Increase the output swing of LM3875,possible??? - diyAudio
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Old 15th August 2007, 01:57 AM   #1
Leolabs is offline Leolabs  Malaysia
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Arrow Increase the output swing of LM3875,possible???

The LM3875 is limited to +/-42V,wonder is it able to increase the output voltage swing by employing 'bootstrap' topology which applied to normal OPAMP???
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Old 15th August 2007, 04:47 PM   #2
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I'm not sure I understand what you want to do. Can you post a schematic or explain more (or both)?

-SL
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Old 15th August 2007, 05:04 PM   #3
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Default Hi Leolabs

You may increase output voltage using "bootstrap",
what you may not increase is the current of the IC which will be
limited.
With load of 8 ohm or higher it may work.
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Old 15th August 2007, 07:43 PM   #4
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Ages ago, someone came up with a Quad style "Current Dumping" output stage for a chipamp. I forget who, but a search should find it.
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Old 15th August 2007, 08:03 PM   #5
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I read in a British mag about an amp that has IC output at low levels and goes to a discrete "booster" stage at higher demand. This is close to what you are asking for. Review said it sounded better at low power.
But I cannot see where more than say 40 volt rms swing would ever be needed. Current delivery is more of an issue with a LM3875 with most speakers. But there are always special cases.

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Old 15th August 2007, 08:22 PM   #6
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Default Re: Increase the output swing of LM3875,possible???

Quote:
Originally posted by Leolabs
The LM3875 is limited to +/-42V,wonder is it able to increase the output voltage swing by employing 'bootstrap' topology which applied to normal OPAMP???
I'm not sure what you mean by "bootstrap topology".

If using a bridged configuration (doubling the voltage swing across a floating load) isn't what you need, you can easily boost the output voltage and/or current, by following the chipamp with a discrete "booster" stage, with feedback from the booster stage's output back to the negative input of the chipamp. (But, if you were going to do that, then maybe you might as well use a high-quality opamp instead of the chipamp, in many/most cases.)

Here are some application notes that have circuits that can boost the output voltage and/or current of any monolithic voltage amplifier, such as an opamp or chipamp:

AN-272 at http://www.national.com

AN18 at http://www.linear.com

And here's one that might also be of interest:

AN14 at http://www.apexmicrotech.com

- Tom Gootee

http://www.fullnet.com/~tomg/index.html
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Old 16th August 2007, 12:15 AM   #7
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I belive Leolabs refers to this circuit:
http://www.edn.com/articles/pdfs/EDN...13/07m4591.pdf as OP-Amp bootstrap.

this techniqe employed in PHONIC Max-250 amp, for example,
is known as class H.
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Old 16th August 2007, 05:21 PM   #8
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The LM3875 can do that as long as the difference in voltage between the two power pins (Vcc/Vee) does not exceed the ratings. So if the +rail goes up then the -rail will have to decrease to be sure the total is never more than the 84V in the datasheet. The question would then be, how good does it perform? The LM3875 doesn't have the greatest PSRR on the negative rail but it would be fun to test and see the results.

-SL
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Old 20th August 2007, 02:50 AM   #9
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Thank you guys for your replies.Dont care about the output current of the LM3875,just want to turn it into a high voltage OPAMP/driver.
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