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Old 10th December 2012, 05:17 PM   #1641
Krisfr is offline Krisfr  United States
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I asked these questions months ago

If I wanted to make a 400 watt into 8 ohm/ 650watt into 4 ohm amp, what would the power supply specs be?
Should each rail have its own transformer and bridge rectifier and which is better

400 100uf caps
40 1,000uf caps
4 10,000uf caps
1 40,000uf cap

should each output transistor have a local cap and a bypass cap of about .1uf ?

Would anyone have an answer now that the spreadsheet is out?

Thanks for any positive answers...

I am trying to make a decision based upon total cost of a project
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Old 10th December 2012, 05:49 PM   #1642
rsavas is offline rsavas  Canada
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I'd say 400x100uF cap's, low ESR/leakage are better, order from PRC = just kidding.
Having a separate PS for each channel is obviously better, in other words a mono-block construction. As Doug Self has discussed in his books, the transformer, filter caps, O/P transistors make up a significant % of the over all costs for a traditional amplfier. Piltron makes some nice transformers. As an example look at the PS design app note from National/TI for evaluating the LME498xxx series of drivers. Lots of information avaliable, keep looking or purchase some of the books like Doug Self or Bob Cordell, they have all the answers to your questions.
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Old 10th December 2012, 06:08 PM   #1643
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Quote:
Originally Posted by Krisfr View Post
I am trying to make a decision based upon total cost of a project
Calculate how much each cap option would amount to (order number discount).
3300-4700uF are often cost effective sizes.

(I hope you meant 40,000uF per rail)
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Old 10th December 2012, 08:33 PM   #1644
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My question here is what about the first current pulse at start up would not a smaller cap then a small resistance less than a ohm then a big cap banks stress the transformer and bridge a lot less. My preference is a choke but they are large heavy and expensive and hard to get in a high current form .
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Old 10th December 2012, 08:45 PM   #1645
rsavas is offline rsavas  Canada
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Quote:
what about the first current pulse at start up would not a smaller cap then a small resistance less than a ohm then a big cap banks stress the transformer and bridge a lot less.
You do not want this topology for a BIG power amp, because of the R power loss & slew in keeping those BIG caps charged up.
If the initial power surge is too much for the switch/fuses/rectifier(bridge), then you need to have a low resistance/high power 30-100ohm/20W in series to charge the BIG caps up slowly, then short out this series resistor (relay/triac) after the initial surge = soft start ckt.

Last edited by rsavas; 10th December 2012 at 08:48 PM.
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Old 10th December 2012, 09:57 PM   #1646
fas42 is online now fas42  Australia
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Quote:
Originally Posted by rsavas View Post
I'd say 400x100uF cap's, low ESR/leakage are better.
+1 ...

Frank
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Old 10th December 2012, 10:24 PM   #1647
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Quote:
Originally Posted by Krisfr View Post
Should each rail have its own transformer and bridge rectifier and which is better
I'd go for separate windings on the same transformer to minimiize mains noise coupling. But then I'd probably go bridged so as to only need one winding.

Quote:
400 100uf caps
40 1,000uf caps
4 10,000uf caps
1 40,000uf cap
Nearer to the top - 400 caps is a large amount of soldering and might not buy much advantage - myself I'd aim for 330uF or perhaps 470uF caps. Let's say 121 * 330uF. Its important to get the layout right to minimize inductance though - see Terry G's posts on that from a few months back.

Quote:
should each output transistor have a local cap and a bypass cap of about .1uf ?
If you think about the current loop you'll notice that that's not really called for unless your zobel network is physically close.
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Old 10th December 2012, 11:01 PM   #1648
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by jacco vermeulen View Post
3300-4700uF are often cost effective sizes.
thats funny, yesterday I looking some of those

Nichicon PW series type, 105 degr, 35-50V, ø18x35, 0.018ohm ESR, 3000mA
ordinary hole mount (not snap in)
about 2 EUR each
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Old 10th December 2012, 11:39 PM   #1649
gootee is offline gootee  United States
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Default New Power Supply Analysis Tool: Attached

New-and-improved version of spreadsheet is attached, v0.98, in both Excel 2007 xlms and Excel 97-2003 xls file formats. Screen image is also attached.

New features:

- optional non-zero start time for load current ("step input" capability)

- user entry of number of final mains cycles to use for calculation of max, min, avg, and rms psu output voltage, p-p ripple voltage, and peak diode and capacitor currents

I have added quite a bit to the related PDF file, too, and will try to post an updated version later today or tomorrow.

Cheers,

Tom
Attached Images
File Type: jpg v0.98scrn.jpg (399.0 KB, 79 views)
Attached Files
File Type: zip Power Supply and Transformer Analysis v0.98_xlsm_2007.zip (846.9 KB, 9 views)
File Type: zip Power Supply and Transformer Analysis v0.98_xls_97-2003.zip (529.6 KB, 5 views)
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Old 11th December 2012, 12:09 AM   #1650
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Quote:
Originally Posted by rsavas View Post
You do not want this topology for a BIG power amp, because of the R power loss & slew in keeping those BIG caps charged up.
If the initial power surge is too much for the switch/fuses/rectifier(bridge), then you need to have a low resistance/high power 30-100ohm/20W in series to charge the BIG caps up slowly, then short out this series resistor (relay/triac) after the initial surge = soft start ckt.
I was talking about low resistance and the time need to charge those large cap is short compared time constant of the bridge . Resistor on the order of less than 1 ohm . see Nelson Pass first watt section for what I was speaking of.
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