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Old 21st January 2013, 11:02 PM   #1
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Default First Build (Stereo LM3886) in need of psu advice

Hello, I have been viewing this forum for a few weeks but this is my first post. I find that people here are very helpful and have found answers to many of my questions but i have a few left. I'm building a gainclone using two LM3886 (one per ch.) @8ohms with an AS-3224 transformer http://www.antekinc.com/pdf/AS-3224.pdf like this http://www.chinabaike.com/uploads/al...204G3L91-0.png but with 10uF film caps not .1uf as one user recommended also I may use more smaller main caps in series in place of one big cap per rail because several people have recommended so. My questions are: 1. I plan to place fuse(s) somewhere i know it should be slo-blo but where should that somewhere be and what current should the ratting be? 2. I might go with 2x 500ish uF per rail is this good? i plan to push this amp to the max from time to time if that means i need more uF so be it 3. What brand of caps and resistors? Im at loss with all the options and im kinda on a buget so something middle of the road or less would be nice but i still what it to sound good 4. Any good websites to order from? I live in the USA Thanks in advance for any feedback.
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Old 22nd January 2013, 12:04 AM   #2
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Looks like you have your bases covered OK here.
The fuse (which is a slo-blow) belongs on the primary side of your TX, along with the switch (DPDT) . Yes you can parallel out a pair of 500uF caps instead of one big 1000uF cap.......fact is you can pile on any values trying to get to 1000uF....in parallel of course. The 0.1uF are there to take out any remaining high frequency switching noise. You could get by just fine with some small caps across each diode(snubbers) to quiet down things.....and then you could delete the second set of 0.1uF caps.
Have you run your proposed PS thru any software? I would recommend "Duncan amps" free PS program....so you can simulate the performance.....and see if your design is up to snuff. Be very generous with voltages & amps......If you caps will see only 24V....use one rated for 63V.....If you diodes will see 2A....get one rated twice or more. Factory designs will run components right to the edge.......as a DIYer we all should be generous......besides, it's just pennies more to "overbuild".

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Old 22nd January 2013, 05:57 PM   #3
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Thanks Richard for your input. So i should go with .1uF not 10 uF as another user said? I don't really want to get into snubbers as im trying to keep it basic at least for now. Im going with four 560uf main caps which of these Capacitors | Aluminum Capacitors | DigiKey would be the best for me (other than the ones that have a min. quantity in the 100's) longer life would be nice but not a must.
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Old 22nd January 2013, 07:55 PM   #4
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Quote:
Originally Posted by Richard Ellis View Post
The 0.1uF are there to take out any remaining high frequency switching noise.
ECQ-P1H104FZW Panasonic Electronic Components | P3872-ND | DigiKey sorry i gave the wrong link would this fit the bill? on the specs. it says "Features High Frequency and High Stability" that what what made it stand out to me

Last edited by Megadeth; 22nd January 2013 at 08:00 PM. Reason: wrong link
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Old 22nd January 2013, 10:57 PM   #5
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Hi megadeth,

If i were you I would use both a 0.1uF and 10uF in parallel, this will help to smooth out a lot more noise than just a single cap. I would double what rick says about 'overbuilding', the one massive advantage of DIY'ing is you can build to much higher specs at 10% of the cost. A few extra capacitors will cost you pennies and go a long way.

Also note that it was pointed out these should be in parallel, capacitors in series are an exercise in futility.
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Old 22nd January 2013, 11:38 PM   #6
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Originally Posted by nannoo View Post
Hi megadeth,

If i were you I would use both a 0.1uF and 10uF in parallel, this will help to smooth out a lot more noise than just a single cap. I would double what rick says about 'overbuilding', the one massive advantage of DIY'ing is you can build to much higher specs at 10% of the cost. A few extra capacitors will cost you pennies and go a long way.

Also note that it was pointed out these should be in parallel, capacitors in series are an exercise in futility.
Thanks for posting. Yeah nannoo your right lol I got my wording backwards parallel not series. So going in order from wall to amp (left to right in the schematic im basing it on from my first post) trans, MRU860's, 2.2kOhm, 560uF, 560uF, 10uF, 1Ohm+.1uF. Is this the right order? If anyone would do anything different or if anyone feels like this is correct feel free to say so.
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Old 23rd January 2013, 06:42 AM   #7
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oh, just to clarify, I didn't mean double the values, just that i agreed with him 100%
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Old 24th January 2013, 04:29 AM   #8
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Quote:
Originally Posted by Megadeth View Post
Thanks for posting. Yeah nannoo your right lol I got my wording backwards parallel not series. So going in order from wall to amp (left to right in the schematic im basing it on from my first post) trans, MRU860's, 2.2kOhm, 560uF, 560uF, 10uF, 1Ohm+.1uF. Is this the right order? If anyone would do anything different or if anyone feels like this is correct feel free to say so.
Your schematic showed 10000uF, not 1000uF. But I would recommend more, probably at least 22000uF per rail. And I would recommend many more but smaller caps in parallel. Use at least ten caps per rail, to get lower inductance and ESR, from paralleling. Use any brand. Nichicon are perfectly fine, for example (actually, they are quite good). Check mouser.com, too, not just digikey.

The music signal that makes the sound that comes out of the speakers is CURRENT that comes DIRECTLY from the capacitors. See the image at: Power Supply Resevoir Size

Use cap-bank PCBs, if possible:

Power Supply Resevoir Size
Power Supply Resevoir Size
Power Supply Resevoir Size
Power Supply Resevoir Size

If you can, just put all of the caps as close as possible to the chipamp power pins. If you can't, then at least pack a bunch of smaller caps around the pins, like ten 220 uF or more. The ONLY thing that really matters is what impedance is seen BY THE POWER PINS. The output impedance of the power supply means nothing, unless the output is right AT the power pins of the chipamp.

Decoupling cap calculations: Power Supply Resevoir Size

By the way, those small-value caps on your schematic will do no good at all, and could do some harm (forming resonant LC tanks with stray inductances), unless they are within less than about 2 mm of the chipamp power pins.

A resistor, with an optional small cap in series, COULD do some good, as a snubber to damp the spiking and/or ringing from the diode turn-off. But that would need to be across the transformer secondary, or the bridge input terminals (or across each diode). But there would be NO possible way to know, yet, what the resistance or the capacitor value should be. It could be done like this: paralleling film caps with electrolytic caps

Last edited by gootee; 24th January 2013 at 04:57 AM.
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Old 24th January 2013, 06:47 PM   #9
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Thanks your post was very informative. So your recommending something like 20x UKW1J222MHD Nichicon | Mouser 10 per rail? From what I understand the caps supply the current to the speakers which varies depending on the signal in. This is best achived by having a large enough bank so that it never runs empty under high demand. More small caps in parallel offer access to more current per given time due to lower impedence because what good is a large bank that can't be easily accessed. I was planing on housing the psu in a separate but close by case. the only components that should be in the psu case are the transformer, 8x Digi-Key Part Search to convert to DC and the 20 caps linked above. What about the 2.2k resistor does it serve a purpose?
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Old 24th January 2013, 06:56 PM   #10
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Please do not quote the entire post just above yours. I have fixed it. tnx
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