Increase the output swing of LM3875,possible???

Leolabs · 15th August 2007, 01:57 AM

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???

SpittinLLama · 15th August 2007, 04:47 PM

I'm not sure I understand what you want to do. Can you post a schematic or explain more (or both)?

-SL

robo7 · 15th August 2007, 05:04 PM

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.

pinkmouse · 15th August 2007, 07:43 PM

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.

Panelhead · 15th August 2007, 08:03 PM

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.

George

gootee · 15th August 2007, 08:22 PM

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

robo7 · 16th August 2007, 12:15 AM

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.

SpittinLLama · 16th August 2007, 05:21 PM

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

Leolabs · 20th August 2007, 02:50 AM

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.

15th August 2007, 01:57 AM	#1
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	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???

15th August 2007, 05:04 PM	#3
robo7 diyAudio Member Join Date: Dec 2003 Location: Kir-on	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.

15th August 2007, 07:43 PM	#4
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

15th August 2007, 08:03 PM	#5
Panelhead diyAudio Member Join Date: Dec 2002 Location: Houston	Commercial amps 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. George __________________ Cheapest is bestest

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

15th August 2007, 04:47 PM	#2
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	I'm not sure I understand what you want to do. Can you post a schematic or explain more (or both)? -SL

16th August 2007, 12:15 AM	#7
robo7 diyAudio Member Join Date: Dec 2003 Location: Kir-on	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.

16th August 2007, 05:21 PM	#8
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	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

20th August 2007, 02:50 AM	#9
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Tested output swing on small chips	johnr66	Chip Amps	0	16th March 2009 12:31 AM
LM4702:increase the output current,possible??	Leolabs	Chip Amps	3	14th January 2008 04:11 PM
Can this set up increase the output power of LM1875 at 4 ohms??	Leolabs	Chip Amps	7	25th November 2007 11:29 AM
how to increase output power on a 400watt amp	db junkie	Solid State	14	11th October 2005 07:20 AM
Need to get the highest voltage swing, which output topology?	EWorkshop1708	Solid State	7	2nd March 2005 06:56 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read