| Mattyo5 |
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! |
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| pinkmouse |
| quote: | Originally posted by Mattyo5
oops, posted this in the wrong spot, moderator, please move this if need be...thanks! |
Done! :) |
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| unitgain |
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. |
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| tlmadsen |
Hi Mattoy5
Have a look at this threads. You might pick up something usefull :)
http://www.diyaudio.com/forums/show...&threadid=19157
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:xeye:
Have fun
Thomas |
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| Mattyo5 |
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 |
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| Mattyo5 |
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 |
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| rampage101 |
| There's plenty of threads on the dc offset, but 20mv will be fine. |
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| Mattyo5 |
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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| Will |
Can the 4 x 22uf caps be omitted ?
If not, what's the best for the job ? Elco 50V non polarised ? |
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| jaudio |
| quote: | Originally posted by Will
Can the 4 x 22uf caps be omitted ? |
If you remove them,you could get more dc offset.
| quote: | Originally posted by Will
If not, what's the best for the job ? Elco 50V non polarised ? |
Use film caps for the input and electrolyic for the caps that are connected to ground. Polarized or non polarzed it is up to you. |
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| tiltedhalo |
| Do not remove the caps, they are the DC decoupling without them your DC offset will cause all outputs to push and pull. I run a 4 parrellel bridge amp. 20mv is to much DC offset, multiple that 20mv across 6 chips, a differance of 10 per chip can cause output differance upwards of an amp per chip then 6 chips 6 amps at 35+- 210 watts of waste.:mad: set all chips in a non inverted formatt. 22.1K .1% 1/4 watts percision resistor soldered directly to IC pins run to a 47uf 10% tantalum capacitor then a 1K ohm .1% 1/4 watt to ground. Make sure all pc board set in parrellel are identical also run a cover ground plane and bypass each chip +/- inputs with a .1uf poly at pins. Use a DRV134 line driver to bridge this allows all parrellel pairs to be identical, this should give you a reliable 200 to 240 watts at about .05% with about 100watts idle= or -..... |
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| neutron7 |
If you make all of the amplifiers the same (inverted or non) and then drive half f them with DRV134 then you can:
1: match all the resistors more accurately.
2: group 2 sets of 3 amps with the closest DC ofset for each side of the bridge.
3: If you decide you do not like it, then you can take the DRV off and the 6 amplifiers will all be in phase. and you could use it for lots of configurations
bpa 200 + 2x single lm3886
3x parrallel lm3886
2x parrallel + 2x single
6x single
etc etc.
"swiss army amp" |
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