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Old 27th February 2008, 08:27 AM   #1
Puffin is offline Puffin  United Kingdom
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Default PS Pin Capacitance LM3886

It seems that the PS pin capacitance is suggested to be between 1000 and 2200uf for a single 3886. I have built a bridged LM3886 and after re-reading the notes it seems that they suggest 4700uf for bridged.

Is the reccommendation an absolute i.e is there any detriment to using higher than 2200uf (or 4700). The reason I ask is that I have some good quality 10,000uf caps. I will give them a go and see how they sound.

I would however be intersted to know if the extra capacitance will just be wasted or whether it will have any other detrimental effect ?
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Old 27th February 2008, 08:35 AM   #2
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No problem, it will work better
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Old 27th February 2008, 09:33 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
when suggesting the smoothing capacitance requirement for a bridged amp I argued that bridged needed 4times as much capacitance as a single amp into the same load. Most came back and said "rubbish".
Try four or more times the local decoupling. it might help.
But remember the fast caps are the ones that go on the pins. The electrolytics can be a little further away.
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Old 27th February 2008, 09:53 AM   #4
Puffin is offline Puffin  United Kingdom
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jerome69 & AndrewT. Thanks for your responses. Andrew I am going to show my ignorance now and ask what denotes a "fast cap" ?
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Old 27th February 2008, 10:51 AM   #5
AndrewT is offline AndrewT  Scotland
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fast cap=low value polyfilm or ceramic or silvered mica.
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Old 27th February 2008, 10:54 AM   #6
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Quote:
Originally posted by AndrewT
Hi,
when suggesting the smoothing capacitance requirement for a bridged amp I argued that bridged needed 4times as much capacitance as a single amp into the same load.

I agree , It's simple math


1 Chipamp into 8 ohm = xAmps
1 Chipamp into 4 ohm = xAmps x 2

2 chipamps into 8ohms bridged(each chipamp "sees" 4ohms) =

xAmps x 4

So your current has multiplied by 4

C = 0.7(I)/ΔE(f)

where C = capacitance in farads, I = dc load current in amperes, ΔE = peak-to-peak ripple voltage, f = ripple frequency (generally 120 Hz for full-wave or 60 Hz for half-wave), and 0.7 is the complement of the rectifier-current duty cycle, which is assumed to be 0.3


http://electronicdesign.com/Articles...ArticleID=9809

IT all depends on the ripple voltage you are happy with at high loads (high volume)

You do have SOME play with music because you don't have a contstant current pull , but If you want to quote an RMS voltage these calculations are important.

am I overlooking something?
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Old 27th February 2008, 11:22 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by tangmonster
I agree , It's simple math
but we are in the chipamp section.
Nobody here wants to admit they learned anything at school.
In the car audio section, peer pressure alone forces them to claim they never went to school
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Old 3rd March 2008, 11:28 PM   #8
gootee is offline gootee  United States
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Quote:
Originally posted by AndrewT
but we are in the chipamp section.
Nobody here wants to admit they learned anything at school.
In the car audio section, peer pressure alone forces them to claim they never went to school
<GRIN!>
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The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
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Old 4th March 2008, 05:14 AM   #9
digi01 is offline digi01  China
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Quote:
Originally posted by AndrewT
fast cap=low value polyfilm or ceramic or silvered mica.

i have use the beacon cap of camera,350v/120uf.they work great.guys can try it
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Old 4th March 2008, 01:29 PM   #10
weissi is offline weissi  Europe
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without all those theoretical aspects aside (poor ripple rejection of LM3886 in the >500kHz region) just put 220F || ~100n caps for every chip you use. But them as near as you can to the PS pins, bulk capacitors can stay more far away then.
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