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Old 21st March 2013, 04:14 PM   #1831
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Quote:
Originally Posted by AndrewT View Post
No.
The stiffness is relative to the load current.
A small low current amplifier will "need" less capacitance than a large high power amplifier, for the same apparent stiffness.
please forgive me for Stating a general rule of thumb kind of value for this range of amps when others are looking at exact values. The side effect of 100,000 uf and larger cap is the turn surge must be controlled but we all know that. I should have explained my previous statement better about the 200k uf amount . Gootee has done some very good work on showing the range of cap need to produce what level of power supply rigidity . Andrew T your point is well taken.
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Old 21st March 2013, 06:00 PM   #1832
gootee is offline gootee  United States
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Default "Vrail" is NOT the "unloaded" value.

For some reason I wrongly labeled the Vrail entry as the "unloaded" peak rail voltage. I apologize for any inconvenience.

The rail voltage should be entered for loaded conditions, for "Vrail".

i.e. From the unloaded rail voltage, one must subtract two diode drops, assuming the diodes are conducting the current that would result if the load had a constant DC voltage across it, with an amplitude equal to the peak voltage of a sine at the maximum rated output power. (i.e. Output power = 1.414x the rated maximum output power.)

If in doubt, just use the largest diode drops that are spec'd for the maximum forward current of the diodes to be used.

For silicon diodes, it would probably be safe to subtract 2 to 2.2 Volts from the transformer secondary's peak output voltage. For Schottky diodes, it would probably be safe to subtract 0.9 to 1.0 Volt.
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Old 21st March 2013, 06:04 PM   #1833
gootee is offline gootee  United States
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Quote:
Originally Posted by Triodethom View Post
please forgive me for Stating a general rule of thumb kind of value for this range of amps when others are looking at exact values. The side effect of 100,000 uf and larger cap is the turn surge must be controlled but we all know that. I should have explained my previous statement better about the 200k uf amount . Gootee has done some very good work on showing the range of cap need to produce what level of power supply rigidity . Andrew T your point is well taken.
No problem. Note that in the latest version of the spreadsheet, you can just increase the percentages in the blue row, near the top, to see larger capacitances. For example, enter 99.9 percent, or 99.99 percent. That makes the PSU try to leave almost no room for any ripple, which makes the needed capacitance value "excessive". You might even have to widen the columns, to see the resulting numbers. (If a number is too long to fit within the column width, it might display as ######## instead of the number. In that case, just go to the very top, where the column letters are, and hover over the line between two columns. When the cursor changes, you can left-drag to widen the column to the left of the cursor.)

Last edited by gootee; 21st March 2013 at 06:08 PM.
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Old 23rd March 2013, 05:17 PM   #1834
1audio is offline 1audio  United States
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Your spreadsheet seems to assume the mains are a pure sine with really low source impedance. They are remarkably low Z with in reality as much as 500A peak available but rarely less than 3% distortion, mostly flattening of the peaks. (From all the other rectified loads on the power lines.) This means two things, first that the peak voltage available will be different from what is calculated from an RMS (or average) meter, and second, not knowing the conduction angle or the real peak current makes it harder to calculate the voltage drops in the diodes, transformer, and wiring. Perhaps you can show what the peak current on the AC side of the supply would be?

Larger caps may be stiffer but with the short conduction angle and higher currents the net voltage on the supply is less, leaving less power available. In reality the difference between 200,000 uF and 40,000uF could be less than a less than a dB of maximum output and the supply and components may last a lot (10X) longer if the caps are smaller. In the '80s there was a fad for huge cap arrays for amplifiers. The real fallout was burned connectors on the power connections and overheated diode arrays. Not to mention the increased EM field from upping the peak current from 10A to 100A.

Of course I will be using it since its very good, well documented work.
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Old 23rd March 2013, 06:36 PM   #1835
gootee is offline gootee  United States
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Thanks, Demian.

I think that everything you said is correct.

I am still just a dabbler in this area. So there may be major omissions and mistakes in anything I have posted. But I have consistently recommended using LT-Spice or some other more-robust method of modeling and simulation.

The spreadsheets I have posted were intended mainly for use by those who are unwilling or unable to invest the effort and time required to make the jump to spice.

Assuming you were referring to the psu simulation spreadsheet that produces a plot of the psu output voltage (et al), you might be able to add resistance and inductance via the two fields for that, which essentially add them in series with the secondary winding.

There are other significant omissions, as well, such as no parasitic inductances except that of the transformer, and no accounting for amplifier inefficiency. On the other hand, the VBA code is accessible.
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Old 23rd March 2013, 07:00 PM   #1836
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Quote:
Originally Posted by gootee View Post
I finally figured out how to calculate the worst-case minimum required reservoir capacitance for a sine signal at less than the fmains frequency, in the presence of rectifier charging pulses.

There's an interesting table of C values versus frequencies, for each de-rated max output power that could be chosen for a given theoretical max power, in the attached spreadsheet.

You can enter your Rload, Vrail, fmains, etc, and it calculates everything else for you. It basically shows what Rated Max Power you would have, for different capacitances, in order to avoid clipping when operating at the rated max output power.

It ALSO shows the lowest frequency sine your amplifier could reproduce without clipping, for a given reservoir capacitance, when operating at the rated max output power.

I also made an MS Word doc that gives the equations used in the spreadsheet, with partial derivations. (I didn't want to take the time to enter the many equations for all of the steps of the derivations.)
All,

The minimum capacitances in the SINE portion of the spreadsheets I posted in posts 1821 and 1828 are a little less accurate than they could be. Currently, they are pretty close, assuming that ESR is very small (which it probably always will be). But even though the results are very close to correct, part of the math was not done correctly so I feel the need to revise it. I will upload a new set of xlsx and docx files, tonight or tomorrow.

Cheers,

Tom
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Old 23rd March 2013, 07:02 PM   #1837
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gootee,

You actual use of math was nice and refreshing.

Of course there are many other issues (including the effects of an almost constant current load on the power supply capacitors) that are of some importance.

If you really want to spend too much time on this then try looking at transformers!

Gold star for the nice work.

ES

P.S. In normal sized power supplies with transformer regulation into a full wave bridge conduction angle is almost 60 degrees!
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Old 23rd March 2013, 09:44 PM   #1838
fas42 is online now fas42  Australia
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I'm a lazy fella, my poor old head would explode trying to follow Tom's maths, so I take the easy way out and use LTspice. It's important that all the realities of an actual circuit are taken into account, including dodgy mains quality, right through to loads comparable to actual speakers -- something I've explored for quite a while -- and it is very revealing as to what's important.

I hope at some stage to do a full analysis of this sort of thing, in sim's, when my energy reserves are topped up a bit ...

Frank
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Old 24th March 2013, 08:51 AM   #1839
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Hello !
I wonder how many Joules are stored in power supply caps of an average solid state amp and of an average tube amp.
I have the feeling that there are many more Joules in the case of the tube amp.
Could this lead to something ?
Thanks for the very interesting 3D
Regards,
gino

Last edited by ginetto61; 24th March 2013 at 09:08 AM.
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Old 24th March 2013, 10:09 AM   #1840
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by simon7000 View Post
...................

P.S. In normal sized power supplies with transformer regulation into a full wave bridge conduction angle is almost 60 degrees!
This seems unusually high.
The pics I have seen all support a MUCH shorter conduction period.
Even the saw tooth ripple on the smoothing caps shows a MUCH shorter conduction period.
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