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Old 13th September 2012, 11:35 PM   #1111
gootee is offline gootee  United States
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Quote:
Originally Posted by AndrewT View Post
16V of drop on the smoothing capacitor !!!.
What about a 4V drop?
or a 2V drop?
or maybe a 1V drop?
(I'm back from travel.)

AndrewT,

The calculation you were looking at was for calculating the ABSOLUTE MINIMUM capacitance. You reacted as if you thought it was giving a recommended design value!

The maximum supply voltage drop at full output power can be as small as one wants it to be, obviously. But that's basically the whole point of this thread! HOW SMALL IS "small-enough"? But mainly, WHY? And, AFTER that, how do we easily predict the reservoir capacaitance needed, for a particular amp and specs?

As you should know, I had been focusing, FIRST (in order to try to find an easy way to set a lower bound, just as the FIRST STEP), on finding ways to predict the ABSOLUTE MINIMUM reservoir capacitance that would keep the charging pulses from causing gross distortion of the output signal.

And with DF96's correct insight, I was able to see that, with the amplifier load in the spice circuit I've been using, the MINIMUM usable capacitance is the one that just barely doesn't allow the voltage rail (at the bottom of the ripple voltage waveform) to force the amplifier to have any less voltage across it than is physically possible.

The calculation you were looking at was for calculating the ABSOLUTE MINIMUM capacitance value.

You reacted as if you thought it was giving a recommended design value!

Changing the subject, momentarily, to what an acceptable lowest-excursion of the power rail voltage might be:

I have no good rule, yet, except that the power rail voltage had better not crash into the absolute minimum, i.e. vload + vamp .

But, what would be so wrong with a worst-case, at MAXIMUM rated output power, that got "kind of close" to that but was guaranteed to never reach it?

No harm no foul, as I see it, in one sense, but I'll also bet that the output distortion figures will be lower when it DOESN'T approach the absolute minimum, as closely.

However, if the distortion difference was very small, between the "huge dips" case and the "1-5V dips" case, then we could possibly have to say that it's fine to have huge dips in the rail voltage.

I posted such distortion figures (distortion vs reservoir capacitance, for C from below the absolute minimum to very large), for several cases, but the distortion figures were not well-calibrated, back then. I do have the calibration method very well-refined, now, for the simulation setup I'm using. I will start posting some plots as I get groups of them finished.

I realize that square wave distortion and sine THD are not necessarily good measures of an amplifier's sound quality. But we do also have reports that lower reservoir capacitance (and therefore larger dips in the rail voltage) "sounds better". Other differences in sound quality seem likely to be due to differences in the transient response (since distortion pretty-much covers the quality of a steady-state response). The transient response will also get looked at, here. Personally, I usually tend to think that the transient response is more important than the steady state response (or, at least, more important to think about).

I realize, too, that I am moving rather slowly, toward the goal of finding a way to pick the value of the reservoir capacitance. Investigating how to at least put a lower bound on it was interesting and maybe it should have been more-obvious to me. But I learned a lot, the way we went about it, and actually verified and validated some simple "well known" ideas.

The next step, as I see it, will be to see if there's a simple rule for calculating the reservoir capacitance that puts the steady-state distortion just beyond the "knee" of the distortion vs C curves, which always start out almost vertical, then curve quickly at the knee and become almost horizontal. (Maybe we just found most of that rule, based on the ripple voltage excursion etc, but we need some easier way to express it, and first need to see how that point relates to the actual distortion numbers, and how their "diminishing returns" behave, vs increasing C.)

After that "knee" point, the distortion doesn't improve much at all and we might be able to assume that there would be no good-enough reason for adding any more capacitance, after that (unless someone can give a convincing argument for not having downward excursions of vrail beyond a certin percentage of nominal, for example).

So I THINK that we'll be looking for a rule of thumb that will relate that distortion knee to the load impedance, output power, and transformer Vrms and VA ratings. I'm hoping that the the distortion plot knee will relate to something that the standard equations already use as a constraint, such as a maximum ripple amplitude (lowest Vrail excursion), which would then be applicable, more or less, to all cases, although it might again relate to the minimum possible voltage across (or impedance of) the amplifier, or whatever is sitting between the rail the the top of the load.

Maybe it's already obvious to some others, here. But it would be nice to show it working out as expected, for at least several disparate example configurations.

By the way, what's a good/reasonable LOW LIMIT for frequency? I have been using 25 Hz. Is that low-enough?

Cheers,

Tom

Last edited by gootee; 14th September 2012 at 12:01 AM.
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Old 13th September 2012, 11:39 PM   #1112
AJT is offline AJT  Philippines
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i suggest 20hz .......
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Old 13th September 2012, 11:44 PM   #1113
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In relation to how 25Hz has been used to calculate, the caps are likely to get charged twice as music plays, worst case is when the charge and discharge are 90 degrees out of phase. Use 50Hz or 60Hz. I showed something interesting earlier Power Supply Resevoir Size ,Power Supply Resevoir Size

Last edited by OnAudio; 13th September 2012 at 11:47 PM.
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Old 13th September 2012, 11:47 PM   #1114
tinitus is offline tinitus  Europe
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my own personal non proven theoretic fact is that the last cap should be big enough to 'make it on its own 99% of the time
the rest are 'just' backup and smoothing, if any other needed

then we can still argue about 'big enough'
but very big caps are simply slow in my opinion

as in many other things in life, going big introduces lots of problems
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Old 14th September 2012, 12:10 AM   #1115
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Quote:
Originally Posted by Nico Ras
This afternoon I fooled with two extremes, 80,000uF per rail capacitors versus a single 4700 uF per rail. Now here is were I get shot down. Bass is better with the big caps but on the other hand the small cap allows me to hear so much more music.
80,000uF timer can be like driving with the parking brake on.
When a charging cap is an open door to noise, and when the power board has such large capacitance to form a slowdown timer, we have to interrupt this action with capmulti, or regulator, or a series schottky/fast diode prior to cabling the power to the amp board, because we don't want the amp board caps to charge with the same slowness as the big power board reservoir.
Materials list:
Schottky Left V+ at the output of the power board
Schottky Right V+ at the output of the power board
Schottky Left V- at the output of the power board
Schottky Right V- at the output of the power board
(Amplifier board power cables attach to the schottky.)
Of course one could use regs or capmulti, which is more orthodox, but schottky or fast silicon diodes are cheaper and easier.
Credit to diyaudio.com member "theprof" for researching this inexpensive repair.
P.S.
Some applications may be partially effective with an ordinary cable instead. My suggestion is greatly applicable to the "all in one" amp board power supply made all together as a single unit. But, my suggestion is less applicable to other layouts that have an interlink cable.
P.P.S.
Other fixes include smaller size low esr caps for amp board (improve quality, reduce size--low esr caps usually win the fight for charge), and/or (sounds just like) adjust your compensations to sharpen up your square wave test for more ear perking fun than rounded.
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Last edited by danielwritesbac; 14th September 2012 at 12:19 AM.
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Old 14th September 2012, 12:14 AM   #1116
gootee is offline gootee  United States
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Quote:
Originally Posted by tinitus View Post
my own personal non proven theoretic fact is that the last cap should be big enough to 'make it on its own 99% of the time
the rest are 'just' backup and smoothing, if any other needed

then we can still argue about 'big enough'
but very big caps are simply slow in my opinion

as in many other things in life, going big introduces lots of problems
But, but, but, a larger total capacitance (whether all in one big cap or in several paralleled) reacts FASTER, in the sense that it takes a smaller (or slower) drop in the "ambient voltage" to make any given amount of current surge out of them.

Or do you mean the rise-time is slower? i.e. the time it takes to go from zero to the discharge level, for a particular lowering of the "ambient voltage".

Or are you thinking about the RC time constant, and the longer, slower decay time of the "e to the minus t over RC" equation for the voltage across the cap? In this case, the longer the better, since that's what keeps the ripple amplitude small, and enables the caps to power the output all by themselves, between charging pulses, i.e. it enables them to last until the next one.

Maybe you are referring to ideas derived from digital circuits, where the decoupling caps are said to need to be "small", to be fast-enough. In that case, they are talking about having low-enough inductance to be fast-enough to provide the fast transient currents. And that does depend on the physical smallness of the capacitors (and the shortness of their connections). They even go so far as to turn the surface-mount caps so that the smaller of the two dimensions is across the decoupling points.

BUT STILL, you would want to use the LARGEST capacitance you could get, in the small package size, for those digital decoupling applications.

So I'm not sure what would be considered to be "slow" about larger electrolytics, UNLESS we are talking about transient response. THEN I TOTALLY agree (because of inductance due to larger dimensions and connection distances). HOWEVER, that job, the "fast" part of it at least, will be handled by the DECOUPLING CAPS, which HAVE to be very near the points of load (to be fast-enough), not by the reservoir caps.

Last edited by gootee; 14th September 2012 at 12:38 AM.
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Old 14th September 2012, 12:14 AM   #1117
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Hi Daniel what is the switching frequency of mains
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Old 14th September 2012, 12:37 AM   #1118
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Quote:
Originally Posted by OnAudio View Post
Hi Daniel what is the switching frequency of mains
60 cycles per second in the USA (from an amp it sounds like 120hz distortion), plus a lot of higher frequency bonus noise from every appliance that you and all of your neighbors who are on the same transformer have either running or switching. The bonus noise is why I've been promoting filters installed at both the primary and secondary windings of transformers. Even if an audio amp has excellent advertised power rejection, it is still better to avoid inserting noise into the audio amp. Filtering off the HF trash is the first step towards realistic digital replay and, surprisingly, that filtering need also includes the audio amp's transformer. Results do vary--some are dramatic and others are not.
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Last edited by danielwritesbac; 14th September 2012 at 12:40 AM.
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Old 14th September 2012, 12:53 AM   #1119
gootee is offline gootee  United States
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Quote:
Originally Posted by Nico Ras
This afternoon I fooled with two extremes, 80,000uF per rail capacitors versus a single 4700 uF per rail. Now here is were I get shot down. Bass is better with the big caps but on the other hand the small cap allows me to hear so much more music.
Quote:
Originally Posted by danielwritesbac View Post
80,000uF timer can be like driving with the parking brake on.
When a charging cap is an open door to noise, and when the power board has such large capacitance to form a slowdown timer, we have to interrupt this action with capmulti, or regulator, or a series schottky/fast diode prior to cabling the power to the amp board, because we don't want the amp board caps to charge with the same slowness as the big power board reservoir.
<snipped>
Daniel,

The RC time constant of the reservoir caps should not be a problem, when charging the decoupling caps on the amp board. You're probably thinking of the slower exponential decay of a larger capacitance's voltage, as it discharges current through a resistance. But look at the very beginning of that curve. There is no slow ramp UP to it. The current starts flowing blindingly quickly, and at its highest rate.

The power rails themselves do have resistance and self-inductance that could slow down the charging of the decoupling caps (not by a lot, though). But larger reservoir caps should only help make it faster and better for the decoupling caps, as far as I can see.

As far as what Nico experienced, there were far too many variables to know why he heard what he heard. What were the rail inductances and resistances (i.e. the physical lengths)? What were the ripple amplitudes on the rails, and how were they changing? What was the average voltage on the supply rail and how much was it changing, due to the music signal? What decoupling capacitances were used at the points of load? What were the transformer Vrms and VA ratings? Etc.

As far as bass is concerned, there IS a minimum reservoir+decoupling capacitance that must be present, for the system to be able to handle the demands for current at bass frequencies, since low-frequency bass waves have periods (1/f), or half-periods if only one polarity is considered at a time, that could make them need current for the entire time between charging pulses. Can capacitors charge and discharge at the same time? I think they must. But I wonder if it would help to offset the voltages at which several parallel caps would each start charging (and discharging).

Cheers,

Tom

Last edited by gootee; 14th September 2012 at 01:08 AM.
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Old 14th September 2012, 01:10 AM   #1120
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Anybody bothered to figure out how much bigger transformer is need for a power factor of 0.5 (big caps) vs 0.7 (normal sized caps)?
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