Modded BPA-200

Mattyo5 · 16th May 2004, 01:50 PM

Ok guys, i think i'm just gonna go for it here. I want to use 6 lm3886 chips in a bridge parallel design for a sub amp. So basically i'm gonna follow BPA-200 w/ 3 chips in parallel instead of 2 for a total of 6 bridge parallel. I noticed in the bpa-200 schematic that one parallel set of chips uses 20.5k resistors in the feedback and 21.5k for the other set of parallel chips. I assume that I do need to follow this. What if i just use 10k inputs and 220k feedback instead for both sets? Anyone have a more complete schematic of the opamp input stage? If this for some reason doesn't work, i'll just turn it into a 6 channel lm3886 amp...but I'd like to try the bridge parallel design first...for the heck of it. Besides adequate power supply, .1% .1ohm output resistors, adequate heatsinking, any other advice? Thanks!

-Matthew K. Olson

oops, posted this in the wrong spot, moderator, please move this if need be...thanks!

pinkmouse · 16th May 2004, 01:58 PM

Quote:

Originally posted by Mattyo5
oops, posted this in the wrong spot, moderator, please move this if need be...thanks!

Done!

unitgain · 16th May 2004, 06:13 PM

It has to do with inverted/non-inverted topology gain. In one case it's Rf/Ri, in the other case it's Rf/Ri + 1.

Why would you want to use higher than recommended nominals? More resistance => more noise.

tlmadsen · 16th May 2004, 08:18 PM

Hi Mattoy5

Have a look at this threads. You might pick up something usefull

200 watts Gäjnklon (Gainclone) - BPA200

I agre with "unitgain". Why do you want to make the gainresistors bigger ?? It will give you more noise and more DC-ofset. Something you DON't need when you are paralleling chips.
(make sure you take care of the DC-ofset some how...)

You don't have to use the same resistors as in the BPA-200, but you most make sure that both your non-invertes and your inverted section have the same gain as explained by "unitgain" otherwise your "bridging" will be running "strange" to say the least

Have fun

Thomas

Mattyo5 · 16th May 2004, 10:36 PM

Well, i'll order some chips, and use the same resistors as the BPA-200 recommends, just more chips

Same opamp too I guess. I guess I can check dcoffset, before I actually tie the chips together. Keep those ideas comin! Thanks guys!

-Matthew K. Olson

Mattyo5 · 18th May 2004, 02:39 AM

What is an acceptable dc offset ...ie what can I live with per chip ? Obviously zero would be best, but would 20mV be ok per chip? Or do I need to use output couple caps or something ridiculous? I'm going to hand match the feedback and input resistors, and I'm not going to bother w/ the dc servo's, but i'm just wondering what an acceptable dc offset is per chip before I tie them together. Thanks!

-Matthew K. Olson

rampage101 · 18th May 2004, 03:28 AM

There's plenty of threads on the dc offset, but 20mv will be fine.

tlmadsen · 18th May 2004, 07:30 AM

I am sorry, but 20 mV DC off-set is way too much for paralled chips, unless you make the output resistors bigger.

See my replay in

Can you tell me how to beef up PA100?

Have fun

THomas

Mattyo5 · 18th May 2004, 03:00 PM

What you said in your other post is that if there is a differential of basically >20mV, then that can be trouble. But, if each of the chips are have +20mV offset (say 16mV, 22mV and 25mV), then the differential between chips would be minimal and they would have minimal current flowing between chips. I have a 6 channel lm3875 amp that is pretty darn close to that, so I doubt getting offset that low is difficult if I handmatch all the resistors. Thanks for the thoughts though guys. Helps

-Matthew K. Olson

Will · 30th November 2005, 07:50 AM

Can the 4 x 22uf caps be omitted ?
If not, what's the best for the job ? Elco 50V non polarised ?

16th May 2004, 01:50 PM	#1
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	Modded BPA-200 Ok guys, i think i'm just gonna go for it here. I want to use 6 lm3886 chips in a bridge parallel design for a sub amp. So basically i'm gonna follow BPA-200 w/ 3 chips in parallel instead of 2 for a total of 6 bridge parallel. I noticed in the bpa-200 schematic that one parallel set of chips uses 20.5k resistors in the feedback and 21.5k for the other set of parallel chips. I assume that I do need to follow this. What if i just use 10k inputs and 220k feedback instead for both sets? Anyone have a more complete schematic of the opamp input stage? If this for some reason doesn't work, i'll just turn it into a 6 channel lm3886 amp...but I'd like to try the bridge parallel design first...for the heck of it. Besides adequate power supply, .1% .1ohm output resistors, adequate heatsinking, any other advice? Thanks! -Matthew K. Olson oops, posted this in the wrong spot, moderator, please move this if need be...thanks!

16th May 2004, 06:13 PM	#3
unitgain diyAudio Member Join Date: Jan 2004 Location: northern israel	20.5k/21.5k It has to do with inverted/non-inverted topology gain. In one case it's Rf/Ri, in the other case it's Rf/Ri + 1. Why would you want to use higher than recommended nominals? More resistance => more noise.

18th May 2004, 03:28 AM	#7
rampage101 diyAudio Member Join Date: Apr 2004 Location: NSW Australia	There's plenty of threads on the dc offset, but 20mv will be fine. __________________ What's a signiture?

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16th May 2004, 08:18 PM	#4
tlmadsen diyAudio Member Join Date: Aug 2003 Location: Roskilde - The vikingships	Hi Mattoy5 Have a look at this threads. You might pick up something usefull 200 watts Gäjnklon (Gainclone) - BPA200 I agre with "unitgain". Why do you want to make the gainresistors bigger ?? It will give you more noise and more DC-ofset. Something you DON't need when you are paralleling chips. (make sure you take care of the DC-ofset some how...) You don't have to use the same resistors as in the BPA-200, but you most make sure that both your non-invertes and your inverted section have the same gain as explained by "unitgain" otherwise your "bridging" will be running "strange" to say the least Have fun Thomas

16th May 2004, 10:36 PM	#5
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	Well, i'll order some chips, and use the same resistors as the BPA-200 recommends, just more chips Same opamp too I guess. I guess I can check dcoffset, before I actually tie the chips together. Keep those ideas comin! Thanks guys! -Matthew K. Olson

18th May 2004, 02:39 AM	#6
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	What is an acceptable dc offset ...ie what can I live with per chip ? Obviously zero would be best, but would 20mV be ok per chip? Or do I need to use output couple caps or something ridiculous? I'm going to hand match the feedback and input resistors, and I'm not going to bother w/ the dc servo's, but i'm just wondering what an acceptable dc offset is per chip before I tie them together. Thanks! -Matthew K. Olson

18th May 2004, 07:30 AM	#8
tlmadsen diyAudio Member Join Date: Aug 2003 Location: Roskilde - The vikingships	I am sorry, but 20 mV DC off-set is way too much for paralled chips, unless you make the output resistors bigger. See my replay in Can you tell me how to beef up PA100? Have fun THomas

18th May 2004, 03:00 PM	#9
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	What you said in your other post is that if there is a differential of basically >20mV, then that can be trouble. But, if each of the chips are have +20mV offset (say 16mV, 22mV and 25mV), then the differential between chips would be minimal and they would have minimal current flowing between chips. I have a 6 channel lm3875 amp that is pretty darn close to that, so I doubt getting offset that low is difficult if I handmatch all the resistors. Thanks for the thoughts though guys. Helps -Matthew K. Olson

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