I can't really buy into Ralph's paper at least the terminology defined. The example of the constant power amplifier as a tube-amplifier with transformer taps, is in my mind no different from the voltage amplifier paradigm presented, the only difference is the taps on the transformer impedance match the output to what is still essentially a voltage amplifier. The output power of those amplifiers will still change as the load impedance changes, perhaps not as much as if there was more feedback to compensate for the low output impedance of the amplifier, but it will still change as it is inherently a voltage amplifier.
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This is too simplistic a view, and I was thinking specifically just related to the basic output stage which does typically behave much like a voltage source, and is usually configured as a voltage follower, and with a light load (lighter than a speaker), behaves as a voltage source, and with load, as a voltage source with an element of constant and variable impedance. One thing you are not getting has to do with the application of
feedback. What I have said in that paper is true if the amp has none-
what you say above is true if the amp has enough feedback to allow it to
behave as a voltage source. Again, I find this is too simplistic of a view. Simply saying 35db is too little feedback without taking into account the frequency response of the feed-forward and feedback paths, not to mention what the inherent feedback is in the output stage if you are considering that separately makes any hard number in the sand questionable. The statement makes assumptions about the linearity of the feedback network as well. Ditto for Gain-Bandwidth, which is one number, but gain at frequency is far more relevant. Instrumentation op-amps may have very high gain-bandwidth, but are useless at 20KHz. This is a bigger deal that it would seem to appear; if the amplifier has
too little feedback (less than about 35dB) the consequence is that the
feedback itself will introduce distortion, mostly composed of higher
ordered harmonics (and some IM). Somewhere in the area of 35dB and north
the amp finally has enough feedback such that is can actually
compensate for the distortion introduced by the feedback itself.
The bottom line is this is all about Gain Bandwidth Product and the
resulting loop gain- both of which have been insufficient in the prior
art. The Benchmark amplifier is one of the very few non-class D designs
that actually gets the feedback into the ballpark. So if you want really
natural sound, you either go with an amp like that or go with an amp
that uses no feedback at all- and deal with the simple fact that it
won't work on all speakers, which is also true of an amplifier that is a
perfect voltage source! So you'll have to audition the speaker and amp
combination in any event. |
Not sure the justification for this statement. Their response w.r.t. voltage, is fairly flat from mids-highs, with usually a bit of a dip at high frequencies. An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output. The brightness is more a factor of their emission shape and how they will interact with most room, and the resultant room response, which will differ from a "point source" dynamic driver. The problem with ESLs is that they typically vary by about 9 or 10:1 in
impedance from bass to treble, but their efficiency doesn't vary in
lockstep as it is supposed to like you see with box speakers. So an amp
that doubles power as impedance is halved is typically way too bright on
most ESLs. Martin Logan got around this (sort of) by making their ESLs
very low impedance in the bass (4 ohms) so they are only 0.5ohms at
20Khz. Even most solid state amps have troubles into that impedance,
thus reducing the brightness that would otherwise manifest. |
atmasphere,
I deleted my old post questioning your article as I misinterpreted what your point was and where you were coming from. Put my head into a different mindset and completely agree in principle w.r.t. what you were communicating w.r.t. constant power for a tube output configuration. I don't have a blind attachment to 0 output impedance / infinite damping factor, and I expect most who do don't even know the details of why and why it may not be best from a system standpoint.
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Doing 4 things at once including posting here :-) ... sorry for my error. I meant impedance is halfed, but had V-squared/R on the brain :-) An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output.
Buddy, you went the wrong way. I know of no amp that doubles power as impedance goes from 4 to 16 Ohms. That is certainly not an ideal voltage source anymore.
No conflation. It was postulated that ESL sound bright due to higher power output to the speaker as the impedance drops. I claimed that was not true, because though the power goes up, the anechoic response w.r.t. constant voltage over frequency is flat to down at high frequencies. The postulate w.r.t. bright due to amplifier interaction is not the reason, the reason is different dispersion and how that interacts with the room and creates a room response that will be bright (if not done right). You are also conflating dispersion with relative differences in amp output
vs. impedance.
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It is not an electrical flaw in the speakers. It is primarily a mechanical design choice. The site you linked is good, but it is easy to draw the wrong conclusions if you don’t read in depth.
A pure current feedback output is not ideal as it will provide a peaky frequency response, and provides no additional electrical damping for the drivers which can be beneficial.
The damping factor the speakers see is never near 0 either due to the crossover impedances.
Neither was is really "wrong", just different and the right answer is probably somewhere in the middle ... though I would say most accurately, the correct answer is speaker specific amplification with an amp per speaker, but that is a tough nut for most companies to crack and not easy to market. Good on SGR tackling it at some level.
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I would expect it is not very high output impedance / current output driven, though it may have a relatively low damping factor and may have some current feedback. The issue is most speakers today are designed for an amplifier with high damping factor / low output impedance. Too high of output impedance and your bass response changes, and you screw up the cross-over points. All that is what I hear with Bakoon amplifiers.Which are class A/B but
which are reputed to be zero negative feedback/high output impedance/
current drive/probably low damping factor amplifiers.In simple terms
they simply sound incredibly clear and pure.Which I have really only
heard elsewhere from expensive SET amps like the |
See my later post where I deleted one of my previous posts ... too much on the brain and wasn't paying enough attention. I was never implying that speaker response is not impacted by amplifier impedance. I was more questioning the "constant power" paradigm, not thinking about specific implementations that incorporate aspects of voltage and current control like the ultralinear tube configuration or mixed output feedback implementations in solid-state. As a speaker designer I can tell you that the difference between the two
amplifier paradigms is significant, whether or not output transformers
are involved. The relationship between the impedance curve and the
speaker’s output level differs depending on the amplifier type, such
that if the speaker’s impedance curve has significant peaks and dips, it
will measure and sound different with the two amplifier types.
Duke |
Considering there are no speakers flat to 0.5db without equalization, let alone 0.1db, are we certain that a super high DF is going to result in the best on and off axis frequency response? What is likely to product worse cone breakup, a high DF or a low DF? According to this article DF=100 produces about 0.5dB variations typically, while DF=200 reduces it to 0.1dB. DF above 200 is inaudible. |
High damping factor directly into a driver as opposed to electrical dampening, can make high frequency ringing of the cone worse exacerbating some breakup modes. From a high level model, high DF can be like holding the voice coil stiff while the cone flexes. With low DF, the voice coil is not stationary and moves with the flex, and the increased resistance of the lower DF dissipates the energy of the motion. Very high level, almost like adding a resistor to a snubber.
I should clarify, I am not saying a huge improvement, but the low DF will in many cases be better.
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No offence to Bruno, but he doesn't sound like a very deep expert on transducers, but this is a critical point: Crossover filters are simply designed with the assumption of a voltage
source. It’s a matter of standardisation. Otherwise how this particular
speaker sounds when connected to that particular amp becomes rather
unpredictable.
It is one of the reasons active speakers have design flexibility that simply don't exist in traditional amp/speaker combinations. How low? For electrical damping of 8ohm speaker difference between
DF=10 and DF=100 will be like 6.8ohm vs 6.08ohm (assuming 6ohm as
resistance of 8ohm coil). It is about 12% difference in breaking
current. Not to steal Atmasphere's thunder especially when I stuck my foot in my mouth yesterday, with current drive for speakers, impedance is infinite, but ideally you want a trade-off, so to atmasphere's point, you can use a combination of voltage and current feedback to achieve something close to constant power irrespective of impedance, but for that to work, you need drivers that match that characteristic. |
You are confusing damping factor with dynamic headroom. Damping factor is mainly a factor of feedback, especially at bass frequencies. Dynamic range is a factor of supply rail voltage and capacitor size.
Given the love of tube amplifiers, saying high DF usually sounds better is debatable.
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