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Old 18th August 2012, 09:42 PM   #631
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Quote:
Originally Posted by DF96 View Post
Given the ratio between DC and RMS current in a cap input PSU you would need transformer VA to be something like 5-6 times W before you begin to approach 'oversize'. 1.5 - 2.5 times simply recognises that real music is mostly quiet so current demand is mostly well below peak, so the transformer has time to cool down between peaks.

The size of your caps depends on how much droop you can stand, which is essentially the same as ripple: droop = ripple pk-pk (roughly). How much droop you can stand depends on how far above peak signal voltage you started from, among other things.

Assume Ipk=sqrt (2P/R), then Vdroop=0.01 Ipk /C; to avoid clipping you need Vdc-Vdroop>= R Ipk. So Vdc>= (R + 0.01/C) Ipk = sqrt (2PR) + 0.01/C sqrt (2P/R)

Stored energy E = 1/2 C Vdc^2, so E>1/2 C (2PR + 0.0001/C^2 2P/R + 0.02/C sqrt (4P^2) )
E >= PRC + 0.0001 P/RC + 0.02 P/C
to keep the numbers simple I will assume R=5 so
E >= 5PC + 0.02002 P/C
so stored energy E must be at least as big as this, but it is a function of C with a broad minmum. We can approximate it as E >= 5PC + 0.02 P/C.
The minimum occurs in this type of problem when the two terms are equal (you can prove this with calculus or trial and error).
So 5C = 0.02/C so C = sqrt (0.004) = 63 246 uF, and then E = 0.63 P.

What does all this mean? I think I have shown that the minimum stored energy you need is 0.63 times output power, but this requires the optimum Vdc. Most people will use more than this, to give some slack, and then you can get away with a smaller cap (and more droop) but more stored energy because of the higher voltage. Conversely you could use a much bigger cap to get less droop and slightly smaller Vdc, but this would also require more stored energy.

Note that this calculation ignores the voltage drop through the output stage so it is a bit idealised. I often make mistakes in algebra, so use this result at your own risk!!
Are you sure you understand what you wrote? Am sure you are not because there so many flaws in your posting.
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Old 18th August 2012, 09:56 PM   #632
DF96 is offline DF96  England
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Thank you for your detailed and helpful comment on my post.
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Old 18th August 2012, 10:19 PM   #633
alayn91 is offline alayn91  France
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Hello,

You spoke about stored energy, ripple and so on.

But, you didn't answer my question: is this a high pass filter or not ??

When you spoke about 13.000 F, you don't play anymore in the ripple recreation courtyard.
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Old 18th August 2012, 10:27 PM   #634
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Also with 13.000uF there is ripple if the current is high enough. No its not a high pass but I dun understand the question why you think its a high pass, since its also not a car or an airplane either.
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Old 18th August 2012, 10:58 PM   #635
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Liching1952, you want to share something incredible with us idiots but you don't seem to know what it is that you want to share. So why don't you just start at the beginning and tell us all the answers that we are dieing to find out.
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Old 18th August 2012, 11:05 PM   #636
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The problem is that you yourself dun know the question. Ripple must low so you use huge capacitors, a capacitor multiplier, smps or what ever it is but you dun know how low the ripple must be.
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Old 19th August 2012, 12:00 AM   #637
MiiB is online now MiiB  Denmark
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the ripple must not be so low (high) that the amplifier clips due to sagging rails...that was why I set the 4V from the rail.. giving room for a mossfet driver. that runs on same voltage but on its own bank...
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Old 19th August 2012, 12:37 AM   #638
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alayn91,
I am not one of the experts here that is for sure, but I think that the answer to your question is that no the power capacitors are not considered a high pass or low pass filter. This portion of the power supply if I am following this correctly is not a part of the audio filter. This is before that portion of the audio circuit that would be considered an active filter. Someone correct me if you like. A Duh, dummy here.

It would be nice if the people who really have the answers and know what they are doing would answer the questions in an intelligent manner instead of being facetious with their answers. For those of us who are here trying to learn it doesn't much help to read a bunch of back and forth innuendo about who understands the question.... Please give us the facts and when something is wrong point it out and tell us why or where the error is being made. As a famous American said in the not to distant past, "Can't we all just get along', (Rodney King).
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Old 19th August 2012, 02:14 AM   #639
fas42 is offline fas42  Australia
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Quote:
Originally Posted by Kindhornman View Post
I am not one of the experts here that is for sure, but I think that the answer to your question is that no the power capacitors are not considered a high pass or low pass filter. This portion of the power supply if I am following this correctly is not a part of the audio filter. This is before that portion of the audio circuit that would be considered an active filter. Someone correct me if you like. A Duh, dummy here.
The power capacitors are in fact a low pass filter, and the better they are at being such the better the real behaviour of the device they're feeding. Ideally they only pass DC, but theoretically they also pass 50 or 60Hz, and some harmonics, the normal ripple people think of. Being low pass means they pass the low frequencies, and block the high frequencies, and the better they are at the latter then the better everything is. The simulations I and Tom did showed that an actual audio signal forcing the amplifier to work hard makes the voltage sag firstly, but also injects audio noise onto the supply voltage, that high frequency stuff up to 20kHz, and beyond.

And that's where the troubles begin. Circuits are notoriously bad at stopping that higher frequency noise from interfering with their operation, which is what this PSRR is all about: the ability of the circuit to reject noise on the voltage rails.

Take an extreme example: a stereo amp with power supply shared between left and right where only one channel is driven hard by a 20kHz signal. The power supply will ripple like crazy at this frequency, and the other channel, of the many power amps that will have very poor rejection at this frequency, then proceeds to output a very, very healthy high frequency tone, and if the feedback operation suffers because of this poor behaviour, lots of harmonics as well. Really terrible, distorted crosstalk, in fact.

Frank
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Old 19th August 2012, 02:51 AM   #640
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fas42, Okay I follow that and should have remembered about the noise from the ac source.

Next question then. If what we are attempting to do is keep the voltage on the +/- rails from dropping or sagging as you say, is there an ultimate value for this. If we just increase the power capacitor value to some excess value then the slew rate becomes important and we have a problem with charging the capacitor bank fast enough.? If to small a capacitor rating the voltage drops. What are we matching the expected output of the amp plus and loss to heat in the output devices?
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