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Old 9th January 2015, 04:20 PM   #1
mravlca is offline mravlca  Slovenia
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Default LM3886 with SMD components

Just wanted to share with you my LM3886 stereo PCB with mainly 1206 smd size components. Some components have Farnell ordering codes in schematic for easier sourcing.

Best Regards,
Aleš
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File Type: jpg LM3886_V2_schematic.jpg (64.0 KB, 674 views)
File Type: jpg LM3886_V2_PCB.jpg (145.3 KB, 684 views)
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Old 9th January 2015, 10:35 PM   #2
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Nice work. Thats interesting layout.
But i see a few problems:
Aren't feedback resistances too low? Thats basically 1K from output to gnd. 10s of miliamps will be flowing through them.
100nf decoupling caps are too far from LMs (C5 & C8).
There is no Mute cap. Danger of pops when turned on.
I would route C7&C6 with separate traces to star point. But this one probably makes little difference.

Why electrolytic cap on input? R2 and R8 can be easilly increased to 47K, and then small 0.68-1uF film caps on input will be enough.

Last edited by valtra103; 9th January 2015 at 10:42 PM.
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Old 10th January 2015, 03:18 AM   #3
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you need caps for both the rails for both the chips.
fb resistor r5 is too far.
rf filter should be close to input socket.
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Old 10th January 2015, 12:15 PM   #4
mravlca is offline mravlca  Slovenia
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Quote:
Originally Posted by valtra103 View Post
Nice work. Thats interesting layout.
Thank you!

Quote:
Originally Posted by valtra103 View Post
But i see a few problems:
Aren't feedback resistances too low? Thats basically 1K from output to gnd. 10s of miliamps will be flowing through them.
If you try it you'll notice that there is nothing wrong with it and the sound of chipamp will vastly improve.

Quote:
Originally Posted by valtra103 View Post
100nf decoupling caps are too far from LMs (C5 & C8).
There is no Mute cap. Danger of pops when turned on.
I would route C7&C6 with separate traces to star point. But this one probably makes little difference.
You're right about C5 and C8 but this was the best option to use only two for both of the LMs. You're also right about routing GND from C5, C6, C7 and C8 to star point, I didn't see it before.
I tested the PCB and I didn't have any turn on/off pops.

Quote:
Originally Posted by valtra103 View Post
Why electrolytic cap on input? R2 and R8 can be easilly increased to 47K, and then small 0.68-1uF film caps on input will be enough.
It is tantalum 10uF/16V capaitor in paralel with 10nF MKP1837 capacitor. I found this combination more neutral than any other foil capacitor and very good sounding.

Quote:
Originally Posted by doors666 View Post
you need caps for both the rails for both the chips.
Why?

Quote:
Originally Posted by doors666 View Post
fb resistor r5 is too far.
You're absolutely right, I should have placed it next to R3 and R9. Trace would be then as short as possible. Still It can be solved with through hole resistor soldered directly to the pins of LMs.
Quote:
Originally Posted by doors666 View Post
rf filter should be close to input socket.
How would this improve the sound quality?

Best Regards,
Aleš

Last edited by mravlca; 10th January 2015 at 12:30 PM.
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Old 10th January 2015, 12:27 PM   #5
Mihkus is offline Mihkus  Estonia
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Not good... Not good at all... Missing HF filters on supply rails. There's more than that tho...
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Old 10th January 2015, 12:36 PM   #6
mravlca is offline mravlca  Slovenia
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Originally Posted by Mihkus View Post
Not good... Not good at all... Missing HF filters on supply rails. There's more than that tho...
Please elaborate why is it not good at all.
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Old 10th January 2015, 01:59 PM   #7
Mihkus is offline Mihkus  Estonia
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I couldn't call it a good design because:
Poor grounding, think and long supply rails, no HF filters on both chips, signal layout should be as compact and as closely together as possible and as far away from output and supply rails as possible and missing mute cap. I'm not an expert on this but just second opinion. Which is all true

BTW, have you tried your PCB's yet? I wouldnt risk. Chips wont blow up but they will oscillate for sure.
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Old 10th January 2015, 02:06 PM   #8
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Quote:
Originally Posted by doors666 View Post
you need caps for both the rails for both the chips.
Quote:
Originally Posted by mravlca View Post
Why?
Because the way you have done it now, you get very poor channel separation.
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Old 10th January 2015, 02:17 PM   #9
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Quote:
Originally Posted by mravlca View Post


How would this improve the sound quality?

Best Regards,
Aleš
I guess because you would want the rf to goto ground asap. Thats why people solder a cap to the chassis input socket also, along with the resistance of the interconnect, it acts like an rf filter.

Last edited by doors666; 10th January 2015 at 02:20 PM.
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Old 10th January 2015, 02:19 PM   #10
mravlca is offline mravlca  Slovenia
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Quote:
Originally Posted by Mihkus View Post
I couldn't call it a good design because:
Poor grounding, think and long supply rails, no HF filters on both chips, signal layout should be as compact and as closely together as possible and as far away from output and supply rails as possible and missing mute cap. I'm not an expert on this but just second opinion. Which is all true

BTW, have you tried your PCB's yet? I wouldnt risk. Chips wont blow up but they will oscillate for sure.
You are clearly mistaken, PCB has excelent grounding because every components ground is routed into star point. Which HF filters you are reffering to, the ones on the input?
I could place components on the input better but hey this isn't bad also. Missing input cap doesn't make desing good or bad. I have one LM3886 without this mute cap for over 10years now and no turn on off pop noise is present.

Yeah I tried this PCB and no oscillations are present, no noises etc. and the sound is awesome. You are forgetting that this chip can work with very little amount of components stable.
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