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28th August 2012, 07:47 AM  #851 
diyAudio Member

So, DF, Terry, Fas, and other experts: Squarewave testing, with peak values equal to the peak sine values previously used, as in post 846, should be comparable to using a sine load current's or voltage's peak value instead of its RMS value, as the DC load current or voltage when calculating ripple and the needed reservoir capacitance using the standard approximate formulas, correct?
So it's like using double the power as a worstcase design margin, and throwing in huge fast transients, too. Last edited by gootee; 28th August 2012 at 08:04 AM. 
28th August 2012, 08:00 AM  #852  
diyAudio Member

Quote:
You want enough headroom, with the transformer output voltage. Then you can use reasonable and predictable capacitance values, and a reasonable and predictable VA rating. If I had cranked up the input voltage for the 200 Watt case, the minimum required capacitance would have dropped quickly. At some point, I might try to parameterize the secondary voltage and fix one or two of the other parameters and see what other types of plots might be useful or enlightening. 

28th August 2012, 08:05 AM  #853  
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Join Date: Sep 2011

Quote:
What I do with squarewave testing is limiting the gain untill the square wave average output power would equal that of a full power sine. But the chance of running into full power square waves is slim in audio, unless you have an affection for playing loud chip tunes of computers of old
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28th August 2012, 10:42 AM  #854 
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Join Date: Nov 2007

Do I read it correctly if I assume that 5400uF is enough for a 300 watt single rail amplifier if it also has a 360va transformer? Um, my understanding needs proofed a bit prior to making a graph with the information. It also seems interesting to compare with some very low power amplifiers. As for line graphs, it seems to need 3 graphs to get the small, correct, large transformer selections represented (in addition to capacitance per watts). I'd do it with an all small transformer graph, an all correct correct transformer graph and an all large transformer graph on a 3d (3 color) type line graph for getting the transformer information visible. If we get enough datapoints, we can fill in the gaps, but the low power data points are currently missing.
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28th August 2012, 11:41 AM  #855  
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Join Date: Nov 2007

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P.S. But I appreciate this audio track because it is challenging to decompress in live time during replay. So, more learning for me.
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28th August 2012, 01:33 PM  #856  
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Join Date: Jan 2004
Location: Italy, Genova

Quote:
first of all you I didn't but apply the logic you pointed out, so no honor at all... Anyway, I shared the idea with an ex University fellow of mine and he warned me against some possible oscillation of the amp saying he couldn't say anything more precise (without being able to measure amp behavior). Now, it's a long time since my last Bode diagram, H(S) poles and zeroes distribution, and so on, and I can't recall/imagine how just moving half of PSU caps near the output device (in my case 2SC2922 and 2SA1216) would add anything to zeros and poles of the transfer function, so impairing stability... (while adding new caps such as bypass and that one in the snubber do affects H(S)) Would some of you be so kind to show me (point to links, or whatever) how/why this might happen and what is possible to do aside from using a signal generator feeding square wave trains and an oscilloscope? Thus said I need anyway to buy the latter... Thank you, Stefano Last edited by seppstefano; 28th August 2012 at 01:37 PM. 

29th August 2012, 12:58 AM  #857  
diyAudio Member

Quote:


29th August 2012, 01:11 AM  #858  
diyAudio Member

Quote:
The RMS Power (dissipated in the load) of the square wave is done as if the negative portion were folded around the axis to be positive. With only one rail, the negative parts would be missing so the power would be half of the total given (half of 300 W). 

29th August 2012, 02:19 AM  #859 
diyAudio Member
Join Date: Mar 2008

Tom @ #851:
Yeah, I think that is precisely what is required. And here's a justification for it using an entirely different approach: With a rectifier conduction interval on the order of 12ms, any LF output (eg 20Hz) is very long by comparison, and its also quite long compared to the cap conduction interval (6~8ms). So the rectifier/filter needs to be designed to handle the peak power, not the average power. And the peak power is twice the average (assuming sinusoidal output), so Voila, there's your factor of two. And thats why the transformer leakage inductance is so important. lovely work BTW, showing quite clearly how excess leakage requires stupidly large amounts of capacitance to reduce rail droop. Given that very few builders will have custommade magnetics, it probably makes sense to develop a design procedure based on xfmr parameters. If you normalise ("PerUnitise" as power systems engineers like to say) the figures you have, you'll pretty much end up with a set of design nomographs (by varying the output power you have achieved the same effective results as varying the transformer parameters). Add in a little explanatory text (eg how to measure & normalise transformer parameters) and its an awesome design tool. And of course ROTs are as simple as picking points on the nomograph that give the "best" (whatever that means) bang for ones buck. 
29th August 2012, 03:35 AM  #860  
diyAudio Member

Quote:
Quote:
Using the same average output power for the square wave as for the sine wave was discussed and tried already. It results in nothing interesting being learned, since the voltage peaks are then too low to challenge the power supply. The idea behind using the fullpeak square waves as the test signal was to find something close to the worstpossible case that the reservoir caps and power supply would ever have to be able to handle, such that if the worst case could be handled without the possibility of any distortion that was due to the power supply "choking on it", then maybe one could "truly" be CERTAIN that any and all music that did not exceed that peak voltage could also be handled without any PSUinduced problem. Also, conveniently, the square waveform covers half of the total time during which a sine peak could have occurred, requiring testing of only two phase angle cases, i.e. 0 and 180 degrees, whereas with a sine as the test signal we would need to test at many, many phase angles, to try to ensure that the worstcase alignment of the test peak with the PSU's charging pulses and ripple voltage waveform's troughs had actually gotten tested. A single sine seems to obviously not be the worst case for testing the PSU's robustness. All we would have to do to make it worse would be to play more than one sine at a time, so that there were more peaks in each unit of time. If we had enough tones playing at once, with different relative phase angles, then the peak level would be completely filled with sine peaks and would look like the top of a square wave. Actually, it could be much worse than that, and far worse than even the square wave, whenever there were multiple tones and their peaks happened to line up with each other in time and all overlap, i.e. all be in phase, even if just for an instant. Then their peak values would be SUMMED, and the resultant peak voltage could try to be MUCH higher than we anticipated, very possibly far above the transformer output voltage. But I guess we are allowed to assume that the volume control will be set at an appropriate level, to avoid such problems. At any rate, any particular frequency of square wave is only about HALF as bad as it could veryeasily get. All we'd need, to make it immediately almost twice (2X) as difficult for the reservoir caps would be a square wave of half the frequency, which would draw from the caps for twice as long at a time. We should also expect that a fixed maximumpeakvalue DC signal voltage could be applied. But I don't think we'd be worrying about the PSU distorting its sound, if that happened. So verylowfrequency square waves with peaks at the expected peak sine voltage seem like a pretty good test. The lowest frequency of square waves that can be handled gracefully might even be a good additional general "figure of merit" to use, for a PSU. For a particular transformer voltage and VA rating, it looks like we should be able to literally "assume", with high confidence, that we should never need to use more than the Cmin value that worked during the peaklevel squarewave testing. I will also note that this type of test seems like it might be relatively easy and practical to implement on our test benches, with a square wave smallsignal source and an oscilloscope: Just add capacitance until the amplifier's output square wave looks goodenough. Then it should be "guaranteed bulletproof" for real music signals. Of course we still need to figure out and agree on what frequency of square wave is "lowenough", for this type of test, and what is an acceptable maximum amount of distortion of the square wave. In my mostrecent (square wave) testing, I moreorless tried to keep any individual instant's peak deviation (i.e. the max error) of the output voltage (versus the theoretical ideal output voltage) to less than 0.1% of the expected ideal peak output voltage, while also rejecting any PSUinduced "gouging" of the output voltage waveform that furtherdeviated by more than 1 mV (while still also keeping the maximum error within 0.1% of the ideal peak output voltage, including the 1 mV or less). The only "problem" I still perceive, throughout the above discussion, is the uncertainty of the degree of "overkill" in the Cmin values obtained with the squarewave method described. (i.e. "A Cmin that's too large is not the real Cmin.") Nico's ORIGINAL GOAL for this thread was related to "how much reservoir capacitance SHOULD be used" (and the possibility of developing a "rule of thumb" for that). So I thought that to just BEGIN the process of finding that answer, maybe I should start by finding a way to determine the absoluteminimum C that would work, i.e. without allowing "obvious" PSUinduced problems to occur. The idea was to try to be able to determine at least one BOUNDARY in the range of candidate C values, and hope that maybe we could narow it down further, from there, somehow, later. So, at this point, even without having complete data, yet, for the many possible configurations of transformer, load, and output power specs, we think that we can at least determine, for a given viable configuration, a minimum C value that will prevent the possibility that the power supply, itself, could mangle the music signal due to insufficient capacitance. And we think that we will also be able to determine what are "good enough" combinations of transformer output voltage, transformer VA rating, reservoir capacitance, load resistance, and output power, but with the "good enough" being ONLY in terms of the PSU being guaranteed to not be so badly designed that it might betray its only purpose and possibly be unable to even support the faithful reproduction of the music signal, at up to the rated maximum output power. In some ways, the progress so far seems embarrassingly minor, given the time and effort that have been applied, so far. And we don't even have nearly enough data produced, yet, to be able to actually use it as I just described. BUT, we MIGHT have almost enough data produced to be able to check whether or not the quickandeasy standard "shortcut" formulas can always (or even just usually, or often) predict comparable worstcase C values, which would be an extremelynice result that would save a lot of simulator time, and simplify things a lot. (So hey, DF96 (or anyone): "Got equations?" How do their predicted C requirements line up against the simulation results? Or... what fixed DC load voltage or current gives the same C value, for one [or each] of the cases?) My point was going to be that while being able to find a lowerbound for C_reservoir seems necessary, and important, it is rather coarse, in the overall scheme of things, here. I am wondering what should be the next step, if we eventually want to be able to have a good and repeatable method for determining what reservoir capacitance "should" actually be used? I guess it's obvious that having a way to find an "upper bound" for C would be nice. If we knew how to measure the "goodness" of the amplifier, we could then know how to raise C until the measurement's improvement rate versus the C's rate of increase could no longer justify raising C any more, or something like that, for whatever criteria we wanted to use (assuming that such a diminishingreturns "convergence" phenomenon would indeed occur). In reality, such an effort could be quite complex and even just developing the methods for doing it, either with simulations or physical testing, might be difficult to achieve with a comfortable (and believable) confidence level. BUT, it helps if we remember that the goal would be to develop a reasonable "rule of thumb", early in that process, we hope, rather than developing the full process and then churning out an exhaustive array of test configurations to guide us, instead. Or is a lower bound all that is needed? On the other hand, we DO ALREADY have the simulation setup that can calculate the sine signal THD or square wave signal distortion as a function of C_reservoir. And I think people already do know what levels of distortion are lowenough that no further improvement could be audible. So that might suffice for judging the steadystate response. Maybe "all" we need is some way(s) to measure the transientresponse's relative fidelity, and probably a way to relate that to audibility. Perhaps the alreadyexisting data on the lowestdiscernable timeofarival differences for sounds reaching ears could be used to declare an initial minimum required transientresponse accuracy (i.e. the maximum acceptable timing deviation). And nonlinearity of phase errors versus frequency would show up as ringing on a square wave. C_decoupling, near each active device, would also have to be considered. Sorry to have belabored all of that for so long. It's been a rough few weeks so I'm probably about "done", for tonight. Last edited by gootee; 29th August 2012 at 03:59 AM. 

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