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Old 24th June 2013, 12:09 PM   #1
Kev06 is offline Kev06  England
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Default Sweet spot for power amp smoothing caps?

I've been looking at smoothing capacitors for my upcoming (solid state) amp builds, which could be up to a maximum of 100w per channel into 8ohms from a PSU with +/-42v rails.

I'm thinking 63v caps to be safe, as 50v doesn't 'quite' allow as much headroom as I'd like for transformer regulation plus mains voltage variations. And I was thinking something like 10,000uf per rail (20,000uf per channel) would be a reasonable capacity. Money is not irrelevant, unfortunately though. Cap prices soon start to spiral with better life expectancies, ESRs and ripple currents, yet I've seen some terrible looking specs at the budget end that I would want to avoid.

Roughly what sort of lifetime and total ripple current (or esr) would people suggest is a sensible sweet spot to aim for, and what sort of pairings are likely to be the best way of achieving it (e.g. 1x 10,000uf, 2x 4,700uf etc).?

Many Thanks
Kev
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Old 24th June 2013, 12:11 PM   #2
DF96 is online now DF96  England
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You could try calculating/modelling it.
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Old 24th June 2013, 12:35 PM   #3
Kev06 is offline Kev06  England
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Quote:
Originally Posted by DF96 View Post
You could try calculating/modelling it.
You may be under-estimating my limitations :-) I have had a go at working out how the ripple affects temperature and so lifetime etc but I'm not really sure what I'm doing, or how real-world it is. Or in fact what 'ripple' effect would stem from driving the speakers with typical music, assuming (perhaps wrongly) that discharging the caps heats them as much as charging..

TBH I was sort of hoping for some rules of thumb or general estimates from experienced builders, which are likely to be rather more trustworthy than my uncertain attempts. I found some for estimating capacitance, but not for ripple and heat/lifetime. Is that because its always very specific to a given setup, or can ball-park ranges be of use?

Thanks
Kev

Last edited by Kev06; 24th June 2013 at 12:38 PM.
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Old 24th June 2013, 01:03 PM   #4
DF96 is online now DF96  England
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Ripple current is hard to calculate exactly, although modelling should give it. For a wild first stab I would assume about 1.5-2 times the DC current draw. Lifetime depends on temperature and build quality. You can affect the temperature through the rest of your design. Build quality depends on the cap manufacturer.

You could try looking at the long thread on reservoir cap size, but recently it has wandered off the point.
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Old 24th June 2013, 01:08 PM   #5
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You could also download PSUD here:

PSUD2

It wil not help you with heat/lifetime issues, but give you an idea about the capacitance needed for a given ripple and/or the ripple for a given capacitance.

The tools is pretty straightforward and the is also a thread here on DIYAudio as well as many tutorials on the web.
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Old 24th June 2013, 02:01 PM   #6
Kev06 is offline Kev06  England
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Quote:
Originally Posted by DF96 View Post
Ripple current is hard to calculate exactly, although modelling should give it. For a wild first stab I would assume about 1.5-2 times the DC current draw. Lifetime depends on temperature and build quality. You can affect the temperature through the rest of your design. Build quality depends on the cap manufacturer.

You could try looking at the long thread on reservoir cap size, but recently it has wandered off the point.
Thanks, thats very helpful; I realise its just a rough ball-park but it helps a lot for checking the sanity of my choices. It also doesn't seem unreachable at the sort of currents I'd be looking at, which is quite reassuring.

Yeah, I've noticed that the manufacturer's life expectancies can vary hugely, though theres also quite a bit of variation within a manufacturer depending on the range and size of the capacitor. I've seen anything from 1,000 to 200,000hrs quoted at the rated temperature.

I'll have a look at that long thread again - I assume you mean this one. I'd found some later pages in my searches but perhaps I need to start reading from earlier on if its wandering.

Quote:
Originally Posted by coolnose View Post
You could also download PSUD here:

PSUD2

It wil not help you with heat/lifetime issues, but give you an idea about the capacitance needed for a given ripple and/or the ripple for a given capacitance.

The tools is pretty straightforward and the is also a thread here on DIYAudio as well as many tutorials on the web.
Thanks, I'll have a go when I'm on my home computer. I'd kind of assumed such things were too advanced for me, but if there are tutorials and threads on it I should be okay.

Thanks again,
Kev
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Old 25th June 2013, 10:30 AM   #7
AndrewT is offline AndrewT  Scotland
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TNT and ESP have sections on PSU.

You won't get 100W into 8r0 from +-42Vdc supply rails.
30+30Vac gives +-42Vdc. This suits ~75W into 8r0.
35+35Vac gives +-50Vdc. This suits ~100W into 8r0.

And using the UK's 240Vac into a 230Vac transformer gives you a slightly higher rail voltage than what all the European and New World Members tell you will happen.
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Old 25th June 2013, 04:32 PM   #8
Kev06 is offline Kev06  England
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Well spotted; I should have checked the wattage myself - I just took it from the ESP site's P3a description, which I was thinking of using at the time of posting: "and with the transistors specified the amp will provide 100W into 8 ohms, at a maximum supply voltage of ±42V. This supply is easily obtained from a 30-0-30V transformer". I see that a tad lower down the page though there's a spec table which quotes it as 90w for the same supply voltage which is different but still above your estimate, so perhaps he's talking about some kind of peak as the driver impedance changes.

Thanks for pointing out my error, a 25% drop in expectation is significant enough to make a difference if I were to choose less efficient drivers; I was already considering the p101 mosfet project instead so maybe thats further support for it - though higher voltage capacitors would be needed.

Cheers
Kev
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Old 25th June 2013, 04:37 PM   #9
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Consider a bank of smaller 63V radial caps. Even good ones don't cost that much, especially if you hit a quantity price break point, and the total will likely be less than for a big screw terminal can. You'll also get better dissipation factor and HF performance.
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Old 25th June 2013, 08:40 PM   #10
Kev06 is offline Kev06  England
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Quote:
Originally Posted by Conrad Hoffman View Post
Consider a bank of smaller 63V radial caps. Even good ones don't cost that much, especially if you hit a quantity price break point, and the total will likely be less than for a big screw terminal can. You'll also get better dissipation factor and HF performance.
I think you're right. I've just been doing some price comparisons on a reputable dealer's website. Looking at just the standard type caps with 2000-3000hr@85c lifetimes, it looks like the best value sizes are between about 2,200uf and 4,700uf or so. Building multiples to my 10,000uf target (give or take) I end up with ESR totals of around 0.021-0.035ohms and total ripple currents of 8A to nearly 13A as standard. By comparison a single 10,000uf cap with suitable ripple current is 2-3 times the price as making it up from a few/several smaller ones.

Its slightly different for caps with more endurance. 105c caps are less economical in the small sizes, so starting around 4,700uf seems better value - but it still adds an extra 50% cost over the 85c ones; 3000hrs (rather than the standard 2000) at 105c is similar but more or less doubles the price of the basic 85c types. If I wanted say 12,000hrs at 85c then its 2-3 times the price of the basic ones.

(though that's just at this one retailer so may or may not be universal)

Looking at a few examples of temperature specifications, keeping an 85c capacitor at 40c seems like it can extend life by 25-30x its rating at 85c. Keeping a 105c capacitor at 40c can be more like 100x. The ripple causes heating though, so its a matter of keeping that down as well as just the ambient conditions. I guess with the internal heating from ripple and ambient temperature inside an enclosure case, 40c for the caps core temperature may be harder than it seems though, even with more smaller caps with more surface area.. perhaps standard 105c caps would be a better bet?

Cheers
kev

Last edited by Kev06; 25th June 2013 at 08:55 PM.
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