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Old 16th January 2004, 03:30 AM   #1
newbie1 is offline newbie1  United States
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Default LM3886 Power Supply Schematic?

Hi,

I am contemplating building a gainclone from the LM3886T chip. I have seen a lot of good schematics for the amp itself, but does anyone have any for the power supply? What about caps? I have seen people use 1000uF up to 10000uF any recommendations? I want to ultimately use this as a subwoofer amp. Does anyone have any opinions on how the the LM3886 would perform as a sub amp?

Thanks
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Old 16th January 2004, 06:23 AM   #2
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Have you read this:
AN-1192
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf
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Old 16th January 2004, 12:35 PM   #3
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The reason people use 1,000uf per rail is if they want poor bass responce, but excelent mids and highs. This is good if you have a subwoofer along with the speakers you drive with the 3886.

Otherwise, I reccommend a least 8,000uf.

Heres a pcb layout I designed. It can take any combination of up to 4 caps per rail:
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File Type: zip power supply mirror.zip (7.1 KB, 566 views)
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Old 16th January 2004, 01:16 PM   #4
newbie1 is offline newbie1  United States
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Quote:
Originally posted by crown300
What size sub? Type of enclosure?

for a 6.5" maybe one amp will do, but I prefer a bridge.
8" definately a bridge.
10" and up, not enough power.
Well, I was hoping to use a 4 ohm 12" sub. But I guess that's not going to happen huh? What are the bridging possibilities of the LM3886 for driving a 12" sub? If I am reading the docs from NS correctly there are configurations with multiple chips that can produce 200W, or is this a pipe dream?

Thanks.
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Old 16th January 2004, 02:39 PM   #5
macboy is offline macboy  Canada
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With a 4 ohm speaker, your options are a little bit more limited. You cannot simply use a bridged amp, since the effective load on each chip will be half the speaker impedance, or 2 ohms. That is simply too low if you expect to get high power output from the chips.

A much better option is to parallel two chips. This will make the effective load for each chip to be twice the speaker impedance, or 8 ohms. Now you just have to design the power supply according the to information for an 8 ohm load. The voltage of the supply is important; check out the graph "Output power vs. supply voltage" in the datasheet. Keep in mind that you have two amps, each effectively driving 8 ohms, so find the power output for an 8 ohm load and your power supply voltage, and then multiply by 2.

Another option is a bridged-parallel design. Both this and the parallel design are deailed in the National app note AN1192. Here, the effective load for each of the chips is the same as the speaker impedance, but you get around 4 times the output power as a single chip. Again, the voltage of the power supply is very important.

Don't forget when choosing a transformer that the final DC voltage after rectifying and filtering is 1.414 times as high as the AC voltage. This is because the DC voltage will be as high as the peak of the AC sinewave, but AC voltages are given as RMS values, which is like an average of the sine wave. So if you want +-35 Volts DC, you need a transformer with two 25 VAC secondaries, or a 50 VAC center tapped.
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