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I was playing around with Cheever's results and plugging some 1Khz test data I have for a pretty good measuring SET amp (KR audio VA340) into his formulae. Interesting results. He claims a TAD of around 100 sounds excellent and most SS amps with feedback fall in the 1000+ range with his equation (a higher number is worse as it relates to % deviation from his derived Aural harmonics.Based on the TAD number I calculated for this amp (around 150) it should sound superb...speaker permitting (it is only a 20 watt amp). My own personal experience with this brand confirms its essentially invisible character (or lack thereof...a very good thing). This TAD suggests that the distortion this amp generates is essentially inaudible at the level I calculated it for.
Now some background:
1)
I found a FFT distortion spectrum for this amp at 1 watt output. The spectrum contains only harmonics up to 5th, with a really tiny 7th.
n=2 -50 db
n=3 -72 db
n=4 -95 db
n=5 -107 db
n=6 below noise floor (ie. -125 db in the measurement)
n=7 -118 db
n=8 below noise floor
n=9 below noise floor
n=10 below noise floorSo essentially no higher harmonics were present. The calculated THD for 1 watt from this amp is 0.31% and the S/N ratio is 98db.
2)
In using Cheever's TAD I calculated his so called Aural Harmonics using a 87 db SPL level. I chose this because 1 watt into an average sensitivity speaker, which JA from STereophile says is about 87 db/watt, gives 87db. I figure you gain 3 db for a stereo pair and if you sit about 3 meters away you have a normal listening level.
I will try to plug in some values for other amps on the market to see how they compare to this and also look a bit at their reputation for sound in the marketplace to see if there is some agreement on them sounding good and having a low TAD.
Follow Ups:
I'm trying to refrain from the usual PropHead hyperbole here. Let's just say my opinion of Cheever and his thesis is not good. So I don't have anything useful to say about computations using his metric.
Hi AndyC,
Well that's fine. You can start by explaining what you find wrong with the thesis. Stick to technical. I mainly wanted to try it out because it is interesting his approach and I wanted to see what numbers come out of it and if they have any correlation with what I have heard with certain amps.Critique of this metric is a valid thread IMO. Afterall, these kinds of metrics are exactly what measuring is all about.
Morricab, I think that this is a good direction to go, but I don't yet completely understand the rating system. I guess that I will have to review Cheever as well, first. I don't think that Cheever is perfect in his prediction of audio quality, based on one method of selection, but it looks better than most.
Hi John,
Yeah I know its not perfect but I want to see where it leads. When I find more time I will calculate for some amps that I know the sound of better and will get a feel for if it is making any sense or not. I will throw one of your halo amps in the mix as well...just don't shoot the messenger if you don't like the result ;). What I real like in his thesis is this derivation of the Aural harmonics. They get brutal for most amps around the 4th to the 7th harmonic. The 2nd and 3rd end up being nearly completely trivial. For 87 dbA (my surrogate 1 watt assuming an 87db speaker) the 2nd Aural harmonic from Cheever's equation is nearly 6%! For the third though already it drops to .07%. At 97db (my 10 watts) the 2nd harmonic is up to nearly 17%! The third is still quite low, around 0.2%.
Personally, I would think that your amp example would be OK for a horn loaded speaker, but marginal for a direct radiator. Your results may vary.
Is that because of the sensitivity difference between the two types of speakers (normally anyway)? Remember though some direct radiator speakers like the Wilson Watt/puppy are about 93 db sensitive (I have a friend with the original X1 that is 95 db/watt). Several speakers from Cabasse and Focal are also well above 90db/watt making 10 watts in a moderate room (think 30m2 or less) more than sufficient for all but the most headbanging affairs. I am getting very good results with my 87 db/watt (equivalent sensitivity) 2.4 meter tall eletrostats. My room is also not very large (25m2).In big American homes obviously if the room is 70 or 80m2 and you sit 6 meters away more power will be needed than a SET can provide unless you are using horns.
Hi.My last project last November : a 2x1W IDHT SET power amp, driven directly from a standard DVD player (without linestage at all!),
can handle a pair of Westlake BBSM-6 referenc monitors (91dB/W/M)
gracefully in a very small living room (condo type), sitting 8ft away.Surprisingly it plays dynamic firecrackers, like Wagner, Beethoven
nicely, not a bit of shy vs its counterpart: 2x100W class A SS dual mono power amp (Swiss Physics 6A). FYI, that tiny 2W tube amp got no global NFB at all.c-J
It has been my experience that many amps with feedback sound dynamically "sat on" like they would wake up if you just crank the volume but it never really comes alive. My amp (KR audio VA350i) also has no feedback (neither does my all tube DAC or tube phono stage for that matter).
Most amplifiers operate within the sound fields of the speakers to which they are connected, and connected to room boundaries such as the floor with resonant mechanical devices such as racks.This model is interesting, but limited in usefulness unless the acoustic environment is part of the measurement discipline.
I am sure that the amps were not hooked up to speakers at all, most likely a dummy load inside the measuring device. So I don't think room vibrations play any role whatsoever in these measurements.
I know from experience that mechanical support is important for amplifiers as well as CD players. Comparing amplifiers on the basis of electrical bench measurements made in the absence of typical listening room acoustic vibrations might lead us to prefer an amplifier (A) with serious microphonic problems to another (B) with perhaps less optimal electrical characteristics, but less severe microphonics. Amplifier B might give more satisfactory performance in the majority of setups.Another question regarding the measurement setup occurs to me. How is the safety-earth connection to be treated to remove its influence from the effective noise floor performance of the amplifiers under test?
Hi.Let alone data measurement, we can actually detect the sonic difference with & without proper support to a component, e.g. LP turntable, CD player, & even amps.
My decadesof listening experience taught me to provde vibration resistive support to our equipment.
My turntable is 'laid to rest' on a heavy-dead-weight DIYed platform built with 2x70lb hollow concrete blocks, triple isolated with rubber supension leg pads, tuned acoustical tip toes & strong steel spikes. Stand alone not touching any other equipment.
All my amps, SS & tube, are DIY fitted with strong steel spikes at the bottom, seated on marble slate & granite blocks. Mass & isoation.
Again all stand alone on the floor not touching any other components.My DVD-audio player & CD players are seated on 3-in-a-set tuned acosutical tip toes point up with the rubber cushioned tip toes' base mounted on suspended playwood board. So triple isoation.
I don't worry too much about microphonics. I am more concerned about
acoustical feedback from the loudspeakers, & structeal vibration from the floor below.Such mass+isolation do improve the sound big bigtime.
c-J
Hi Al,
I get the feeling you are overestimating the influence of external vibrations. I think the problem is that it is not really possible to make these FFT measurements when the amp is hooked to speakers. If you could then you would also get a convolution of the amp output with the back EMF from whatever speaker you have it hooked up to. Many speakers are highly reactive and would throw considerable energy back into the amp and therefore also the test measurement. You could simulate a speaker load, like the NHT dummy load, but again I think this throws too many other variables into the mix.Obviously a suboptimal ground could cause a noise problem making the results dubious. I was using each FFT spectrums lowest "noise" floor as the minimum value in calculating the TAD. If I had gone with the lowest value for all the amps (the Audio Research in this case I think) some of the amps would have fared very poorly with a high TAD that may or may not be reflective on their sound quality. This needs to be worked out by more experimentation. Remember, this is not my theory, I am merely seeing how useful it is in implementation.
Use two amps driven in parallel (by separate buffers): one connected to the dummy resistive load for the sake of easy measurements, and one connected to a standard speaker in some standardized acoustic environment. Both amps are exposed to the vibrations from the speaker.This does not resolve the back-EMF question, but would include any amp microphonic behavior in the characterization data. By 'microphonic' I mean any measureable change in amp behavior caused by acoustic vibrations, not just breakthrough. The differences in measured response in the presence of acoustic vibration versus those in silence would be valuable to the amp designer as well as to those seeking to purchase higher performance equipment.
The back-EMF question is so large that it deserves its own set of measurements and model. So does the safety-earth noise issue.
"If it is audible, it must be measureable" is completely different from "if it is inconvenient to measure, it must be inaudible."
It is actually going to be misleading and atypical to measure amps just on a bench, without the presence of audio vibrations, as it is similarly misleading to measure it into just a resistive load.Both the amps behavior in the presence of vibrations, and it's reaction to back EMF from the speaker are NOT trivial and insignficant things to be swept under the rug.
Yes, it does throw in more variables, but it could still lead to a series of amplifier comparisons using the same acoustic and load conditions, at least the performance would be ranked relative under near identical conditions, if the environment was adequately controlled.
Ever see the SPLIF architecture by Altmann? See:
http://www.altmann.haan.de/splif_page/This simple circuit change can transform a dirt ordinary text-book power amp from a mediocre sounding amp into something that sounds like it turned into an iron-fisted control amp like a Krell. His circuit virtually eliminates the back-EMF from getting into the feedback system. It also tends to sweeten up the HF's a bit, although it does depend on the loudspeaker crossover topology and the tweeter, whether or not it is a three-way, etc., in other words, the improvements are somewhat loudspeaker dependent, with the more difficult loads improving the most.
Typically, when this kind of issue is raised with objectivists, they tend to ignore it or try to dismiss it as irrelevant/insignificant.
However, there are an awful lot of anecdotal comments about vibration control, as well as the known problems from back-EMF, etc.
WE can pretend, or we can dealw ith the real world variables and issues.As for the Cheever approach, it has a great deal of potential merit, but I tend to look back to the orginal Olson data he copied into the thesis, rather than use the algorithm, less complexity and more direct application of the measured data.
BTW, has anyone got a copy of that original Olson data, the citation in the thesis is incorrect, it is not in "Music, Physics, and Engineering" by Olson (Figures 2-2 and 2-3, citation 28 in the thesis), at least, it is not in my copy.
Jon Risch
I guess what would be needed is a way to map the extremes, from completely non-reactive (ie. a resistor like load) to extremely reactive (simulating an electrostatic speaker?) and obtaining the necessary data under these conditions and then doing comparative listening as well to attempt to establish the correlation.In practical terms, if you wanted to do such things with real speakers in order to understand the room vibration aspect you could hook up a pair of Apogees (I don't know of a more purely resistive speaker) on one end and Soundlabs (probably about the most reactive) on the other end. Then, not only do you get the opposite loads you also get extremely high resolution to hear subtle effects.
The subjective listening tests can be conducted at the same time as the measurements are being made most likely.
Hi.There got to be a dummy load standard that all desingers & builders can employ universally to ensure apple to apple comparison of data.
Resistive dummy load is not ideally the best vs the realworld speaker load situation, but at least it is consistent & share the common concrete platform of data comparison.
We have seen reactive dummy loads, or technically, called lousdspeaker simulators, are getting designers attention as such loads reflects the more closely the performance of an audio amp under realworld speaker loads.
The question is: what reactive standard should we adopt which can reflect the cloest to the realworld speaker loads?? Are are zillions of loudspeakers exiting. So which designs should be taken as reprentative enough ???
Are we opening a huge can of worms ??
cj: ""
Are we opening a huge can of worms ??""Hell ya. (and that's good)
Personally, I wouldn't worry about real world speaker loads, given the amazing variability.
All one must do is consider the I/V space that needs to be exercised and at what frequencies. Asking for full current in all 4 quadrants at 20Khz, for example, is very dangerous.
Perhaps the load should be capable of approaching 75% of the output SOA in all four quads.
But that's only first order loads. While trivial to do this with a honkin inductor, it does not address the hf current that can be pulled simultaneously..without that one, one does not exercise the damping factor in all quadrants.
Cheers, John
+/- 60 degrees of phase shift for the maximum should do it. Can there be negative resistance in a crossover?
d.b.
db ""
Can there be negative resistance in a crossover? ""Not in real time. Storage of energy within reflex or tl could conceivably go 360 degrees +/-, so I'd ignore it.
We need to stress the amp in all four quadrants, and within each quadrant, stimulate in all four directions. Once that is done, then measurements can be formulated and standardized.
A trivial task with an active, programmable load. But impossible with a passive one comprised of simple storage elements....there's no time delay.
"We need to stress the amp in all four quadrants, and within each quadrant, stimulate in all four directions. Once that is done, then measurements can be formulated and standardized."I'll disagree with some of this. If negative resistance is not going to be an issue, then exercising an amp in the 2nd and 3rd quadrant is for academic exercise only.
The last time I was looking at speaker impedance plots, the max was about +/- 60 degrees for worst case. Home theater speaker folks are deliberately designing for less than +/- 30 degrees so as not to stress receivers. Subwoofers have their own supplies so the receiver is not affected. High End doesn't matter anymore as the market is now so small it is nearly back to 1950's as a hobby market.Summing it up, time/phase issues are going to be issues of the source, whether it be recording or format, or loudspeakers. I don't see amps/preamps being an issue in that regard, if I'm understanding you correctly.
d.b.
db: ""
I'll disagree with some of this. If negative resistance is not going to be an issue, then exercising an amp in the 2nd and 3rd quadrant is for academic exercise only. ""Your question was about crossovers exhibiting negative resistance. The answer was no. That does not mean that a typical load does not traverse either quadrant 2 or 4. (Note, these are the quadrants you actually meant, as 1 and 3 are the quadrants which a resistive load will traverse. The distinction of quadrants 1 and 3 are as follows: if a positive signal is being applied to the load, the positive rail output devices will be the ones which current is passing through (quadrant 1), and the same with negative signals (quadrant 3). When a reactive load is applied to the amp, there will be times when the output is positive, yet the negative rail devices are the ones with current within them (quadrant 2), and a negative out with the positive devices having current (quadrant 4).
This is why reactive loads can be so difficult for the SOA of an amp...the voltage across the devices carrying current can exceed rail voltage, but yet the device must carry current.
The bottom line is, for speakers with any reactance, all 4 quadrants will be exercised. One can easily see this with a scope...run x axis as voltage, y axis as current...a resistor will plot a line from upper right to lower left...an inductor will show an ellipse or circle, same with a capacitor. Any plot which is not a simple line is showing reactance. (note, you can use a tek type 576 curve tracer(I think that was the model) set to ac mode to show this as well).
db: ""
The last time I was looking at speaker impedance plots, the max was about +/- 60 degrees for worst case. ""Any E/I phase shift requires all four quadrants operation. That is the definition of feeding a reactive load.
db: ""
High End doesn't matter anymore as the market is now so small it is nearly back to 1950's as a hobby market.""It does appear to be rather unhealthy, however, that was not within the context of our discussion..that of reactive loading of an amplifier, with the consequential issues.
db: ""
Summing it up, time/phase issues are going to be issues of the source, whether it be recording or format, or loudspeakers. I don't see amps/preamps being an issue in that regard, if I'm understanding you correctly.""I do not know what you are "summing up". While I have real issues with the fact that all the recorded source material has had imaging cues butchered beyond recognition, this discussion was about the output stage of a power amp, the fact that it exercises all four quadrants, and that a passive load will be unable to exercise the output stage adequately to allow measurement of the output parameters under such stress.
Cheers, John
"When a reactive load is applied to the amp, there will be times when the output is positive, yet the negative rail devices are the ones with current within them (quadrant 2), and a negative out with the positive devices having current (quadrant 4)."Are you sure about this? I have never seen any plots that would indicate that the above is correct.
"This is why reactive loads can be so difficult for the SOA of an amp...the voltage across the devices carrying current can exceed rail voltage, but yet the device must carry current."
I think this is more of a rarity than one might think. I am going to presume for a moment that you are talking about the peaks of music which are not very fast transients, so the rate of change is generally pretty manageable, and they are still well within SOA of the amp. Well er, uh, most reasonably well designed amps.
My general response to the above is that crossover/speaker design in general has realized that high reactive/inefficent loads limit their sales so designers are now more careful about this than in the past. I'ts not the major issue it was 10 or more years ago.
John: These days I don't think that much about this stuff, I'm tending mostly to other things, like ripping close to 500 CD's (uncompressed) and putting them on my hard drive, and trying to find the time to repaint the kiitchen.
d.b.
db: ""
Are you sure about this? I have never seen any plots that would indicate that the above is correct.""Guess you don't get out much, eh?? Don't you remember Time Tunnel?? That huge lissajous? :-)
The equations are easy. When you plot the voltage and current of a reactive load, it doesn't go through the origion. It's also easy to see on a scope.
The easiest way is on a 576 curve tracer in AC mode..but it's still easy with a speaker and music. After all, it's just phasor analysis.. Make a simple current viewing resistor, say 100 milliohms, and use it's drop for the scope vertical, the amp output voltage as the horizontal, and any speaker as the load..if it's pure resistive, it'll be a straight line through the origin..regardless of the musical content.. But any reactance will cause the line to form an ellipse..that means operation in 2 and 4 as well as 1 and 3..but be careful of the .1 ohm resistor, it'll also have a non trivial b dot error component at typical audio frequencies, so any measurements you get will be useless unless you make the CVR correctly.
As for speakers and reactance?? Guess you don't listen to those resonators much, eh? One note wonders made for cars.
Sheesh, even the SWTPC 250 had SOA protection which extended into quadrant 2 and 4..waaay back then..
Amp design/system reactance is important in two applications. First, when the user is beatin the livin hell outta the amp...which I tend to do...:-( ..second, when one is nitpicking the daylights outta the image reconstruction parameters. (which I can't be bothered with given today's recording/delivery technology).
Cheers, John
Cheers, John
O.K., if it does go into the 2nd and third quadrants that implies negative resistance. How can that be?
Curve tracers? I haven't seen one of those in at least twenty years!
d.b.
db: ""
O.K., if it does go into the 2nd and third quadrants that implies negative resistance. How can that be?""
It doesn't imply negative resistance. It states reactive phase shift, stored energy. Going into quads 2 and 4 is a simple reaction of the load to changes in the output that the load cannot follow.Think of charging an inductor.. When the voltage is positive, the inductor current increases..this continues until the sine lobe hits zero volts..at that time, the inductive current will be at a maximum. Then (here's the important part), when the amp voltage goes negative, the current within the inductor will go down..but being an inductor, it's current cannot change instantly, but will ramp down based on voltage and inductance..so when the amp voltage goes negative, the inductor current is POSITIVE...going down, but positive nonetheless..
So, negative voltage, positive current...that be quadrant 2..
db: ""Curve tracers? I haven't seen one of those in at least twenty years!""
I'd take a pic of the one here, but this site doesn't support jpegs. I still use it on occasion to test diodes and the such. It works great for my 3 inch diameter, 11 pound diodes (I'd send a pic (one next to the monitor), but this site...sigh.... Guy in the next room might be buying 1500 cree silicon carbide schottky's for a repair of lots of switcher supplies..I man the 576 for him..
Whow Nellie! If your talking a near pure inductive load you are correct. But a loudspeaker driver will have the voice coil inductance and resistance so you should never see the full effect of what you describe. To take it even further, the Back EMF of the driver will have a good deal of that dissapated by the series choke in the crossover.
d.b.
No, I am not talking about just a pure inductance.Any reactance whatsoever will do it.
VC inductance is one part, another is the energy stored in the acoustic system, like the cabinet.
The series choke has both inductance and resistance. Series inductors are not normally designed to dissipate excess energy coming back from the driver. It is the amplifier which has to absorb that.
An amp is designed to do one thing. Provide a desired voltage in time.
It has to do this regardless of what the load current is. Again, at all times.
The biggest problem with load reactance is, it can drive the current in wonderful ways that the amp has to be able to absorb.
Supply regulation and loop stability are all that I can offer at this time. To tell you the truth, it may be all anyone can offer.
d.b.
db: ""
Supply regulation and loop stability are all that I can offer at this time. To tell you the truth, it may be all anyone can offer.""
Nah, there's more. Heck, even SWTPC figured it in back when velociraptors were chasin me.At the immediate level, load reactance can seriously impinge on the secondary breakdown region of bipolars..it can really go bad when the devices are made with a thin base region for gain, as that impacts the zistor's ability to turn off in the heat of the moment.. Many drive circuits cannot supply sufficient alternate direction current to shut off a zistor that enters heavily into second breakdown. It's really neat to watch this effect, I can detail it if you wish..
At the listening level, it can impact the mids/highs if the bass driver causes the output zistors to become unruly, the drive circuit can become overwhelmed short term..
And of course, the transient thermal excursions are a reliabilty concern.
Are you saying that back EMF can then prevent the transistor from shutting off properly or suffer from secondary breakdown? Would this be due to the extra heating from current being pumped into the transistor going "the wrong way"? The phase of this current would be something approaching 180 degrees out of phase, would it not?
morricab: ""
Are you saying that back EMF can then prevent the transistor from shutting off properly or suffer from secondary breakdown?""Yes. For inductors, it's the stored energy which manifests as a pseduo constant current, and it will continue to supply that current even if the output goes to the opposite polarity. Perhaps it's easier to envision a large capacitor as load..have the amp ramp slowly to the negative rail, then once the load cap is charged, change the output to the positive rail..the cap doesn't move quickly, so the output stage draws lots of current in an attempt to move the cap. The pass devices trying to pull the cap up are from the positive rail, so the devices are dissipating both rail voltages and whatever current being forced..in the case of an inductive load, it's quite easy for the pass devices to move the out to the other rail, however, since an inductor cannot change it's current as quickly, it continues to have current in the same direction for a while. A more controlled current, but the passes still have to hold it for double rail voltage momentarily. (it's easier for most to envision caps rather than inductors, but they are both reactive storage mechanisms.)
If this brings the outputs close to the secondary breakdown, poof. (bipolar discussion). As devices approach this dangerous region, several mechanisms happen..as you raise the C-B voltage, the neutral base region gets thinner (early effect), increasing the gain of the device. Also, the leakage collector to base increases, this leakage is amplified by the device gain.. And thermally, gain is also going up..these conspire to make the output devices "cramp up"..this is secondary breakdown.
..Consider a darlington config output..if nothing is put in to pull the output bases down when this "cramping" happens, the device will clamp on. As this starts, the only way to stop it is to pull the base down hard. If the only thing the base sees is an emitter, then that driver has to go reverse breakdown base-emitter to get any current into the base. (note, I've been using current nomenclature loosely for ease of discussion). That'll probably toast the driver e-b junction.
This seems to be primarily a problem with bipolars because if I am not mistaken what you are describing is a form of thermal runaway. FETs tend to have the opposite effect under thermal stress and reduce gain if I am not mistaken. Sorry if I am not talking in quite a technical language, I understand the issues conceptually quite well but the actual jargon I am not so experienced in using. Keep talking away though because I appreciate the lessons. Now if I am also not mistaken many (if not all) BJT amps require extensive thermal tracking to prevent such a situation from getting out of hand. A Class A mosfet amp would not require this thermal tracking as they are more or less self clamping.I find it a relevant topic because I use a big pair of full-range electrostatic speakers. These are about as reactive (mainly capacitive) as a speaker is likely to get. A very large percentage of the signal fed into the speakers is sent right back into the output stage of the amp. Now I am using a tube hybrid amp (with the tubes on the output unusually) without negative feedback. I think the tubes (or the output transformers) are able to dissipate this back EMF without trouble and of course there is no breakdown possible. What I have found though is that a lot of SS amps sound very anemic on these speakers, with my modest 30 watts sounding much more powerful. I am wondering if the speaker is regularly putting the output stage under a lot of stress or if a significant portion of the back EMF is making its way back into the input and being reamplified (as described by Otala as IIM distortion).
morricab: ""
This seems to be primarily a problem with bipolars because if I am not mistaken what you are describing is a form of thermal runaway""While I did say "bipolar discussion", I did neglect to say the word thermal runaway...sorry bout that. I was describing thermal runaway as a result of secondary breakdown in a bjt...
The thermal tracking for most bjt amps has a far slower time constant. The silicon die and the thermal structure have several time constants, ranging from the 100 uSec range for the junction, to the millisecond range for junction to case, to the seconds from case to heatsink. So the thermal tracking schemes typically correct in the second or so timescale. Some schemes can be faster, like the sensefets, or that new device with an onboard sense device.
I could guess as to your amp's reaction to it's load, but that'd be a guess. I'm talking with Dan (and anybody else who's interested) about a test scheme for amps which cover reactive loads. Sent him a first pass jpg (then of course, modded it into a better scheme) for a setup. Rev 2 has a programmable resistive load, as well as a completely independent reactive load control, from capacitive through to inductive. That type of testing regimen is what is needed to get to the bottom of why you hear differences between amps..It may also be the root cause of tube vs solid state sound...who knows..
Hopefully, it can get posted here in a new thread..I fear that I will not be able to do it at AH, as I am "this close" to being banned there because I posted my displeasure with having some of my analysis misrepresented as part of an AH article. Apparently it is allowed to misrepresent the words of another, and there is no rebuttal allowed on an open forum. In my humble opinion, that is an abuse of the term "open discussion".
I'm a bad boy..:-)
Well as long as DB can remain civil I am more than willing to dive in and learn (hopefully contribute a bit as well but I have to think more deeply on these things first). I appreciate a sane discussion for once on this forum.
Ah, yes...now I recall some kind of animosity between you two. I honestly do not know why, nor am I really interested to know why.I agree, I hope civil discourse continues..
Can you post diagrams here?
If that's your only complaint then Motorola has been making TO3 pkg transistors for years that will take that stuff. Go to On Semiconductor and look up MJ15003, MJ15004, MJ15024, MJ15025, etc. etc. Parallel a couple or more of these and you can satisfy any audio requirement. Heatsink them right, and they will last longer than you and I will.
Sheesh; and I thought for a minute you where you were going to ask more hard stuff.
d.b.
There's a difference between just tossing more silicon mass at the problem, and engineering it. Anybody can put a dozen 15003's in to do the job required of one if engineered correctly..sheesh.But the question is, what's the damping factor at mid/hf while the device is dissipating reactive energy?? Does the drive accurately keep the outputs in line at all frequencies during sweeps through quads 2 and 4?
That's the question..not can the outputs survive..
First of all Damping Factor is really a misnomer, it should be output impedance, that is if you're Dick Pierce.
I've measured into a resistive load over power and frequency and got a minimum of 12 milliohms to a maximum of 25 millohms.
I am not convinced just yet that output impedance would change into a reactive load, or at least change significantly.
Cheer up, if it doesn't I won't ask you to eat your shirt collar.
d.b.
shirt collar..he he..bet some here have no clue what that refers to..How did you measure the hf damping factor with an inductive load being driven at a low freq?? Yah I know, you just used single tone, didn't ya...so old school there dude..you gotta get into the future..
Best way I know of is a second amp, with a load resistor between them.
And two signal sources and a set of summers..
I can detail ya the setup if your really interested.. Unfortunately, it does start to define new standards of test and performance..
Yeah, I'm old school. Now my amp is a sliding bias and the output transistor bias will vary with signal voltage. The higher the signal voltage the higher the DC Bias. Now, as us circuits guys know, the higher DC bias will reduce output impedance. So your theory may well be more appropriate to AB designs with a "static" DC bias is my guess.
What do ya think there John;
d.b.
P.S. Post your technique, I will archive it.
nt
db: ""
So your theory may well be more appropriate to AB designs with a "static" DC bias is my guess.
What do ya think there John;""I think you are correct...you are just guessing...:-)
(sorry, couldn't pass that one up...)
Honestly, the discussion was without regard to topology. I do not know which topology will be better or worse.
db: ""
P.S. Post your technique, I will archive it.""Which part of "I can't post pics here" did you not understand???
Duh...
(boy, your given me such material today)...:-)
btw, I'm just preppin ya for your test..
I could put a drawing together for ya, might take a day or two..but I'd hafta e-mail it to ya..
I am not understanding how the current could go into the 2nd or 3rd quadrant if the resistance is always positive.
Am I missing something?
d.b.
db: ""
I am not understanding how the current could go into the 2nd or 3rd quadrant if the resistance is always positive.
Am I missing something?""Yes, big time.
You have to consider the return of stored energy within the load back to the amp.
You are forgetting that.
O.K. but that would oppose the energy going out, the feedback would further oppose the energy coming back in, as it would feedback the output voltage,and since when do amplifiers like this sink current? and I still don't see how we could ever go to negative resistance in this application.
I don't get it John:
d.b.
nt
Jon R. Fig. 9.71 seems to give the info that Cheever is implying in citation 28. This was originally derived in Olsen's 'Acoustical Engineering' Fig. 12.42 on p. 596.
Hi.To play a fair game, i.e. without any prior sub-conscious bias based on the measurement, I think it is crucial to first listen before you measure so that whatever measured out would serves as a sorta third party guideline to approve or disprove what you've found subjectively.
Likewise, when I walk into an audio boutique or an audio shows, I would sit down for a few minutes to listen without any questioning.
If the sound was good, then I would start for serious audition tests with my own test CDs & then start to ask questions whatever equipment were being played.Without looking at your THD spectrum analysis in detail, the figures you quoted look pretty even in decending orders, suggesting pretty good sounding.
But it is only one watt O/P at 1KHz where any decently built audio amps, SS or tube, PP or SE irrespective, should sound good & be measured good.
I think what makes the sonic difference is where any audio amp starts to clip or overloaded. Does Cheever's TAD cover this clipping
criteria?c-J
First off, I have listened to the Manley's and KR VA340 from the inital list I provided.I only recently found these measurements for the VA340 but I had Kronzillas previously for review and found them the best sounding amps I have yet heard. Not being able to afford them I bought the next model down the scale from KR, the VA350.
I didn't make the measurements. I merely am reporting the data from the graphs.
Most listening is done at not much more than this one watt.
"But it is only one watt O/P at 1KHz where any decently built audio amps, SS or tube, PP or SE irrespective, should sound good & be measured good."
It all depends on the higher order harmonics where Cheever's ratios get pretty severe. An amp with noticeable distortion in the higher orders gets pretty heavily penalized with his TAD calculation.
I provided a TAD for the Manley at 10 watts, which is approaching clipping for this amp. I also show a TAD for the KR at 10 watts, also close to clipping. The TAD increases significantly, however, with most speakers 10 watts is already quite loud (especially with compressed rock music) so it may not be as noticeable as at lower powers.
The TAD has a term in it for dbA (spl level). It is in the aural harmonics term of the equation. As to dealing with clipping the story should come out of the dramatic increase in higher order harmonics and this will make the TAD get very large.
"But it is only one watt O/P at 1KHz where any decently built audio amps, SS or tube, PP or SE irrespective, should sound good & be measured good."
And yet you should know from listening that his is not the case. Also, most feedback amps will have a similar TAD regardless of power (as their THD is pretty constant up until clipping)...it may even get worse with lower power (sometimes you see THD + noise increase at lower powers for these amps).
The new Audio Research Ref 110 stere TAD = 929Belles 350A TAD = 649
Bel Canto REF 100 TAD = 24897 (There is an issue of where to take the noise floor on this one so it could be substantially lower)
KR VA340 at 1 watt TAD = 190 (not 150 sorry for the slightly incorrect value above)
KR VA340 at 10 watts TAD = 3900 (this is near clipping for this amp)
Vincent Hybrid monoblocks TAD = 406 (not bad!)
Manley Mahi Monoblocks TAD = 37446 (again this amp is significantly more linear at 1 watt than at 10 watts, which is near clipping. However, no 1 watt data are available.)
It is not so easy to determine what is the true noise floor of the amplfier in many cases. As a result these are estimates and nothing more. One thing is for sure, many many many amps do NOT have a clean noise floor. Most seem to have a lot fuzz that I guess could be IM with the power supply and distortion or the 1Khz fundamental.
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