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Old 26th July 2011, 03:59 PM   #201
CeeVee is offline CeeVee  Portugal
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Yes Patrick, i wasn't getting into physics....foil surface area is much higher.
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Old 26th July 2011, 07:36 PM   #202
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Old 26th July 2011, 11:11 PM   #203
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Ah, a pinhead (pretty obvious)
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Old 27th July 2011, 06:12 AM   #204
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Nice hat. I tried one just like it and funny thing was, I could still hear the voices after I took it off
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Old 27th July 2011, 12:45 PM   #205
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Quote:
Originally Posted by CeeVee View Post
Yes Patrick, i wasn't getting into physics....foil surface area is much higher.
It is better to use bigger caps? Sound quality not affected by ripple?
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Old 27th July 2011, 12:52 PM   #206
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Are you asking a question as indicated by the "?" at the end of each sentence?
Or are you telling us to use bigger caps?
And that ripple does not affect SQ.
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Old 27th July 2011, 01:02 PM   #207
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Sorry, I was asking. Is it better to use bigger caps then?
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Old 27th July 2011, 01:06 PM   #208
AndrewT is offline AndrewT  Scotland
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Ripple does affect Sound Quality (SQ).
Incorrectly sized capacitors will affect performance. Impaired performance is likely to affect SQ
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Old 27th July 2011, 01:13 PM   #209
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Thanks, Andrew. Little things does make a different, as far as hifi is concern.
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Old 27th July 2011, 02:57 PM   #210
CeeVee is offline CeeVee  Portugal
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To quote ESL :

Capacitor Ripple Current
The manufacturers' ripple current rating is the maximum continuous ripple if the quoted life expectancy of the capacitor is to be achieved (usually 2000 hours, but 12000 to 26000 hours for some manufacturers). The ripple current rating is for the capacitor operating in an ambient of 85°C (or higher for high temperature types) and the maximum ripple current can be increased by up to 2.5 times as the operating temperature is reduced (2.5 times at 30°C), though going above about 1.5 times is risky because the ESR increases as the capacitor ages and causes more heat for the same ripple current. [3]. Personally, I prefer not to exceed the quoted ripple current rating.

Capacitors in power supplies feeding Class-A amps should be operated well within their ripple current rating. In a Class-AB amp, the maximum ripple is at maximum output which only occurs occasionally (if at all!). Occasional excursions up to or even above the maximum ripple current will not significantly affect the life of the capacitor. In a Class-A, however, the ripple is at a maximum whenever the amp is switched on. If the ripple current is at the maximum for the capacitor, the life expectancy would be 2000 hours (for the normal types). This equates to a life of less than 2 years if the amp is used for 3 hours a day.

A formula for calculating ripple current would be very useful, but unfortunately (despite claims made in some articles I have read), it is almost entirely dependent on the series resistance provided by the incoming mains and the power transformer. Any formulae that do exist are only true for "sub-optimal" values of capacitance (in other words, the cap is too small to be useful). The summary (below) has some guidelines that may be useful, but be aware that these are guidelines only - the final outcome has so many variables that it is impossible to give an accurate prediction of capacitor ripple current.

Remember that large capacitor values will have a smaller surface area per unit capacitance than smaller ones, so the use of multiple small caps instead of a single large component can be beneficial. There is more surface area, the ESR will be lower, ripple current rating higher, and the combination will most often be cheaper as well. This is an "all win" situation - rarely achieved in any form of engineering. An example is worth showing - the following details are from an Australian electronics retailer's catalogue:

Value (uF) Voltage Size (mm) Dia x H Surface Area (mm²) Ripple Current (mA) Price (AU$)
1,000 63 16 x 32 1659 1,400 1.95
2,200 50 16 x 35 1810 1,900 2.85
4,000 75 30 x 80 7634 4,600 14.50
8,000 80 35 x 76 8467 3,460 18.95
10,000 100 51 x 85 13779 8,100 32.95
Table 8 - Capacitor Comparison
For the sake of the exercise, we will assume that we need 8,000uF at a minimum of 50V, and with a ripple current rating of 7A - this is more than adequate for a 100W amp, and meets all the design criteria.

A single 8,000uF 80V cap could be used, with a price of $18.95, ripple current of 3.46A and a surface area of 8467 mm² Overall, a simple solution but ripple current rating is 1/2 what I want, so it is excluded.

Two 4,000uF 75V caps will cost $29, but have a ripple current of 9.2A and a surface area of 15,268 mm². Considerably more expensive, but very good performance.

Four 2,200uF 50V caps cost $11.40, ripple current is 7.6A, and surface area is 7,240 mm². The most economical, but there is a minor performance deficiency by comparison with the previous and next options (but you do get some extra capacitance). This would be my choice for most systems, as it meets or exceeds all requirements.

Eight 1,000uF 63V caps will cost $15.60. The ripple current is 11.2A, and surface area is 13,272 mm². For performance vs. price, there really is no contest. More effort is required to mount them, though.

Naturally, you could also use the 10,000uF cap, but why would you do that?
I shall leave it to you to do your own comparisons, but most of the time you will get similar results. This is a very good way to reduce the size requirements as well - it is far easier to fit a number of small caps into an enclosure than a couple of large ones, and since there is an overall improvement in specs as well as a price advantage, it is an elegant solution.

It is worth pointing out that historically, filter capacitors are the number one cause of power supply failure. This is almost always because of the effects of temperature and ripple current, and close attention to this is very much worth your while.

Last edited by CeeVee; 27th July 2011 at 03:00 PM.
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