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Old 19th April 2012, 10:44 PM   #1
ZLyzen is offline ZLyzen  United States
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Default Quick question on ps requirements, mainly cap sizes

Hello,

I am building a 2.1 self-powered speaker set and have landed on a transformer
Antek - AN-4225
I'll be able to get almost 35V out of the rails with this but I was wondering 2 things

Can I use one with a smaller vac rating (and smaller pricetag, size, weight, etc)

Would 2 4700uF caps in parallel PER RAIL (so 4 total for the psu) be enough for the rough power demands?

there will be:

-2 satellite speakers (8Ω) that have 25Wrms max
-they will be powered by a 40Wrms max into (8Ω) chip amp from national semiconductor, or texas instruments now I guess

-1 Sub (8Ω) 80Wrms max
-This will be powered by a bridged L4780TA

-also a few very small opamps (about 4) that will be for processing/filtering the low level input

Any input would be great thanks!
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Old 19th April 2012, 11:06 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

The short answer is yes to both. A 140/160VA transformer is more
than adequate and 200VA plenty. for 4 PS caps I'd used separate
bridges and caps for L+R and the sub, makes sense to me.

rgds, sreten.

Based on D.Selfs analysis of PS requirements for music, not test, signals.

TBH you'd probably be OK with 100VA, but I'll get jumped on saying that.
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Last edited by sreten; 19th April 2012 at 11:24 PM.
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Old 19th April 2012, 11:46 PM   #3
ZLyzen is offline ZLyzen  United States
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Okay, I was hoping you'd say that, seeing as I don't think I'll ever be near max and almost always under 50% of these speakers potential I'm really leaning towards the 100Va, plus the 200Va is HUGE. I think I'll do the 100 and see how it goes, thank you so much!
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Old 19th April 2012, 11:56 PM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by ZLyzen View Post

...' plus the 200Va is HUGE .....
Hi, And the 400VA you linked to isn't ? , rgds, sreten.

Typical listening levels are usually well below clipping,
even at clipping levels typical music signals spend 80%
of the time below 20% of maximum output, except
in the bass which is much more compressed.
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Last edited by sreten; 20th April 2012 at 12:26 AM.
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Old 20th April 2012, 02:02 AM   #5
ZLyzen is offline ZLyzen  United States
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Well that does concern me a little as the majority of the power will be drawn from the sub, but how would I go about calculating such values of power consumption?

If anything I'm looking for a quick crash-course and not trying to steal all your time!

I've though long and hard about how I could come up with power drawn and the amperage I get is very high, hence my starting with the 400va
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Old 20th April 2012, 04:30 AM   #6
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Expected power levels are always a wild guess. The conventional wisdom is the sub or bass will get the most, but its entirely program and volume dependent. Get into some male or even female vocals and almost everything can go to the midrange. Similar thing with piano, huge peak power can be needed. IMO, your divisions make sense, since there's only one sub, but I bet you clip the rest of the system before the sub unless you're into movie soundtracks.
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Old 20th April 2012, 06:54 AM   #7
ZLyzen is offline ZLyzen  United States
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Also, from looking at the fact that the speakers are 6~8 Ω shouldn't I focus more on amperage than voltage when choosing a toroid? For instance won't I need the amperage more than the available swing? and if i calculate that a speaker will pull 1.4A is that .7A per rail?
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Old 20th April 2012, 09:31 AM   #8
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My understanding is it is 1.4A per rail but only for a half cycle so the heating effect is the equivalent of 0.7A rms per rail , and its the heating effect we are interested in .

As a rule of thumb i apply this :-

your total max power is 160W say at 70% efficiency
so total required max power is 160/70 X 100 = 228W

I personally would go for a 200VA transformer because i like to cover most eventuallities ( 200VA ignores 4ohm working ) and yes it probably is overkill for real music handling .

Look at it another way a 200VA could be an investment as transformers are only going to get more expensive .
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