Bifwynne, if I may, not trying to split any hairs, but the term SOA has a very specific meaning for ss devices. The Safe Operating Area defines the combinations of current and voltage across the device, that it can withstand for a certain time before being destroyed. For instance, a power transistor might be rated for 100W dissipation, but that's not the full story. 10 amps at 10V, being 100W, would be OK indefinitely. BUT, 1 amp at 100V would NOT be OK indefinitely, although it is also 100W. That 1A/100V combination can only be withstood for, say, 100 milliseconds, and then it will let out all the magic smoke*.
Now over to amplifiers: they also have an area where they will operate as designed. As you noted, drive the level too high, they will clip. Drive too much level in too low a load and they may eventually overheat and break. One common accident on the test bench is full power at 20kHz - not many amps can hold out on that, and of course during music reproduction, they don't have to. But all this is not referred to as 'the SOA' of the amp.
* The reason is that at higher voltages, the current tends to hog a small part of the transistor die, so that small spot will heat up much more that the rest of the die. That small spot cannot handle the 100W the whole transistor could handle. It blows. |
Every once in a while I remember to use my memory :-).
One of the speakers used here was the earlier ML Aerius and not the OPs ML Aerius i, nevertheless it might be germane:
http://www.stereophile.com/reference/810/index.html |
I would like to thank Jan Didden for laying this furfie "NF feedback DOES NOT effect Z=out or damping factor" out in the open, in his usual calm and palpable way. His is a bottomless pit of correct information over at the much more techincal forums, along with the usual others like Nelson Pass, John Curl, and many others, there should be more of them to police bad info on forums. http://www.wordsense.eu/furfie/Cheers George |
George don't make me blush :-).
Anyway. Atmaspere's statement that the INTERNAL Zout doesn't change has another side to it. It suggests that you can open up an amp and point to a component and say see, there's the output impedance. This is not the case. Apart from a few minor things like the resistance of the cabling from PC board to speaker connector, the Zout is sort of virtual. One cause for instance is the fact that the gain of the output transistors drops when you request more output current, so the output voltage drops a bit and that is seen as 'Zout'. As Bitwynne noted, what nfb does is counteract the drop in output voltage so it appears that Zout is now less. But the 'internal Zout' and the Zout with nfb are equally virtual. |
Jandidden, keep in mind this thread is not about transistor amps. But more importantly you have to be careful here not not violate something called Kirchoff's Law. Actually you will find that such is impossible as it is a law of physics, and not part of legal code :) This law is known also as the law of energy conservation and simply states that the amount of energy in an electrical network is equal to the amount of energy going into that network. What feedback does not affect output impedance: Now if we take two output circuits, one with a high impedance, for example the single ended output of a tube preamp, and that of a transistor power amplifier we will see that due to the lower output impedance of the amplifier that it will drive a lower impedance. Now if what you and George say is true, that adding negative feedback lowers output impedance, it then follows that if we add feedback to the preamp circuit its output impedance will become so low that at some point we can drive a 4 ohm load effortlessly. But what we find is that is not the case. Now you may argue that the preamp output is not relevant, so let's take the case of an SET with a 10 ohm output impedance on the 8 ohm tap. If what you say is correct, its output power will increase if feedback is added when asked to drive a 4 ohm load off of that tap. But again, what we find is that the 4 ohm output power is unchanged. The reason for this is if negative feedback really did decrease the output impedance, the resulting circuit would have the increased current to drive a lower impedance. That of course would violate Kirchoff's Law. Of course, the real way to provide for greater current ability is more output devices, larger heatsinks, output transformers, power transformers and the like. IOW, what is happening is that the term 'output impedance' as used with the Voltage Paradigm is a charged term that actually refers to servo gain in the output circuit and not the actual impedance of the devices involved (all types of which have an impedance greater than zero ohms). One might state that the issue here is semantic- I point it out simply because its use in the context of teh Voltage Paradigm leads to a lot of confusion- but that is how the audio industry is set up. If you are having trouble following this, it is because you are operating within the Voltage Paradigm. The word 'paradigm' has to do with a platform of thought; quite often viewpoints outside of that platform are hard to think about or might be considered blasphemous. A further note- people have accused me of making up the two Paradigms (voltage and power) that I mention in the article at this link: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpIn case I run into that I refer them to this google search on the Fisher A-80 amplifier https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+a-80+amplifier&ie=utf-8&oe=utf-8The very first hit you get at this link is a YouTube image of the damping control of the Fisher amplifier. It is marked 'Constant Voltage' at fully counter clockwise, 'Constant Power' at noon and 'Constant Current' at fully clockwise. This is because there was a time when not all loudspeakers conformed to the voltage model, despite George's and Bruce Rosenblit's remonstrations. Such speakers are still made today, and in increasing numbers. Any speaker than can be driven successfully by an SET will be an example. ESLs are also an example as it will be found that their impedance curve is not based on the resonant impedance of a driver in a box. If you could show that the impedance curve of the speaker (which in most ESLs varies by about 10:1) is also an efficiency vs frequency curve then you would have an argument that the speaker is a Voltage Paradigm device. IOW the ML ESLs are a low impedance Power Paradigm loudspeaker. |
Atmasphere, your conclusion that someone (certainly not me!) would think if you lower the Zout to zero in a preamp, it can effortlessly drive 4 ohms, is rather far-fetched. As I'm sure you know, Zout has NOTHING to do with what you can or cannot drive, and I would expect that most people here would be well aware of that. A D-type battery cell has a Zout of less than an ohm but nobody would expect it to be able to start their car. :-) . I really have no idea what gave you that particular idea.
BTW I gave the example of ss amps' Zout being caused by for instance the transistor gain droop with increased load current. In a tube amp, there are causes for 'Zout' like the internal output tube(s) resistance and transformer losses that cause a drop in output voltage if you increase load current (for instance by connecting a lower impedance speaker). My point was that this Zout in the amp is largely 'virtual' in the sense that you cannot point to a resistor in the amp that would be the Zout. Now what nfb does is making the drop in output voltage with more load less so, thus causing the Zout to appear lower.
Actually it is quite easy to measure Zout on your amp. Just connect an 8 ohms resistor as load and set Vout to say 8 volts. The Iload is therefore 0.5 amps. Now connect a 4 ohms resistor and let's say the Vout drops to 7 volts. The Iload is now of course 7 V/4 ohms = 1.75 amps. So we now know that Vout drops 1 V if the current increases 0.75 amps. Therefore Zout is 1 V / 0.75 amps = 1.3334 ohms. Easy, no? |
Atmasphere, I read your last post again - are you trying to say that even when nfb decreases the voltage drop due to increased current output (the Zout), that doesn't mean that the amp can all of a sudden deliver more power? If so, I fully agree. The amp has it's intrinsic limits in output current and output voltage and nfb cannot change that.
Even with nfb, when the output voltage goes up, it eventually gets up to the supply voltage and cannot go higher of course. Same with the output current - the current capability of the amp cannot increase due to nfb. What nfb will do is to keep 'trying' (if you will excuse this antropomorphism) to keep up the output voltage right up to the supply voltage. This is, of course, the reason why an nfb amp generally clips much sharper than a non-nfb amp. |
My 2nd post above, where I say: 'The Iload is therefore 0.5 amps' should be of course 1 amp (from 8 V across 8 ohms). Duhhh. :-) |
^^ Ah- good.
And we now also see why the Power Paradigm is still around some 60 years after it was supposed to be supplanted by the Voltage Paradigm: and that is because amplifiers operating under the Power Paradigm tend to make less odd ordered harmonic distortion.
This is partially because nfb will add trace amounts of odd ordered harmonics, even when used in large amounts.
The human ear/brain system is very sensitive to these distortions, more so than anything else, as it uses higher ordered harmonics to determine how loud a sound is. As a result, nfb will make an amplifier sound brighter than it really should be (regardless of tube or transistor) because the ear/brain system translates distortion as tonality.
This is why two amps on the bench might have the same bandwidth but one can sound bright and the other doesn't. |
"...IOW the ML ESLs are a low impedance Power Paradigm loudspeaker..."
Perhaps I'm missing something here, but do these graphs seem to suggest something else?
http://www.stereophile.com/reference/810/index.html |
Unsound ... I found that same point confusing too, especially in light of ML's recommendation to use an amp that increases power as speaker impedance drops. That sounds like a Voltage Paradigm/SS amp. But our other "enthusiastic" techie members wore me out. Glad you picked up on that point too. |
Unsound and Bruce, as you may have noted TJN's article that Unsound referenced addresses how impedance interactions affect frequency response at the speaker's INPUT terminals. In itself that says nothing about what the speaker's acoustic output will be like in response to those inputs.
I have no knowledge as to whether optimal frequency response of the acoustic output of the Martin-Logan Aerius would result from a flat frequency response at its input, or from a frequency response at its input that is non-flat in some manner. But see the comments in my previous post regarding the Quad ESL-57, and take a look at its impedance curve that I referenced.
Best regards,
-- Al |
So, to recap, what did we learn:
- output impedance of a 'box' is *defined* as the change in output voltage divided by the current change that caused the voltage variation;
- nfb makes the output voltage change smaller for the same current change, so *by definition* lowers the output impedance.
We also saw that nfb does not change the 'internal' output impedance of an amp - nfb operates strictly outside the amp proper. Also, we saw that (to a first order) this change in output impedance due to nfb does not change the amps output capability. Lowering the output impedance towards zero does not all of a sudden produce an amp that can deliver gobs of power in very small loads. The as-designed amp capability does not change.
A corollary we did not discuss but should be quite obvious: since damping factor is *defined* as load impedance divided by output impedance, output impedance drop due to nfb consequently increases damping factor in the same measure.
This was a good exchange! |
Yes, fascinating thread, even for a non-techie like me.
If I may ask a corollary question: is there a relatively simple rule of thumb to determine whether a speaker might prefer a Power Paradigm or Voltage Paradigm amp? |
:) Yes- damping factor is different from "output impedance" in that it gives you an idea of the servo performance of the circuit, it it has any. Unsound, what you see from that graph is the woofer driver in a box, combined with the impedance curve of the ESL. The Aerius is a hybrid speaker. Interestingly, we also see that as ML speakers go, this one is less severe, dropping to a fairly manageable 2 ohms at 20KHz. If I can draw your attention to the letter that is appended to this article, you can see from it why the ZERO works as well as it does, effectively lowering the output impedance of a zero feedback tube amplifier into the range where it might conform with the parameters laid out in the letter. The response plots initially do seem to suggest that the speaker is Voltage Paradigm, since such amplifiers seem capable of producing flat frequency response. I feel its important to point out that the amplifiers in question all show a peak in response as I forecast, which seems to start up at about 7KHz, and increasing with frequency as the impedance of the speaker continues to drop. Since the ear can hear a change in a spectrum of frequencies much better that it can with a single frequency, we now see that there is a correlation between the measurements and subjective listening experience wherein the amplifiers are causing the speaker to have brightness on the top end. As I have mentioned before, this is a common complaint when combining ESLs and transistors. The whole point of the Voltage Paradigm when it was proposed way back in the late 1950s and into the 1960s was that it would eliminate tonal coloration due to frequency response errors. What we see from these measurements is that isn't happening. Now the Power Paradigm does not make any such guarantee. Instead, it seeks to keep the kinds of distortion to which the ear is most sensitive to a minimum. This is does because the ear translates such distortions to tonality, the idea being that there is a tipping point in the brain where the tonality of distortion can be more pronounced than actual frequency response errors. If this pans out correctly, it may well mean that the result will seem more linear, even when the frequency response does not seem to be quite as flat. IOW, this whole thing has to do with how we perceive sound as opposed to how we measure it. Its a classic argument, and as I pointed out in my article about the Voltage and Power Paradigms ( http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php) this argument has been around ever since the Voltage Paradigm was introduced. |
Wrm57, the easiest rule of thumb is ask the designer what sort of amplifier they designed the speaker for. |
Al, thank you. I thought I might have missed something. Still, the last paragraph seems to suggest what I think most could reasonably expect:
"...Figs.5-7 show the measured impedances of the 2Ce, Angelus, and Aerius. Note that the Sonic Frontiers' frequency-response deviations when driving these loudspeakers show the same general trends as the impedance magnitudes of each respective loudspeaker. That is, the peaks and dips in the responses correspond closely to the peaks and dips in the impedance plots. The impedance plot therefore gives a general indication as to just how a given loudspeaker's response will change when used with an amplifier having a high output impedance.—Thomas J. Norton"
Unless there is some sort of internal eq or the impedance actually compensates for the drivers-crossover/speaker systems inherent deviations from flat frequency response (something that has been done to some degree in crossover designs from other manufactures, but something I've yet to hear attributed to ML designs), I'd hazard a guess that it might be reasonable to extrapolate that the sound output to somewhat mimic the impedance/frequency graphs provided. |
Atmasphere, perhaps I'm not seeing the same things you are?
And, Bifwynne's post re: MLs amplifier recommendations seem to suggest amplifier characteristics that are at odds with your post
"...IOW the ML ESLs are a low impedance Power Paradigm loudspeaker." |
04-09-14: Unsound
Unless there is some sort of internal eq or the impedance actually compensates for the drivers-crossover/speaker systems inherent deviations from flat frequency response (something that has been done to some degree in crossover designs from other manufactures, but something I've yet to hear attributed to ML designs), I'd hazard a guess that it might be reasonable to extrapolate that the sound output to somewhat mimic the impedance/frequency graphs provided. As I indicated I have no specific knowledge of the behavior of Martin-Logan speakers. But keep in mind that application of a voltage which is constant as a function of frequency will result in power delivery which progressively increases as impedance decreases. (In saying that I'm oversimplifying a bit by ignoring phase angle effects). As you'll realize, what a speaker basically does is to convert some fraction of the electrical power supplied to it into acoustical power. And as Ralph pointed out earlier, the efficiency of an ESL (power out vs. power in) does not decrease in step with impedance as frequency increases, but instead remains pretty much constant as I understand it. So in the absence of specific indications to the contrary I see no reason to expect flat frequency response in (in terms of voltage) to result in flat frequency response out. As I see it the usefulness of TJN's measurements is that they provide insight into the DIRECTION in which tonality will be affected as a function of amplifier output impedance. But they provide essentially no insight into what output impedance will be optimal. Best regards, -- Al |
From the Marin Logan website:
Quote:
However, it is important that the amplifier be stable operating into varying impedance loads: an ideally stable amplifier will typically be able to deliver nearly twice its rated 8 Ohm wattage into 4 Ohms, and should again increase into 2 Ohms. Quote:
Cheers George
|
Where did ML ever pick up that crazy paradigm?
I guess it's always easier to put the burden on the other guy :-) That's a pretty safe bet no matter which way one thinks technically. |
..unless there is nobody out there willing to play along..... |
However, it is important that the amplifier be stable operating into varying impedance loads: an ideally stable amplifier will typically be able to deliver nearly twice its rated 8 Ohm wattage into 4 Ohms, and should again increase into 2 Ohms. 'Stable' means that the amp won't oscillate when presented with the load in question. Atmasphere, perhaps I'm not seeing the same things you are? Perhaps. Take a look at Fig 4 at the link you provided. |
04-11-14: Atmasphere'Stable' means that the amp won't oscillate when presented with the load in question. Stable is a given with any amp, you may have missed the rest of the Martin Logan quote for the type of amp they recommend for their speakers. And to get these sort of figures, the amps need low output impedance, to get it. So here it is again. " Be able to deliver NEARLY twice its rated 8 Ohm wattage into 4 Ohms, and should again increase into 2 Ohms." In other words for the uninitiated, what's needed is an amp that can ALMOST double it's 8ohm wattage for each halving of load. EG: 8ohm-100watts 4ohm-200watts 2ohm-400watts Cheers George |
Funny to read this thread about autoformers.
Probably 6-7 years ago, I added a McIntosh SS amp to my arsenal that uses autoformers. At the time, I was also using Steve McCormack's platinum modded amps too. I compared them and shared my thoughts with Steve about the improvements possible with adding an autoformer as an additional mod for his amps and suggested the Paul Speltz product.
He noticed the same thing and, apparently, even wrote about it. I've been inactive in this hobby for quite some time, so it's fascinating to see this recent discussion of something I was dealing with when I last was “involved" in the hobby.
I echo everything Atmasphere is saying and can vouch first hand...not because I was prejudiced or persuaded by forums or audiophile "authority"...but because I discovered this phenomenon organically...almost by accident. |
If you want a tube amplifier to mate with ELS speakers, that excels effortlessly and with outright awesome dynamic contrasts, very low level resolution, go the the CAT and never look back. |
This is all well beyond my understanding and wondering can someone comment on a question concerning the voltage vs power model? I believe I read two, perhaps more, points of view?
Concerning an ESL such as the Quad "57" referenced earlier with a SS amp "doubling" the amps output as the impedance is halved (assume)
25W into 8ohms
50W into 4ohms
100W into 2ohms
would not the output be also "halved" as the impedance rises above 8ohms
12.5W into 16ohms
6.25W into 32ohms
If I understand correctly than, for the Quad ESL this means only 6.25W/12.5W in the lower frequencies while 100W into the higher frequencies?
Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)
.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db
so the SPL remains constant into all impedances?
Thank you everyone for all the previous information, so very informative!
Lee
|
"If I understand correctly than, for the Quad ESL this means only 6.25W/12.5W in the lower frequencies while 100W into the higher frequencies?"
No. Its the opposite.
If I understand correctly than, for the Quad ESL this means 100 watts in the lower frequencies while 6.25W/12.5W into the higher frequencies?" |
The quad has an impedance in excess of 32 ohms under 100HZ, with just under 2ohms impedance at about 10KHZ.
Lee |
That seems to be the opposite of what it is for most other speakers. Maybe there's something unique about that particular design. |
Going out on a limb here by getting technical than I should. But I recall reading posts from some our tech members that described the "common" ESL impedance function to be like capacitor. That is high impedance at low frequencies and low impedance (sometimes very low) at high frequencies. Indeed, some of the posts said that only amps that are stable driving such loads need apply.
I think another important question to think about is whether the ESLs were voiced to be driven by a SS or tube amp. As Ralph Karsten (Atmasphere) has explained in his white paper, most SS amps operate under the Voltage Paradigm, i.e., these amps have a very low output impedance and maintain constant output voltage. Under this model, power (i.e., watts) delivered to the speaker will inversely vary with the speaker's impedance, i.e., less watts at high impedances; more watts at low impedances.
By contrast, many tube amp have higher output impedances and operate under the Power Paradigm. In such cases, power (watts) will not vary as much with changes in speaker impedances.
The main point is that as long as the amp is operating within its safe operating range, it's more important to know whether the ESL was voiced to be driven by a SS or tube amp.
Btw, I think Ralph has suggested using ZERO autoformers to simulate higher speaker impedance. The theory is that Zeros can help amps manage the very low speaker impedances presented by some ESLs at high frequencies. |
Maybe that's why my VAC 30/30 worked so well with my SL-3's.
If this is the case for ESL's, that leaves me with 1 question. Does it matter? To make low notes, the amp needs to make the speaker move a lot of air. Not so for high frequencies. So, for example, if the amp needs to produce a low note at 2 ohms, or if it needs to produce a high note at 2 ohms, wouldn't the amp have an easier time with the high note even though the resistance is the same? |
Generally yes ... reason ... there's much less energy called for in the high frequencies. If your music source required the amp to push 100 watts of power into the tweeter, not sure what would blow first ... your ears or the tweeter.
Most of the power demands placed on an amp are in the bass/low midrange. Much less power demands in the upper end. |
Once again from the Martin Logan website. They should know their own speakers.
"Quote:
However, it is important that the amplifier be stable operating into varying impedance loads: an ideally stable amplifier will typically be able to deliver nearly twice its rated 8 Ohm wattage into 4 Ohms, and should again increase into 2 Ohms." Quote:
Cheers George
|
George, that's a pretty clear clue that ML advises that the owner should use a rock stable SS amp. I don't know how low the impedance gets ... but that is a factor to consider when picking an amp.
This issue has come up before .. possibly in this thread. I'm a tube guy, so I'll let the SS guys weight in. I seem to recall that Ralph thinks Zeros can help in these situations. Never used em' so I stop here.
|
The fact is they want an amp that are stable and also that can double it's wattage from 8ohms down to 4 ohms and yet still increase even to 2ohms, (which means an amp that can do good current)
This means that if an amp cannot do this, then at the low 1ohms or 2ohms that their (ML) speakers present to an amp, then that amp will then start to behave like a tone control, and not stay flat in frequency response over the entire impedance range of speaker it's driving.
As simulated speaker load graphs will show you of amps that cannot deliver current at those low loads, therefore their frequency response will not be flat.
All you have to do is look at Stereophile speaker simulated load graphs of tube amps especially to see that they cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms.
If you need proof I will post links for you to see what happens, but I think you should go and look for yourself.
Cheers George
|
Great posts by Bruce (Bifwynne), with which I fully concur. 09-13-15: Georgelofi
... amps that cannot deliver current at those low loads ... cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms. This is true, but I would emphasize the word "into." The frequency response characteristics of the signal at the input terminals of a speaker that will result in flat frequency response in the acoustic output of the speaker will depend on the design of the particular speaker. As Bruce said, it is "important to know whether the ESL was voiced to be driven by a SS or tube amp." And in that regard it is worth noting that the Quad ESL57 was designed before solid state amps existed. Although admittedly, as I believe you (George) mentioned earlier in this thread or in another similar thread, the vintage Mark Levinson ML-2 solid state amp in particular, rated at only 25 watts or thereabouts into 8 ohms but capable of supplying huge amounts of current into low impedances, is considered by many to have been a good match for the ESL57. While at the same time that speaker has been and still is used with tube amplification by many audiophiles. Regards, -- Al |
George, you posted "[a]ll you have to do is look at Stereophile speaker simulated load graphs of tube amps especially to see that they cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms."
I generally concur, but as I am sure you know, not all tube amps are made the same. For example, take a look at JA's bench measurements of my amp, the ARC Ref 150:
http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements#52FW4Aq5RbfXP6dv.97
As you can see, the amp's voltage output plot is a bit wavy when a simulated load is plugged into the amp's 8 ohm taps. That said, JA mentioned that "[t]he figures for the 8 ohm tap [ranged between] 1 and 1.4 ohms; for the 4 ohm tap, they [ranged between] 0.55 and 0.87 ohm." I surmise that the same simulated load would be less wavy if plugged into the amp's 4 ohm taps.
Now ... an actual "off the bench" report from me. If you get a chance, check some of my posts on the "DEQX Game Changer" thread. I bought a DEQX PreMATE, which effects both time domain alignment adjustments and room equalization correction.
To set up the DEQX, actual mic'd measurement are taken at the listener position. The FR of my speakers was frankly a mess. I surmise most of the FR aberrations were caused by room anomalies, not by my amp's output impedance interactions with the speaker. Btw, I drive my speakers off the 4 ohm taps.
One other point of interest. I sent Al my DEQX data files for review and comment. I think Al might concur with my observations. And right now, my speakers are pretty well adjusted via the DEQX. I am enjoying a very pleasant musical experience.
So, based on the foregoing, I suspect that if my amp/speaker combo was checked in an anechoic chamber, my tube amp/speaker FR plots would measure pretty close to the results obtained if my speakers were driven by a high quality SS amp under similar conditions.
Last point. The reason my amp's output impedance is low'ish and output voltage somewhat constant is because ARC uses a prudent amount of negative feedback. There is also some sort of local negative feedback effect achieved by reason of a circuit configuration between the power tubes and the output trannies. Ralph or Al can better explain how that works.
Kudos to all for the good comments.
Cheers,
Bruce |
"important to know whether the ESL was voiced to be driven by a SS or tube amp."
I own myself a pair of ML Monoliths III's with quite new fresh panels.
If I drive them with a my pair of Rogue 120 tube amps, they have highs but they are very distant and too polite, if I drive them with my solid state amps (similar to big Krells) low output impedance and gobs of current the high are where they should be and you know all about cymbal crashes.
And this thread is about the right amp for ML's not Quad 57, which I agree were most probably voiced with low power tube amps.
For Martin Logan to mention an amp should double into 4 from 8 and then increase again into 2ohm, this means current. And they also want one that's stable into capacitive loads like ESL's.
And lets face it a stable amp can be a 5 watter and have no current ability, so long as it doesn't ring or oscillate it's stable, current ability has nothing to do with stability into capacitive loads as ESL present.
Cheers George
|
Unless the amp is being tortured by the load and sensitivity of the speakers:
See this article:
http://www.stereophile.com/reference/707heavy#ovW5G38gcYb8whWE.97 |
Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)
.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db
so the SPL remains constant into all impedances?
Can anyone please comment on the above as to its accuracy? If correct, I dont understand this electrical concept as it pertains to how the speaker plays various notes at the intended (recorded) levels?
Considering a very simplistic example, if a bass note and mid range piano note are recorded at the same level and are played at the same moment, and assuming the speakers impedance at the bass note frequency is different from the speakers impedance at the piano note frequency, how then are the level of these two notes played through the speaker at the intended playback level? At the moment these two notes are played, does the speaker draw a constant output, (voltage/cureent) from the amplifier? If so, lets say it is 1W. If the above speaker sensitivity question is true, how can both notes be played at the same level at 1W being drawn from the amplifier?
Thank you, Lee |
Simple ... it's a function of the transducer system itself. That is what is meant by how the speaker is voiced.
Take a simple example. A speaker system may present an impedance load of 20ohms at the mid/tweeter x-over point. Yet, assuming the drivers are phase coherent at the x-over point, and if the speakers are well designed, the FR should be flat over the x-over point. Similarly, a speaker's impedance function even outside the x-over points may fluctuate. But still, the driver system emits a level FR SPL. It's about how the speaker was voiced.
There are some, but not many, speakers whose impedance function is near linear. Unless the impedance level is either ridiculously low or high, and the phase angle plots are not wacko, such a speaker could very well be both SS and tube (Power Paradigm) friendly. Just throwing this out there, but I think Maggies might represent a somewhat level 4 ohm load and pretty benign phase angles. I'm sure there others. |
Hvowell, said earlier:
Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)
.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db
so the SPL remains constant into all impedances?
The reason this is not so is because in the case of an ESL, the impedance curve is **not** an efficiency curve. The fact is that the impedance curve is based on a capacitor more than anything else. This makes ESLs fundamentally different from dynamic drivers in a box. In an ESL, the efficiency is fairly constant despite the impedance. Generally speaking, ESLs have an impedance curve that varies by about 10:1 over the range of the speaker, which makes any one of them tricky for all amps. The reason they work at all with solid state amps is due to the negative feedback loop in the amplifier which allows the amp to adapt somewhat (reduce its output at higher frequencies) to the lower impedance at high frequencies. Despite that its a common complain that a transistor amp will be too bright on an ESL (not the least of which is that negative feedback in and of itself can cause brightness; likely a topic for another thread). For example the Sound Labs, early Quads and Audiostatics are easily driven with tubes as the matching transformer in the speaker sets the bass impedance fairly high (usually over 16 ohms). OTOH, Martin Logan use the matching transformer to set the bass impedance at about 4-5 ohms, making the 20KHz about 0.5 ohms. This is why they want an amplifier that can double power reliably into low impedances, else they would simply not be getting any highs at all! Note: George is mis-using the word 'stability' and 'stable' in his comments (it may not be his fault; this is a common mis-use of the word). Stability in an amplifier is how well the amplifier resists *oscillation*. For example our smaller amplifiers, like the M-60 (which is an OTL) are perfectly stable driving 4 ohms even though they don't make as much power doing so. OTOH some transistor amplifier are unstable driving capacitive loads (IOW they can go into oscillation) even though they can double power as the load impedance is cut in half. We have customers driving Martin-Logan ESLs quite successfully using either our old Z-Music autoformers from years ago or the ZEROs. Both allow our amps to drive the ML ESLs with ease with no high frequency softness. I've heard one of these systems; if anything to me the combination might have been a little bright, but some of that may have been caused by the solid state preamp that was also in the signal chain. |
Atmasphere,
Thank you very much for your response.
So, if I understand correctly, for the most part an ESL will be as efficent (output approximately the same sound pressure level or volume) at a given amplifier output, say 1W, regardless of the impedance? I assume at the impedance extreme of the ESL (less than say 2ohms) the SPL will be reduced?
I hear reference of "less musical energy" at the upper registers. Effect being these upper frequency sounds are by nature "softer/lower" in volume compared to lower frequencies and less amplifier output is required?
Again my apologies for my simplistic understanding.
Lee |
Sorry guys amp that can't keep it's end up at those 1ohms loads, will not have a flat frequency response from 20hz to 20khz no matter what level it's being played at. It becomes a tone control, end of story.
Cheers George
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George,
Would it also be true than that an amp that can't make power into higher impedances is also a "tone control" of sorts?
Respectfully,
Lee |
An amp like say a Krell or similar, will remain almost flat in it's response across the frequency spectrum.
All you need to do is look at the simulated speaker load graphs of amps at 1/3 power on Stereophile to see what happens to them if they have, high'ish output impedance or can't deliver current, many tube amps exhibit this, and even many Mosfets, they are far from the ideal flat.
And the simulated speaker load that Stereophile uses is not as savage as what we're talking about here with the ML's, and things like Wilson Watt/puppy, Alexandria etc.
Cheers George
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Here is an Audio Research Reference 150 considered by many to be a tube amp with better drive than most. It's only being asked to drive 2.83v hardly taxing. Look at the black trace in the link, this is Sterophile's simulated (rather easy speaker load), the 150 is having a hard time staying flat.(I can show you an impedance graph of their simulated speaker load, I have it somewhere it's not as nasty as the ML's) Then look what happens to it when it has to drive a straight 2ohm load (green trace) it's really starting to take a dive in the upper mids/highs. What do you think will happen to the highs into a 1ohm capacitive load of the ML's http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements#dgk5iym3Akuwqg5H.97Cheers George |
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Again, though, as I indicated in my previous post George's comments mainly address frequency response at the amplifier output/speaker input. And flat frequency response at those points does not necessarily mean that the acoustic output of the speaker will have flat frequency response.
Ralph (Atmasphere) stated above that "in an ESL, the efficiency is fairly constant despite the impedance." ("Efficiency" referring to acoustic power out vs. electrical power in). I believe that is generally true. But even if we assume that the efficiency of a given ESL is just a little bit closer to being constant across the frequency range than it is to mirroring the impedance curve, then the minimal variation of amplifier output voltage as a function of load impedance that is characteristic of almost all solid state amps (assuming they are operated within the limits of their voltage, current, power, and thermal capabilities), and hence the increase in power delivery that will occur as impedance decreases, will result in greater frequency response variation in the acoustic output of that ESL than would result with a tube amplifier (operated within its capabilities) whose output impedance is some relatively high value, and whose output voltage therefore varies significantly as a function of varying load impedance.
Apologies for the long sentence :-)
Regards,
-- Al
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