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Old 18th July 2012, 10:22 AM   #1
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Default Power Supply Resevoir Size

Assuming a 100 watt amplifier driven at 100% duty cycle/factor what size reservoir capacitor would be required in the power supply considering that E = ˝CV˛. A 100 watt being 100 J/s then:

If one would select a rail voltage of 50V one arrives at a reservoir capacitance of 80,000 uF.

I do not find any rule of thumb on this topic on the forum and since amplifier sound characteristic is directly affected by the power supply, maybe creating some "standard design rule" with some plausible reasoning would be most helpful to the DIY community

I would appreciate comments from our power supply specialists on the forum regarding this matter.
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Old 18th July 2012, 10:26 AM   #2
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You need to assume the amount of ripple that you are trying to achieve.

It is impossible to have zero ripple.

Ripple V = I / (f * C)

Therefore lets assume I = 4A and f = 120Hz (Full Wave Rect).

If you require a ripple voltage of 100mV.

C req = 4 / (120 * 0.1) = 333000uF (unrealistic - but so is 100mV ripple at 4A)

If you are using 50Hz mains, Full Wave Rectification and allow for 1V of ripple C becomes 40000uF.

Last edited by KatieandDad; 18th July 2012 at 10:34 AM.
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Old 18th July 2012, 10:42 AM   #3
brig001 is offline brig001  United Kingdom
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I usually use C = (It) / V where:
C = capacitance required (Farads)
I = maximum current
t = 10msec for 50Hz and full wave rectification.
V = acceptable pk to pk ripple voltage

formula comes from Q = CV and Q = It
It's a bit of a simplifcation, but seems near enough and on the safe side.

HTH,
Brian.
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Old 18th July 2012, 10:50 AM   #4
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Posts 2 and 3 use exactly the same formulae as f=1/t
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Old 18th July 2012, 11:14 AM   #5
DF96 is offline DF96  England
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There was a long discussion in this thread; look from around page 630 onwards. There is a rule of thumb, but in the end it is just down to what ripple you want.
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Old 18th July 2012, 12:28 PM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Nico Ras View Post
I do not find any rule of thumb on this topic on the forum and since amplifier sound characteristic is directly affected by the power supply, maybe creating some "standard design rule" with some plausible reasoning would be most helpful to the DIY community
Bigger isn't always best.
As you say, amplifier "tuning" or the sonic characteristics of an amp can be altered by its PSU design.

Again this is where objectivism and subjectivism might not agree.

A) The amp is better technically with the low ripple massive PSU.
B) The amp sounds better with a smaller "faster" PSU.

So which do you build ? A or B



Some good general useful info here... but it doesn't tell you how godd an amp will sound with a given PSU.

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Old 18th July 2012, 12:35 PM   #7
PChi is offline PChi  United Kingdom
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It's also worth looking at the ripple current through the capacitors and checking versus the rating and life / temperature. Crude caluculations or a simulation in LTSpice can get values or build and measure.

A very crude rule of thumb for power supplies is 2000 uF per amp (of load current). I guess a minimum for Power Amplifiers.

A decent power amplifier should be fairly tolerant of ripple voltage. Their ripple rejection is likely to vary alot depending on design.

It also depends on what output current the amplifier is limited to or expected to deliver.

So lots of variables.
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Old 18th July 2012, 01:16 PM   #8
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Do you think it is only ripple that is responsible for a better sounding amplifier, or is there more to it.

The reason for my inquiry came from the week-ends escapades. I had little to amuse myself with so I fooled around with a JLH class A that I knocked together and was discussed on one of the threads here a few months ago.

At that time I swapped between a 300VA smps and standard 300VA transformer supply with 20 000 uFcaps and a remarkable "improvement" resulted while running off the smps.

On the week-end I replaced the SMPS with 2 x12V 22.5Ah gel batteries and the"improvement" again, chalk and cheese.

The SMPS has a 220uF capacitor on a 315V rail, which equates to 10.53 Joule while the 300VA full wave rectified transformer has 20 000uF on a 24V rail and equates to 5.7 Joule.

According to the peak ripple equation the standard supply should be 0.65V at 1.3 amp load, the SMPS is measured 30mV at 1.3 amp, while there is zero ripple with a battery.
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Old 18th July 2012, 01:42 PM   #9
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Quote:
Originally Posted by PChi View Post
A very crude rule of thumb for power supplies is 2000 uF per amp (of load current). I guess a minimum for Power Amplifiers.
I have also heard of this rule of thumb, and we are on the opposite ends of the earth - is there a reasoning behind the rule (I was told this story in the 60s, but at the time it was only 1000uF/amp maybe easier calculations) Assuming all things equal, could the ripple only be responsible for "smearing" the sound. Does amps with better PSSR sound better than amps with lesser PSSR using the same power supply,.......
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Last edited by Nico Ras; 18th July 2012 at 01:55 PM.
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Old 18th July 2012, 01:52 PM   #10
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Mooly, you built a few amps in your time, don't say maybe this or that, say what you experienced as being "better", we could be on to what the real difference are between amps.
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