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Old 17th January 2007, 06:19 PM   #1
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Default Just a quick power supply capacitance question

While making a preamp I decided I would pretty much rebuild my first chip amp (LM3886) on slightly neater and probably better laid out, though probably still not particularly "good", PCBs.

Currently I have 4400uF per rail, per channel (so 17,600uF in total). It's a dual secondary 500va transformer, with a single 35A metal bridge rectifier on the secondaries, then the filter caps are sort of... in parallel I from there. That is, there's two wires coming off each bridge rectifier "DC" tab, one goes to one set of filter caps, the other wire to the other set of caps. Forgive me for being a bit rubbish at explaining, but I hope you get the idea. Each channel is also regulated separately. One regulator PCB per filter cap PCB.

I'm pretty much just wondering whether it might actually be better to just have one big lot of filter caps for both channels instead?

I've made the filter cap board a bit bigger to accomodate up to 20,000uF per rail this time, but I actually only currently have 2200uF caps. Three of these will fit on the board, per rail, so that would be 6600uF per rail, and 26,400uF in total. I would still like each channel regulated separately, but I think I can still do this "in parallel"?

Or I could keep it separate and have 13,200uF per channel... will it really make much of a difference? If not, I am tempted to just have one big lump of caps for both channels instead, as it should be slightly tidier.
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Old 17th January 2007, 09:37 PM   #2
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What is the difference in sound between 1000 uF per rail and what you have now?
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Old 17th January 2007, 11:13 PM   #3
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Quote:
Originally posted by Nuuk
What is the difference in sound between 1000 uF per rail and what you have now?
I don't have any 1000uF's to try, so I don't know Supposedly, because it's regulated, massive capacitance before the regulators doesn't make the highs and things disappear so much? That's what I read anyway... By the chip I only have 100uF per rail.

That said, to test the new amp board I used 3x680uF per rail, with 13v rails. I haven't plugged speakers in to that though. It clips dead easy, and a square wave from my sound card (which isn't the best of square waves to start with admittedly) looks really quite evil! I'm hoping the square wave will look more respectable with more capacitance at the PSU though. It should do. I think.

Is there any particular reason why I might want to stay with separate capacitor 'banks' per channel, rather than just one big lot for both channels?
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Old 18th January 2007, 08:19 AM   #4
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Quote:
Is there any particular reason why I might want to stay with separate capacitor 'banks' per channel, rather than just one big lot for both channels?
That's down to whether you want a dual mono supply or not. I prefer dual mono myself.
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Old 18th January 2007, 09:47 AM   #5
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Quote:
Originally posted by Nuuk


That's down to whether you want a dual mono supply or not. I prefer dual mono myself.
Ok.

Another question!

I currently have it set up (found the picture I was looking for on your site, better than my description) like http://myweb.tiscali.co.uk/nuukspot/...su_options.gif the second one on there. Suppose I keep it like this, would I be able to hook up a third regulator to the same set of two bridge rectifiers, but this third regulator regulating to a lower voltage than for the power amps (as it's for a preamp)?

I tried, rather crudely, to simulate this in LTSpice / SWCAD III, but it seems like I can't do this, as the 0v from the bridge appears to "turn in to" the negative regulated rail? I'm confused again
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Old 18th January 2007, 10:20 AM   #6
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I hope this doesn't sound snobby but in my opinion we are talking hi-fi here. As such, I wouldn't run a pre-amp/filter supply from the same supply that is being modulated by the power amp.

A separate supply isn't that expensive and will pay dividends!
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Old 18th January 2007, 11:04 AM   #7
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Quote:
Originally posted by Nuuk
I hope this doesn't sound snobby but in my opinion we are talking hi-fi here. As such, I wouldn't run a pre-amp/filter supply from the same supply that is being modulated by the power amp.

A separate supply isn't that expensive and will pay dividends!

I actually have a smaller transformer, but having it powered off the power amp supply would mean.... less wiring mess and much more space. Looks like I might have to though.... I can't figure how I could possibly get it to run off the same supply.

For that matter, I'm now even confused as to running multiple regulators, regulating to the same voltage, off the same supply. Like in the diagram I linked to, the second configuration, is that correct (not trying to question your knowledge or anything, just confused!)? Paralleled from after the bridge rectifiers, like:

Click the image to open in full size.

Notice the two negative rails are joined? This is how I currently have it. Maybe this would explain why one of my regulators boards gets hotter than the other too? Wouldn't it be better to parallel before the bridge rectifiers, so that there's four bridge's instead of two? One set per regulator board? Or does it actually not make any difference?


EDIT: sorry, made the screenshot more readable.
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Old 18th January 2007, 12:06 PM   #8
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Please read this !
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Old 18th January 2007, 12:16 PM   #9
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Uh, that doesn't answer my question at all? Or I am just blind...?
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Old 18th January 2007, 12:31 PM   #10
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Quote:
Notice the two negative rails are joined?
Sorry, I thought that was your question!
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