is not exactly in fact to be desired. Doug McDonald |
#5
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the impedance to be any good?
In article ,
"Arny Krueger" wrote: It has been my contention that power amps that double their power as impedance halves are the product of manipulation of specifications for marketing, not how real-world audio technology actually works. A pure voltage source (not a bad thing for an audio amp to be -- up to a point) will indeed double its power output every time the load impedance is halved. This is OK for 16, 8, 4, maybe 2, and possibly 1 ohm loads. But every time the load halves, the current doubles (because the voltage is constant), so it's probably a good idea to have some mechanism (other than the flying leads inside the output devices) that prevents the thing from trying to deliver a megamp or two into a micro-ohm load. Isaac |
#6
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"isw" wrote in message
There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). Simply not so. While modern power amps behave like constant-voltage sources, they only do that within bounds. The bounds are set by gross losses in the output stage and power supply that won't go away and can't be corrected by negative feedback. It is important to understand, though, that you *must* limit the lower limit of impedance for which the amp is allowed to be a voltage source, or things will melt... Every power amp has limited output power, even at 8 ohms. |
#7
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
On Thu, 20 Aug 2009 06:39:20 -0700, isw wrote
(in article ): In article , "Arny Krueger" wrote: [quoted text deleted -- deb] There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). It is important to understand, though, that you *must* limit the lower limit of impedance for which the amp is allowed to be a voltage source, or things will melt... Isaac Since Power = V squared / R, I don't see how any power amp can be "divorced" from it's load. Anyone can see that power increases as R gets smaller and that a dead short (R=1 Ohm. It won't be zero because no solid-state output device has zero "on" resistance) results in power being, essentially, the same as V squared. Since it's doubtful that any amp can supply that much voltage, the power supply becomes the limiting factor. For instance: assuming a V swing of 100 Volts * 100 volts squared is 10,000/8 = 1250 watts and 10,000/4 = 2500 Watts and 10,000/2 = 5,000 watts etc. Since power is also a product of current (voltage, current and resistance are, as we know, inexorably linked together) as in P = VI and I squared times R. The power supply has to be able to deliver enough current across the load to develop sufficient voltage to produce the power associated with the example given. I.E. in real applications, the current limits of the amp's power supply will insure that less and less voltage is developed across the speaker as the load impedance drops, but at some point, usually before the power supply taps out, the collector current In a bi-polar output device (drain current in a FET) is going to exceed the device's rating (I = E/R) and the transistor is going to fail. So the truth is that in order for any amplifier to keep doubling it's power as the load impedance drops, it would have to have a power supply capable of maintaining the amp's rail voltages regardless of load by supplying unlimited current (and only Morbius' ancient Krell in "Forbidden Planet" could manage that) and it would require unlimited I sub c on the output transistors. Even wildly over-built amplifiers like the previous poster's SA/4e cannot meet those requirements. Of course you can parallel output devices to increase current handling, but there's a practical limit to that as well. |
#8
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
On Thu, 20 Aug 2009 16:55:38 -0700, Arny Krueger wrote
(in article ): "isw" wrote in message There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). Simply not so. While modern power amps behave like constant-voltage sources, they only do that within bounds. The bounds are set by gross losses in the output stage and power supply that won't go away and can't be corrected by negative feedback. It is important to understand, though, that you *must* limit the lower limit of impedance for which the amp is allowed to be a voltage source, or things will melt... Every power amp has limited output power, even at 8 ohms. Because P =VI and where V (rail Voltage) is a constant, I, current, must double every time R (load impedance) is halved (P= I squared times R) to maintain V. All amps have a current limit, but up to that point, power does, indeed, double every time the load impedance is halved. |
#9
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"Arny Krueger" wrote: "isw" wrote in message There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). Simply not so. While modern power amps behave like constant-voltage sources, they only do that within bounds. Well, that's what I *thought* I was saying... Every power amp has limited output power, even at 8 ohms. Of course; limited by the available supply voltage, if nothing else. Let's try this: Acting like a voltage source is a very good thing for an audio amplifier to do. IF there are circumstances where the amp fails to act as a voltage source, then it's probably not the right amp for the job. Put another way: If your amplifier fails to act as a pure voltage source IN ANY WAY while you're trying to play music through it, you need a different amplifier. Period. An amp failing to act as a voltage source is guaranteed to cause distortion. (This applies to domestic reproduction; not professional sound reinforcement situations). Isaac |
#10
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"isw" wrote in message
Let's try this: Acting like a voltage source is a very good thing for an audio amplifier to do. True within limits. Those limits are often known as "Power ratings". They are generally strongly dependent on load impedance. IF there are circumstances where the amp fails to act as a voltage source, then it's probably not the right amp for the job. Every real world amp has circumstances where it fails to act like a voltage source. Run it outside its ratings. Run it into too-low of an impedance. Don't give it enough power line power and/or voltage. Put another way: If your amplifier fails to act as a pure voltage source IN ANY WAY while you're trying to play music through it, you need a different amplifier. You've got at least one other option which is to obtain more effcient speakers. Period. An unqualified statement, and therefore likely to be false on the face of it. In fact, its false. An amp failing to act as a voltage source is guaranteed to cause distortion. (This applies to domestic reproduction; not professional sound reinforcement situations). Professional sound reinforcment systems follow the same rules of electrical engieneering basic as home hi fi. I can state unequivocally that An amp failing to act as a voltage source is guaranteed to cause distortion, whether its home audio or pro audio. |
#11
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
Arny Krueger wrote:
I can state unequivocally that An amp failing to act as a voltage source is guaranteed to cause distortion, whether its home audio or pro audio. Well, yes, if you include frequency response changes as distortion. But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Fed into a pure resistance it will have no distortion whether you measure voltage at output terminal or current. Fed into a reactive load it will have frequency and phase response distortion. Fed into a nonlinear response load (e.g. a diode) it will generate harmonic and intermodulation distortion, even measured as voltage. Doug |
#12
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"mcdonaldREMOVE TO ACTUALLY REACH wrote
in message Arny Krueger wrote: I can state unequivocally that An amp failing to act as a voltage source is guaranteed to cause distortion, whether its home audio or pro audio. Well, yes, if you include frequency response changes as distortion. Calling frequency response changes "Linear Distortion" is accepted practice. But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Fed into a pure resistance it will have no distortion whether you measure voltage at output terminal or current. Actually, there will be a broadband frequency response error - a level shift. Fed into a reactive load it will have frequency and phase response distortion. Fed into a nonlinear response load (e.g. a diode) it will generate harmonic and intermodulation distortion, even measured as voltage. True. |
#13
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"Arny Krueger" wrote: "isw" wrote in message Let's try this: Acting like a voltage source is a very good thing for an audio amplifier to do. True within limits. Those limits are often known as "Power ratings". They are generally strongly dependent on load impedance. IF there are circumstances where the amp fails to act as a voltage source, then it's probably not the right amp for the job. Every real world amp has circumstances where it fails to act like a voltage source. Run it outside its ratings. Run it into too-low of an impedance. Don't give it enough power line power and/or voltage. Put another way: If your amplifier fails to act as a pure voltage source IN ANY WAY while you're trying to play music through it, you need a different amplifier. You've got at least one other option which is to obtain more effcient speakers. Which, if I'm not mistaken would (should) let the amplifier work without overloading (i.e. act like a voltage source again). Period. An unqualified statement, and therefore likely to be false on the face of it. In fact, its false. OK. Please explain how an amplifier can behave NOT as a voltage source and not generate distortion in the signal. An amp failing to act as a voltage source is guaranteed to cause distortion. (This applies to domestic reproduction; not professional sound reinforcement situations). Professional sound reinforcment systems follow the same rules of electrical engieneering basic as home hi fi. I can state unequivocally that An amp failing to act as a voltage source is guaranteed to cause distortion, whether its home audio or pro audio. I stand corrected. I tried to make that comment too succinct and failed. What I was intending to say is that while there are possibly good reasons to allow amplifying gear to operate NOT like a voltage source in professional sound reinforcement applications, there's really no good reason to allow non-linear operation when you're trying to do a good job of reproducing music in a home. Isaac |
#14
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"mcdonaldREMOVE TO ACTUALLY REACH wrote: Arny Krueger wrote: I can state unequivocally that An amp failing to act as a voltage source is guaranteed to cause distortion, whether its home audio or pro audio. Well, yes, if you include frequency response changes as distortion. But of course they are; they represent a failure of the amp to be a voltage source, no? But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Well, that makes it a voltage source with a one ohm series resistance in the output. Not too different from a real-world situation where speaker leads, crossover inductors, and voice coils have quite measurable resistances. Fed into a pure resistance it will have no distortion whether you measure voltage at output terminal or current. Well, if you vary your pure resistance, you'll see that (because of that one-ohm resistor) the gain of the amplifier appears to be a function of load; the lower the load resistance, the lower the output voltage. Would you define that as a sort of distortion? I'm not sure, but am inclined to; certainly it violates the "straight wire with gain" notion. Fed into a reactive load it will have frequency and phase response distortion. True in theory, but we're talking about *audio* amplifiers here. Do you know of any actual loudspeakers that have enough reactance to cause a significant problem (let's ignore electrostats here, because they usually require special treatment in other ways)? Fed into a nonlinear response load (e.g. a diode) it will generate harmonic and intermodulation distortion, even measured as voltage. Again true in theory, but how many loudspeakers have diodes inside (excepting back-to-back zeners used for protection; they should never conduct except under pathological conditions)? Isaac |
#15
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"mcdonaldREMOVE TO ACTUALLY REACH wrote: isw wrote: Ideally, if you want an amp that can drive any speaker load you can come up with, an amp that doubles power from 8 to 4 and doubles again to 2 will probably be more versatile than one that can't. There is no such thing as a simple amplfiier that doubles power when you halve impedance. Vastly overbuilt whales like the SA/4e prove this out. That's not so; any amp that is a voltage source will do precisely that (and being nearly a perfect voltage source is pretty automatic for a decent solid-state amp). Actually thats not true. Solid state amps are indeed near perfect voltage sources until the voltage drop across a near saturated bipolar transistor becomes non-negligible, or a FET nears its low limit of resistance. Then the effective series resistance will cut power at high current (low load impedance). Now of course this can be fix by installing a high quality fixed voltage limit of the output sage. In other words, very hard fix-voltage clipping. Then down to some low load resistance value, the power output really will be inversely proportional to load resistance. This is the opposite of "soft clipping" and is not exactly in fact to be desired. IMO if your amplifier clips (i.e. if it *ever* fails to be a voltage source while you're using it to listen to music), you need a bigger one (or, as Arnie said, more efficient speakers). Isaac |
#16
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
isw wrote:
In article , "mcdonaldREMOVE TO ACTUALLY REACH wrote: But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Fed into a reactive load it will have frequency and phase response distortion. True in theory, but we're talking about *audio* amplifiers here. Do you know of any actual loudspeakers that have enough reactance to cause a significant problem Oh yes ... absolutely. Loudspeakers are highly, exceedingly reactive. Normal ones expect to be driven from a voltage source. I should add that speakers are reactive even if you consider a single driver with no external electronics. The mechanical part of the electro-mechanical system has resonances etc. and these back up into the electrical system and look like reactance. Doug |
#17
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
In article ,
"mcdonaldREMOVE TO ACTUALLY REACH wrote: isw wrote: In article , "mcdonaldREMOVE TO ACTUALLY REACH wrote: But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Fed into a reactive load it will have frequency and phase response distortion. True in theory, but we're talking about *audio* amplifiers here. Do you know of any actual loudspeakers that have enough reactance to cause a significant problem Oh yes ... absolutely. Loudspeakers are highly, exceedingly reactive. Normal ones expect to be driven from a voltage source. I should add that speakers are reactive even if you consider a single driver with no external electronics. The mechanical part of the electro-mechanical system has resonances etc. and these back up into the electrical system and look like reactance. I don't doubt that there are reactances associated with speakers -- reactances are *everywhere*. But, do you know of any speakers with sufficient reactance *at audio frequencies* to cause problems such as frequency response errors or phase errors? (Sure, I know there might be crossovers inside; presumably they are doing the "right thing" -- whatever the designer intends). Isaac |
#18
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"mcdonaldREMOVE TO ACTUALLY REACH wrote:
True in theory, but we're talking about *audio* amplifiers here. Do you know of any actual loudspeakers that have enough reactance to cause a significant problem Oh yes ... absolutely. Loudspeakers are highly, exceedingly reactive. No, they are not "exceedingly" reactive, not by any reasonably objective definition of the term. If, for example, be "exceedingly reactive," you mean that over their specified bandwidth (or over the full audio bandwidth of 20 Hz to 20 kHz), the magnitude of the reactive portion of the impedance exceeds the resistive portion the majority of the time, then no loudspeaker system meets that definition of "exceedingly reactive." Even if you use a definition that is far more generous, like the reactive portion of the impedance dominates over at least, oh, 20% of the speaker's bandwidth, then maybe 1 in 100 models and probably 1 in 1,000 speakers sold may barely meet that qualification. For the VAST majority of speakers hooked to the VAST majority of amplifiers, the impedance almost never is dominantly reactive. The phase angle of the impedance almost alwyas approaches, but exceedingly ever wanders outside the +-45 degree limits that signal the objective border between "domanantly resistive" and "dominantly reactive." Normal ones expect to be driven from a voltage source. This is true. I should add that speakers are reactive even if you consider a single driver with no external electronics. As is this. The mechanical part of the electro-mechanical system has resonances etc. and these back up into the electrical system and look like reactance. Generally true. One thing many commentators and, regrettably, self-appointed luminaries and designers in the audio industry fail utterly to comprehend is that, to the amplifier, there is no difference between a speaker with its mechanical components forming a collection of resonant systems and an equivalent collection of resistors, capacitors and inductors. Magical, special and myetrious attributes are assigned to masses attached to voices coils moving in magnetic fields and "back EMF" that are, pgysically, NO different electrically than a parallel resonant tank circuit consisting of an inductor, capacitor and resistor. Electrically, they behave EXACTLY the same way, and elctrically is the ONLY way the amplifier knows about the speaker. -- +--------------------------------+ + Dick Pierce | + Professional Audio Development | +--------------------------------+ |
#19
Posted to rec.audio.high-end
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Do power amps need to deliver twice the power into half the
"mcdonaldREMOVE TO ACTUALLY REACH wrote
in message isw wrote: In article , "mcdonaldREMOVE TO ACTUALLY REACH wrote: But consider a perfect amp with a one ohm resistor hidden inside it between the feedback loop and the output terminal. Fed into a reactive load it will have frequency and phase response distortion. True in theory, but we're talking about *audio* amplifiers here. Do you know of any actual loudspeakers that have enough reactance to cause a significant problem Oh yes ... absolutely. Loudspeakers are highly, exceedingly reactive. In the cosmic scheme of things, not exceedingly. For example, consider a standard Fluorescent balast. Normal ones expect to be driven from a voltage source. Agreed. I should add that speakers are reactive even if you consider a single driver with no external electronics. The mechanical part of the electro-mechanical system has resonances etc. and these back up into the electrical system and look like reactance. Right. |
#20
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Do power amps need to deliver twice the power into half the
Dick Pierce wrote:
For the VAST majority of speakers hooked to the VAST majority of amplifiers, the impedance almost never is dominantly reactive. The phase angle of the impedance almost alwyas approaches, but exceedingly ever wanders outside the +-45 degree limits Sorry, I meant to write "only exceedingly rarely" instead of the gobbldeygook I did. -- +--------------------------------+ + Dick Pierce | + Professional Audio Development | +--------------------------------+ |
#21
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Do power amps need to deliver twice the power into half the
In article ,
Dick Pierce wrote: Dick Pierce wrote: For the VAST majority of speakers hooked to the VAST majority of amplifiers, the impedance almost never is dominantly reactive. The phase angle of the impedance almost alwyas approaches, but exceedingly ever wanders outside the +-45 degree limits Sorry, I meant to write "only exceedingly rarely" instead of the gobbldeygook I did. But some are and some go to very low impedances at some point of their frequency response. I'm not sure if double is required, but it can't hurt. I have a pair of Apogee Divas hooked up to Classe power amps. I don't recall the specifics, but I remember they get to some ridiculous low impedance and the Classe amps will double output when impedance drops in half. They work with no audible strain. I can't say something less wouldn't work as well, but I know anything way less, like a Dynaco Stereo 70, won't because I fried one playing with it. -- Robert B. Peirce, Venetia, PA 724-941-6883 bob AT peirce-family.com [Mac] rbp AT cooksonpeirce.com [Office] |
#22
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Do power amps need to deliver twice the power into half the
Robert Peirce wrote:
In article , Dick Pierce wrote: Dick Pierce wrote: For the VAST majority of speakers hooked to the VAST majority of amplifiers, the impedance almost never is dominantly reactive. The phase angle of the impedance almost alwyas approaches, but exceedingly ever wanders outside the +-45 degree limits Sorry, I meant to write "only exceedingly rarely" instead of the gobbldeygook I did. But some are and some go to very low impedances at some point of their frequency response. I'm not sure if double is required, but it can't hurt. I have a pair of Apogee Divas hooked up to Classe power amps. I don't recall the specifics, but I remember they get to some ridiculous low impedance and the Classe amps will double output when impedance drops in half. They work with no audible strain. You missed the point in several dimensions. First, they may well get to a few femtoOhms impedance (they don't). That does not necessarily make them "exceedingly reactive." That makes them low impedance. Low impedance and "exceedingly" reactive are NOT equivalent. Even normal impedance at some frequencies and low at others does not constitute "exceedingly reactive." Over the substantial portions bandwidth of the system, does the phase angle of the impedance exceeding +-45 degrees (it does not)? If not, they are not "exceedingly reactive." In fact, the impedance curves of the Divas is probably as close to proving my point as anything you could have picked: the present about as close to a resistive load over the entire bandwidth as any speaker could imaginably do without restoring to complex compensation networks. Their impedance varies from about 3.5 to about 4,5 ohms, and the phase angle seldom exceeds +-20 degrees. Secondly, Apogee Divas are but one of several hundreds of high end speaker models. And, given the numbers of them out there, the comprise a vanishing, insignificant minority of the speakers available. So even if the impedance phase angle spent most of its time between +- 45 degress and +- 90 degrees (which is most assuredly does not), that does not mean that, as the poster claimed, "speakers are highly, exceedingly reactive." That generalization is simply at variance with the known physical facts. -- +--------------------------------+ + Dick Pierce | + Professional Audio Development | +--------------------------------+ |
#23
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Do power amps need to deliver twice the power into half the
Dick Pierce wrote:
In fact, the impedance curves of the Divas is probably as close to proving my point as anything you could have picked: the present about as close to a resistive load over the entire bandwidth as any speaker could imaginably do without restoring to complex compensation networks. Their impedance varies from about 3.5 to about 4,5 ohms, and the phase angle seldom exceeds +-20 degrees. Secondly, Apogee Divas are but one of several hundreds of high end speaker models. And, given the numbers of them out there, the comprise a vanishing, insignificant minority of the speakers available. So even if the impedance phase angle spent most of its time between +- 45 degress and +- 90 degrees (which is most assuredly does not), that does not mean that, as the poster claimed, "speakers are highly, exceedingly reactive." That generalization is simply at variance with the known physical facts. We simply took different views on "exceedingly reactive". I suppose you are right, though you have to admit 90 degrees is "exceedingly reactive" and I'd say 80 degrees was too. I was considering 60 degrees, which some speakers reach (and not at ultrasonic frequencies), "exceedingly reactive". I do agree that most speakers stop at roughly 45 degrees. In any case, unless you put in an 8 ohm series resistance, it does not matter. Doug McDonald |
#24
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Do power amps need to deliver twice the power into half the
"mcdonaldREMOVE TO ACTUALLY REACH wrote:
Dick Pierce wrote: In fact, the impedance curves of the Divas is probably as close to proving my point as anything you could have picked: the present about as close to a resistive load over the entire bandwidth as any speaker could imaginably do without restoring to complex compensation networks. Their impedance varies from about 3.5 to about 4,5 ohms, and the phase angle seldom exceeds +-20 degrees. Secondly, Apogee Divas are but one of several hundreds of high end speaker models. And, given the numbers of them out there, the comprise a vanishing, insignificant minority of the speakers available. So even if the impedance phase angle spent most of its time between +- 45 degress and +- 90 degrees (which is most assuredly does not), that does not mean that, as the poster claimed, "speakers are highly, exceedingly reactive." That generalization is simply at variance with the known physical facts. We simply took different views on "exceedingly reactive". Luckily, there are objective definitions for reactivity on which we can reliably depend. I suppose you are right, though you have to admit 90 degrees is "exceedingly reactive" And, there does not exist a single speaker anywhere that comes close to 90 degrees total phase shift. None. That is something that can be said with certainty. and I'd say 80 degrees was too. And I have, in the thousands of speaker systems I have measured, to the best of my memory, never encountered a speaker that reached 80 degrees. I was considering 60 degrees, which some speakers reach Very few speakers exceed 45 degrees, and then only over very narrow ranges of frequency. (and not at ultrasonic frequencies), "exceedingly reactive". I do agree that most speakers stop at roughly 45 degrees. No, they may approach it, but few if any ever make it there in any substantive fashion. Simply stated, if the phase shift of the impedance is between -45 degrees and +45 degrees, the impedance is dominated by the resistive portion. Only when it exceeds +-45 degrees does the reactive portion of the impedance dominate. And it is NEVER +-90 degrees and it NEVER exceeds +-90 degrees. The reasons why are left as an excercise to the student. :-) In any case, unless you put in an 8 ohm series resistance, it does not matter. Or, you hook it to some amplifier designed by some incompent high-end loonie, of which there are a disturbing number of examples. -- +--------------------------------+ + Dick Pierce | + Professional Audio Development | +--------------------------------+ |
#25
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Do power amps need to deliver twice the power into half the impedance to be any good?
"Arny Krueger" wrote in message
... It has been my contention that power amps that double their power as impedance halves are the product of manipulation of specifications for marketing, not how real-world audio technology actually works. **Of course. It is impossible for an amplifier to manage such a feat in the real world. OTOH, many manufacturers deliberately understate the 8 Ohm (and 4 Ohm) power levels, such that it APPEARS that they are doubling their maximum power into successively lower impedances. Here's a real world data point for the controversy about power amps needing to be able to put out twice the power for half the impedance: I ran some bench tests on what may be one of the most overbuilt power amps in the history of audio - the Threshold SA/4e from the early 1990s. No visible effort was spared to build a robust 100 wpc power amp, as this article shows: http://www.thresholdlovers.com/artic...?lng=en&pg=394 The Threshold SA/4e weighs about 110 pounds, according to my calibrated biceps. You really want two people to tote it around, and it facilitates this by having two comfortable handles on each end. In contrast, in similar tests a Behringer A500 which weighs about 15 pounds puts out about the same amount of power at clipping at 8 ohms, but puts out about 144 watts at both 4 and 2 ohms. This is what I would expect from a typical contemporary mainstream amplifier. Among other features, the Threshold SA/4e amplifier has extremely soft clipping. While most power amps, even tubed power amps have a fairly well-defined clipping point on the bench, not so with the SA/4e. In the end I decided to measure its output power at about 0.1% THD @ 1KHz, as observed with a RTA. The actual measured power is as listed he http://cgi.audiogon.com/cgi-bin/auc....82027&auc&3&4& Namely 112 Watts per channel @ 8 ohms with both channels driven @ 1000 Hz, 196 Watts @ 4 ohms, 264 Watts @ 2 ohms. This is still very far from doubling power as load impedance halves, particularly at 2 ohms. **Of course. That would be an impossibility. Saturation effects in output devices precludes such a thing. BTW: Did Mr Pass ever claim such a thing about his amplifiers? -- Trevor Wilson www.rageaudio.com.au |
#26
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Do power amps need to deliver twice the power into half the
In article ,
Dick Pierce wrote: You missed the point in several dimensions. First, they may well get to a few femtoOhms impedance (they don't). That does not necessarily make them "exceedingly reactive." You are correct. -- Robert B. Peirce, Venetia, PA 724-941-6883 bob AT peirce-family.com [Mac] rbp AT cooksonpeirce.com [Office] |
#27
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Do power amps need to deliver twice the power into half the
The best scientific explanation of real world power in amplification:
http://www.youtube.com/watch?v=EbVKWCpNFhY [ For people who don't want to click through without some context -- it's the "Goes to 11" scene from "This is Spinal Tap" -- dsr ] |
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