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Old 21st January 2005, 05:53 PM   #1
breez is offline breez  Finland
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Default LM3886 / LM3875 supply voltage considerations

It seems to be the consensus here that with 4 Ohm loads one should use +/- 25V or so supply with LM3875. If I look at the datasheet (LM3875) and "Output Power vs Supply Voltage" graph I see that at +/- 30V the output power has dropped dramatically to 4 Ohm load. Can someone explain why this happens?

What about LM3886? The same graph for this chip doesn't show the drop in output power, but the graph doesn't extend beyond +/- 30V for 4 Ohm load anyway. Would there be a similar drop in output power with eg +/- 35V supply?

I have 8 Ohm speakers, but of course I might have some 4 Ohm ones in the future so I'd like to build my amp with them in mind while still having respectable wattage to 8 Ohm loads. Probably going to use the LM3886 in any case (LM3876 ie. LM3875 with mute is also available here though).
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Old 22nd January 2005, 01:22 AM   #2
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LM3875 can't supply enough current to drive 4ohm speakers well, especially at higher power.

The stereo amp I built using 2 3875 chips is amazing for the price I paid, easily comparable to amps costing many times as much money as this cost, but only when driving an 8 ohm speaker. I hooked up a 4-ohm woofer to it, and at low volumes, SPIKE would kick in and cause a loud popping sound from the speaker.

I'm running at a +/-36VDC supply, which doesn't help, but in any case, LM3875 isn't a good choice for any 4-ohm unless you parallel 2 chips.

If your speakers are 8 ohm you should definately go for an LM3875, because I find the sound far superior to a 3886 amp.

And, you don't want the mute feature. It's a pain to work with and also is first more things to go wrong, and second, adds more to the amp reducing the quality. In fact, when using an amp without a mute, except for the filtering caps, you can build it with only resistors, rather than capacitors also.

Finally, since you want 4-ohm ability, you could always use 2 chips per channel and parallel them.
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Old 22nd January 2005, 01:39 AM   #3
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why guess -- go to the National Semi website, the Overture Series design tool --

you'd be surprised what you can do with any of these chips with a little moving air.
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Old 22nd January 2005, 01:39 AM   #4
Stocker is offline Stocker  United States
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The short answer is the same as the long answer: too much POWER, captain! a higher rail voltage into a lower impedance load, equals more current. Higher voltage, higher current, higher power yes?

The chips can only put out so much POWER, period.

Turn the 4-ohm speakers down low or have a separate power scheme... or avoid them.
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Old 22nd January 2005, 08:16 AM   #5
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Quote:
Originally posted by jackinnj
why guess -- go to the National Semi website, the Overture Series design tool --

you'd be surprised what you can do with any of these chips with a little moving air.
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf

Hey, guys, you must talk decibells also. 30% powerloss is 3 dB hardly audiable! 10 times the power only 10 dB more!
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Old 22nd January 2005, 03:08 PM   #6
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PerAnders == National Semi has upgraded to series 15 of the interactive design tool -- the new version has the LM4780 -- and there is a second sheet with some interesting power data graphs.
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Old 22nd January 2005, 05:00 PM   #7
homer09 is offline homer09  Canada
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Quote:
Originally posted by soundNERD
If your speakers are 8 ohm you should definately go for an LM3875, because I find the sound far superior to a 3886 amp.

And, you don't want the mute feature. It's a pain to work with and also is first more things to go wrong, and second, adds more to the amp reducing the quality. In fact, when using an amp without a mute, except for the filtering caps, you can build it with only resistors, rather than capacitors also.

Is this statement about the 3875 being far superior based on some direct comparison (ie. identical amps differring only in the chip?)
i'm presently trying to chose between both chips, to build a non-inverting amp. I would like to know in what implimentations you find the 3875 outperforms the 3886 and what you notice.

isnt the mute feature completely independent to the signal path?
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Old 22nd January 2005, 05:22 PM   #8
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I used the same transformer (+/-36VDC) and the same speakers (B&W DM580 I think). No, I didn't try them at the same time, but when I listened to the 3886 I remember hearing hissing, there was a turn on/off thump, and I noticed higher distortion at lower volume. The sound, although much better than the kenwood 100x2 amp I had in there at the time, didn't seem that great. It seemed to be weak with bass, and too much treble.

When I tried my 3875 stereo amp, I was unable to do a direct comparason because I had already used the 3886 amp boards in something else I've built. But I instantly noticed that the bass was perfect with the music, not too loud or boomy its annoying, but not too weak either. And, the high and mid frequencies sounded very good.

There were only slight differences with the amps. the 3886 amps were built on a PCB, while the 3875 was assembled P2P. And, except for the mute, the circuits were almost identical. They are both built using the schematic from the datasheet. The only difference was I didn't use the capacitor *Ci on the LM3875 amp.

I believe the mute circuit is in the signal path somehow, otherwise I don't know how it would mute it.

This is just my opinion, so I would see what others think before choosing one. Or, since the chips are so cheap, you can just build them both and see what you prefer.
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