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Old 5th August 2008, 10:02 PM   #1
roger-k is offline roger-k  Norway
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Default The size of electrolytic capacitors?

I know there is already written a lot of this but a guideline will be fine.

Is there any rule of thumb to estimate the size of the electrolytic capacitor in a class A-amp, in relations with power and linevoltage?

I know the bigger the better but sometimes I think there is a overkill of uF.
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Old 5th August 2008, 11:35 PM   #2
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Default Re: The size of electrolytic capacitors?

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Originally posted by roger-k
I know there is already written a lot of this but a guideline will be fine.

Is there any rule of thumb to estimate the size of the electrolytic capacitor in a class A-amp, in relations with power and linevoltage?

I know the bigger the better but sometimes I think there is a overkill of uF.

Overkill? Your kidding right? This is DIY audio man! We aren't happy until the power amp turn-on surge pulls down our sector of the power grid .

Seriously, I too am interested in the answer to this question. Something like "X x Class A watts = N uF" as a rule of thumb would come in handy.

Bill
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Old 6th August 2008, 08:13 AM   #3
roger-k is offline roger-k  Norway
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kidding and kidding... The point was not too much uF, but a easy guideline or instruction to decide the size of uF.
Any?
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Old 6th August 2008, 11:34 AM   #4
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I have seen many suggestions over the time.....
The one I normally stick to for Class A/B says 2.000uF per output Ampere, so 10 Ampere out = 20.000uF per Rail.
I always end up with twice the calculated just to be sure
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Old 6th August 2008, 11:44 AM   #5
AMV8 is offline AMV8  United Kingdom
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Hi
You ask for a rule of thumb and not a calculation for the size of electrolytics in power supplies.

I do normally try to calculate my needs but this is my rule of thumb;
A class A amplifier of 100 watt continuous output would need 100,000uf to give a ripple of 0.6 volt on the amplifier power rails. You can also reduce ripple in the power supply by using inductors in the power supply.

When I listen to a typical 50 watt class A amplifier I normally find that I can not distinguish any change in sound if the power supply capacitance rises above 50,000uf. Also I can detect only a little change in sound for the same amplifier if the capacitance drops to 25,000uf. Others may have more sensative hearing and may need more capacitance.

You did not mention the transformer, but I think that the same pure class A 100watt amplifier would draw 500 watt all the time and would need a 1,500 watt transformer.

I recommend you read the article by Nelson Pass on Pass DIY entitled "Power supplies"


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Old 13th August 2008, 08:41 AM   #6
roender is offline roender  Romania
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Whay don't you use capacitance multiplier PSU?
http://sound.westhost.com/project15.htm
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Old 13th August 2008, 08:44 AM   #7
h_a is offline h_a  Europe
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Quote:
Whay don't you use capacitance multiplier PSU?
Well, he could also go all the way to voltage regulation. It's just not very practical with all this additional heat - especially for ClassA. That and the additional parts that require another pcb usually scare people away and let them revert to adding caps instead - it's much easier. Oh yeah, there are also people that decline voltage regulation per se.

Have fun, Hannes
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Old 13th August 2008, 09:07 AM   #8
roender is offline roender  Romania
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Quote:
Originally posted by h_a


Well, he could also go all the way to voltage regulation. It's just not very practical with all this additional heat - especially for ClassA. That and the additional parts that require another pcb usually scare people away and let them revert to adding caps instead - it's much easier. Oh yeah, there are also people that decline voltage regulation per se.

Have fun, Hannes
It is not the same as voltage regulation ... in capacitance multipier topology, disipated power is not very high.
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Old 13th August 2008, 09:22 AM   #9
Nordic is offline Nordic  South Africa
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http://www.tnt-audio.com/clinica/ssps3_e.html
There is a list of tables to towards the bottom of this page with some suggested minimums...

On page 2 of that document it offers another solution

"There is another approach. It is accepted that one needs 1-2 joules of energy per every 10W of output power. For a 50W/8 Ohms amplifier, we need 10-20 joules of energy to be stored. We can use a formula, 1/2CVsquared (where C equals capacitance and V equals voltage, the voltage being squared) to calculate that 15,000uF, fed by say 33V (worst case, full load on) allows for 8.16 joules per capacitor, or 16.3 joules per channel - enough to fry quite a few unsuspecting speakers in the low end class, and even some in the midrange class. If 33V is our worst case, we can assume 36-38V supplies with the load off (say 37V), which means that just before a transient, we will in fact have some 20.5 joules stored in each channel's capacitors. "
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Old 13th August 2008, 09:22 AM   #10
AMV8 is offline AMV8  United Kingdom
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Hi

Whay don't you use capacitance multiplier PSU?

I do sometimes use a capacitance multiplier. Typically that would be in pre amps where I need a higher voltage than my transformers could provide and in the front ends of power amps where I need a higher voltage than in the output stages. They are very effective.

I have tried/used a voltage multiplier for the output stage of a power amp. JLH was a believer in this concept.

However I have found that a capacitance multiplier in the output stage of the power amp is not as convenient or as effective as simply using capacitors. ( or capacitors plus inductors if you need less ripple ). For high power output amps the arrangement becomes more complicated than simply using capacitors and does not work as effectively in clearing up detail on complex music passages. But for lower power amps like the JLH it works OK.

Just my observations.

Don
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