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Old 20th September 2003, 03:17 AM   #1
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Default LM3886 amplifier

I would like to tackle the challenge of building my own power amp for my speakers. My current one, a Kenwood, sounds decent, but doens't allow my high quality speakers to sound as good as they can (B&W series 500). So, I began looking for a way to build a new amp and came thought of using a bridged 3886. I know a single 3886 sounds excellent, so I would think that in brided mode, running about 200W, they would sound even better. I would like to get an opinion on how well this would work. I plan on using the schematic on page 1 of National semi's datasheet, and using the Simpelest ever bridging adapter at www.sound.westhost.com. I have created a graphic of the PCB, and it is attached. If you are wondering about the many extra pads, they are for jumpers. I would make 2 of these boards, as one board does each channel. Will this blow away the sound quality of the Kenwood 2 channel tumer, or will it not sound as good? Thanks for your opinion

-Mike
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Old 20th September 2003, 05:36 AM   #2
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Mike lots of info here on the site and great guys around who have built them.......jump in and get building because there is support here
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Old 20th September 2003, 02:32 PM   #3
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Yeah, when I searched, I found a lot about the 3886, but not much about it in bridge mode. I heard that they cannot drive 4-ohm in bridge. Is that true? Also, I didn't find much about the sound quality compared to a cheap commercial amp? Is there a noticable increase of quality, ie. volume before it distorts?

Thanks for your help, Mike

Also, does anybody know where I could find that paper that lets you print PCBs from a laser printer for the chapest? I choked when I saw DigiKeys price for only 5 sheets.
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Old 20th September 2003, 04:54 PM   #4
Mark Kravchenko
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Default 4 ohm Bridge LM3886

to supply enough current into 4 ohms you must create a bridged/parallel amp. I think it's explained well in the newest app note or in the first. To many application notes floating around in my head. Look and or ask some of the gurus on this site and you will get some answers.

Mark
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Old 20th September 2003, 05:27 PM   #5
paulb is offline paulb  Canada
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Quote:
Originally posted by soundNERD
Yeah, when I searched, I found a lot about the 3886, but not much about it in bridge mode. I heard that they cannot drive 4-ohm in bridge. Is that true? Also, I didn't find much about the sound quality compared to a cheap commercial amp? Is there a noticable increase of quality, ie. volume before it distorts?

Thanks for your help, Mike

Also, does anybody know where I could find that paper that lets you print PCBs from a laser printer for the chapest? I choked when I saw DigiKeys price for only 5 sheets.
Two LM3886s in bridge mode into a load are the same as each LM3886 driving half of the load. So bridging into a 4-ohm load would be the same as each amp driving a 2-ohm load, which is not recommended.
I can't say for sure on the sound quality compared to a cheap commercial amp (how cheap?) , but I would guess much better.
You should be aware that these amps apparently sound terrible if they are overdriven, anywhere near clipping. If you want lots of power, there are probably better choices; I find the whole parallel / bridging arrangement too complex and can see it causing you a lot of grief.
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Old 20th September 2003, 06:13 PM   #6
joensd is offline joensd  Germany
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Hi Mike,
as others already pointed out it doesn´t always make sense to bridge an amplifier.
You should read the AN-1192 application note from National Semiconductors to understand what it´s all about.
Have a look here as well for a calculation tool : Spreadsheet from National

I can´t seem to find infos about the B&W 500 series but usually B&W-speakers have efficiencies of 90dB+.
You probably are quite satisfied with single chip configuration.
Choose something like +-35V-rails for 8Ohm speakers and about +-28 for 4Ohm speakers.
If you´d choose to go single chip and wire everything p2p go for the LM3875 as you´d only need 5 pins to connect.

Cheers
Jens
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Old 21st September 2003, 01:31 PM   #7
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If I birdge them, won't I, for sure, get louder and crisper sound with much less distortion into 8-ohms? That was my understaning. Are there any chips that can deliver much more power, and still manage to sound very good? I would like to stick with the 3886, since I have already spent hours desiging the PCB layout on the computer, and I have heard that the sound is excellent. Somebody was mentioning about the datasheet. The datasheet I have has no circuits for bridge/parallel. Could anybody e-mail me the one that does? Thanks.

Also, about my speakers, the data is at www.bwspeakers.com, then search for DM580. There is a pdf file with specs.

Thanks, guys, for all of your help. I am rather new to electronics, so if I ask some dumb questions, please don't laugh or anything.
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Old 21st September 2003, 05:27 PM   #8
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I should think that the 3886 should have no problem with a 4 ohm load or it would be a very poor design indeed. In practice, speaker impedance can vary quite a bit over audible frequencies so most amps have been made with enough damping factor to deal with it.

Before the mono/stereo/bridge switch, bridging used to be done all the time in PA or SR installations using 2 amps, 2 interconnects and some solder. From your source, you would run a Y-split into 2 cables. One of the cables would be rewired so that polarity was reversed. The speaker pos would be connected to the amp pos with the "true" polarity and the speaker neg connected to the other amp pos terminal. Both amp's neg is NOT connected.

Unless the LMx8xx cannot handle the halved impedance, there should be no reason why they cannot be hard-wired in a bridged configuration.

My question then, is this: Can you "fool" the bridged amp output with an 8 ohm power resistor in series with the speaker.

:)ensen.
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Old 21st September 2003, 05:37 PM   #9
joensd is offline joensd  Germany
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Mike,
here´s the link for the AN-1192.
Go to the bottom of the page, there you can download the application note. It should clear things up.
With a bridged amplifier you´ll get more output power but having for example a 4Ohm speaker with it the amp would be current limited. Play a little with the calculation tool.

Quote:
My question then, is this: Can you "fool" the bridged amp output with an 8 ohm power resistor in series with the speaker.
If I don´t get wrong what you´re trying to do you can rather use a single chip (so not bridged) and "omit" the power resistor.
If you wanna heat up your room with the power resistor or keep your cup of tea warm it would be a solution...

Cheers
Jens
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Old 21st September 2003, 05:44 PM   #10
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GC User tool at Nat Semi

Just saw this interesting thread about NatSemi tools for LMx8xx chips.

:)ensen.
- who likes hot tea while listening to music... ;-P
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