LM3886 Power supply based on AN1849 - diyAudio
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Old 17th November 2013, 01:24 PM   #1
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Default LM3886 Power supply based on AN1849

This is about PSU for LM3886 Amplifier, based on National (now TI) Application Note AN1849, I checked the schematic, thinking of removing relay and related circuit to avoid Mains connection on PCB,
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Old 18th November 2013, 02:30 AM   #2
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Here is schematic and PCB Layout for same..
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Old 18th November 2013, 10:20 AM   #3
AndrewT is offline AndrewT  Scotland
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Your schematic is riddled with capacitors that you don't need. Worse, some of them could promote supply rail oscillation.
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Old 18th November 2013, 10:34 AM   #4
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Quote:
Originally Posted by AndrewT View Post
Your schematic is riddled with capacitors that you don't need. Worse, some of them could promote supply rail oscillation.
Thank you for pointing out, I will check and remove some caps.
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Old 18th November 2013, 11:02 AM   #5
FoMoCo is offline FoMoCo  United States
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I don't think any of the capacitors will induce oscillation, but it is a waste of money to have them there. The worst thing is that it shows a lack of understanding beyond reading an app note.
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Old 18th November 2013, 05:24 PM   #6
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Thank you AndrewT and FoMoCo ,
I will go through Application note, try to understand better, then remove caps which will not make any difference with or without it..
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Old 24th November 2013, 09:31 AM   #7
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I've read this app note, and the companion to it and I understand what they are trying to do with this supply. Section 3.1 of AN-1849 describes what the components do and it does explain that you can get away without some of them.

But...this is a reference design. NatSemi wanted to show off their product under the best conditions possible so they went hog wild.

The PSU is over the top but so is the reference design for the amp it's designed to power. Please see AN-1850 LME49830TB Ultra-High Fidelity High Power Amplifier Reference Design. THD+N for the LME49830TB at 10W, 1kHz is <0.0008% with sufficient bias. That's quiet for a class AB amp!

In short, it all comes down to determining the merit of installing $5 worth of caps for a bit less HF ripple on your rails...It's your call. Unless your amp is of similar design caliber to the one in AN-1850 you're unlikely to be able to measure let alone hear the difference.

I wouldn't be so quick to dismiss AN-1849....Bob Pease was still at NatSemi when that was first written and published. He must have seen it; he was in charge of their analog department. That man knew what he was doing; he was a legend in analog circuits.

Finally, these caps will not and cannot cause instability when placed in the power supply as shown in AN-1849. If anything, they'll increase stability by lowering the supply impedance at high frequencies. (the ESL of the large filter caps allows HF to pass by relatively unimpeded until it hits those smaller caps which eat it up)

Last edited by hardwareguy; 24th November 2013 at 09:55 AM.
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Old 24th November 2013, 09:41 AM   #8
AndrewT is offline AndrewT  Scotland
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C1, C2, C5 & C6 are likely to increase the risk of ringing in the PSU.
C5p, C5n, C8p and C8n are likely to increase the risk of ringing in the PSU.
That's 8 capacitors that should be removed.

The snubbers (C9p, R3p,C9n, R3n) at the output are in the wrong place.
Move one of them to across the rectifier ~~. discard the other.
If there are two separate secondaries, then use two snubbers, one across each secondary.
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