First Build (Stereo LM3886) in need of psu advice - Page 2 - diyAudio
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Old 25th January 2013, 03:35 AM   #11
gootee is offline gootee  United States
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Using 10 caps per rail would be fine, I'm sure. I am going to use 100 per rail, with two proper arrays on a 2-sided pcb, mostly as described in the links, except using two halves of one PCB. (But I am pretty far out there, sometimes. I mainly just want to measure the results. It might be way into overkill but I want to find out.) I am also going to use the LM3886 board as a sort of daughterboard, just above where the two cap array power planes almost touch, so that the power and ground pins for everything can go straight down into the power and ground planes of the cap arrays, which should only be about 3/16ths of an inch away, or maybe 1/4 inch (about 4.76 to 6.35 mm).

And without any onboard caps, the 3886 pcb can be made to be pretty small. I'm still figuring out how to mount everything, with heatsinks, while still getting good current diffusion through the cap arrays and lowest impedance at the chip pins, etc etc.

Yeah, it sounds you have the right ideas about the caps. But you also have to consider the high-frequency needs, which probably weren't covered in my linked post about calculating the needed cap values. Look at this one:
paralleling film caps with electrolytic caps

And most of the rest are linked from here:
Power Supply Resevoir Size

It turns out that you need HF currents, too. When quantified, it enables calculation of a maximum inductance that can be tolerated, as seen by the chip's power and ground. That usually equates to a maximum LENGTH of conductors that can be used, from tghe caps rto the pins, and/or how many of what value caps to parallel.

You could probably just add a few smaller caps near the pins, actually; maybe three 68 uF per power pin. But the layout is more difficult than it seems. See links.

Cheers,

Tom
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Old 25th January 2013, 04:27 AM   #12
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If its really your first build and you've done nothing simpler before, then consider going for a much simpler chipamp like TDA8561. Yes its lower power but boy is it easier to get going The decoupling is much easier with small caps nearby the pins because those smart boys at Philips knew what they were doing and put the power pins right next to each other on the package. Thirdly its bridged, so no tedious power supply grounding issues - it runs on a single supply (I'm using a laptop SMPSU). A fourth point is the pin spacing fits straight in to 0.1" prototyping board so you don't need to get a PCB specially made for it.

My final point to recommend it is the awesome sound - miles ahead of LM3886 when done right - meaning with the power supply really low impedance like gootee is suggesting.
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Old 28th January 2013, 08:05 PM   #13
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Thanks for the links ill look into them more. As for the TDA8561 i already ordered the LM3886 but my dad wants also want to build an amp too so ill look into that for him. As for the 2.2k resistor in the link of my first post does it do anything besides constantly draw power should i include it or not?
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Old 28th January 2013, 08:17 PM   #14
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The 2.2K resistors are "bleed" resistors. They will discharge the smoothing capacitors once you turn the power off.

I don't know what your supply voltage is, but if it were say 30V then they would only be using around 13mA of current.

Tony.
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Old 28th January 2013, 11:40 PM   #15
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Originally Posted by wintermute View Post
The 2.2K resistors are "bleed" resistors.
Tony.
Alright thanks for the quick reply that makes sense I will be sure to include them.
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Old 5th February 2013, 11:52 PM   #16
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Last question hopefully. I have made changes in caps thanks to peoples advice but the psu is still like the link in my first post: two bridges, with two separate rails one with +PGND and the other with -PGND at the end and what looks like a connector between the +PGND and -PGND. What I am confused about is how to wire it. Do I connect +PGND, -PGND, the purple wire of the transformer (link to is also in first post), and the chassis all directly to earth (3rd round prong in the US)?
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Old 6th February 2013, 12:13 AM   #17
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Quote:
Originally Posted by Megadeth View Post
What I am confused about is how to wire it. Do I connect +PGND, -PGND, the purple wire of the transformer (link to is also in first post), and the chassis all directly to earth (3rd round prong in the US)?

Now I am confused, "connect +PGND, -PGND, the purple wire of the transformer ...and the chassis all directly to earth"

You can EARTH, yes. Connect chassis to AC third prong ONLY. Make darn sure it is a REAL Earth! Yes, you can buy a cheap tester at your local DIY shop.
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Old 6th February 2013, 12:50 AM   #18
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Now I am confused, "connect +PGND, -PGND, the purple wire of the transformer ...and the chassis all directly to earth"
Are you saying I was correct and should connect all four of them to earth? The case and the purple wire which is a shield go to earth that much I know. Its the +PGND and -PGND that confuse me do i connect them to the earth ( 3rd round prong in the US) or are they something entirely different?
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Old 6th February 2013, 12:56 AM   #19
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Wired with two rectifiers as per the diagram in your first post http://www.chinabaike.com/uploads/al...204G3L91-0.png , NONE of the transformer secondary wires should be connected to your chassis earth! Yes you should connect the +PGND and -PGND to the chassis earth (otherwise known as safefty earth). This point is your zero volts reference.

Ah I see now. The purple wire on the transformer is a static sheild, in this case yes I would say it is fine to connect it back to the Chassis earth point.

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Old 6th February 2013, 01:11 AM   #20
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Great I already have one rail done can't wait till its done. Thanks to every one who posted on this thread you help make it happen.
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