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Old 21st December 2006, 01:55 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Default Have a look my PCB pls..

Hi all,

I have worked on a stereo LM3886 GC project for a while. After my LM1876 (on test board) fiasco i decided to design a good PCB for my LM3886s... I seperated signal gnd (on right side) and power gnd (on left side).
I kindly request you to say something about my PCB design..
- Does it work?
- Is star signal gorund point (on AF clamp, pin 2) ok? Or should i make gorund plate on all right side?
- Is C6 on NFB line necessary? Or may i ommit, so connect R3 directly to gnd?
Is there anything i made mistaken? Or have you got some advices to add or modify?

Click the image to open in full size.

Thanks a lot for your helps in advance.

With my best regards,
zgr.

PS: I use one power supply for all. And will put a stereo 10K potentiometer before amplifiers. All system includes only these..
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Old 21st December 2006, 03:54 PM   #2
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For what it's worth, the layout looks quite similar to how I laid mine out (only yours is so much neater / looks so much nicer!)

I was worried about oscillation, but I am convinced now, after sticking a > 100nf cap across the output and seeing what I believe was full blown oscillation, that it's not. It seems to work rather well indeed.

EDIT: I don't think C6 is strictly necessary, it'll virtually get rid of any DC offset. I didn't make room on my PCB for this capacitor, and had to bodge them on. I couldn't tell the difference in sound quality, but YMMV. I'd probably keep the space there, and if you don't want it stick a jumper bar / wire in there instead.

EDIT: I'm not sure about the ground plane/plate surrounding the signal stuff. I've 'cleaned up' my board a bit for if / when I build another, and I now have the power ground covering the whole board. I haven't tried it, so I have no idea what kind of effects it'll have. My thinking was that any inteference would be more likely to choose to enter the big ground plane surrounding the signal stuff, so it would be better? It'll definately be easier on the etchant though

Also not sure about the squiggly spkr/power to signal ground on mine. I wanted to see if I could have them all connected and not have hum. If it doesn't work, it's dead easy for me to just cut the traces and return the signals back individually

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Old 22nd December 2006, 07:42 AM   #3
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You mean you applied that design and have no noise or oscillation. Is it? And on your PCB you havent got seperated gnds so it is not necessary.. But i can see a long line (like UHF antenna) between power and signal gnds.. Why is it there?
By the way i am developing my PCB and will put 470nF and 2,2ohm in parallel between signal and power gnds, as recommended by Nico Ras
Thank you friend.
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Old 22nd December 2006, 08:14 AM   #4
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This is my last;

Click the image to open in full size.
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Old 22nd December 2006, 10:08 AM   #5
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Quote:
Originally posted by Dxvideo
You mean you applied that design and have no noise or oscillation. Is it?
Yes and no. As I said, I haven't used that exact PCB yet, but one very similar.

Quote:
And on your PCB you havent got seperated gnds so it is not necessary.. But i can see a long line (like UHF antenna) between power and signal gnds.. Why is it there?
They're not separate because, well, ideally I didn't want to be running separate star ground wires everywhere. If it hums (as my first board did), then its dead easy to just cut the trace and separate them out.

It's a long squiggly line because I figured a long thinner trace would mean more resistance. It might be a bad idea (probably is, I dont know anything about PCB layout), the shape of it might pick up interference etc etc etc. I don't really know, but again... it's easy enough to just cut the trace if there's humming / buzzing

Quote:
By the way i am developing my PCB and will put 470nF and 2,2ohm in parallel between signal and power gnds, as recommended by Nico Ras
Thank you friend.
A resistor separation is probably a much better idea than a long thin squiggly trace thing as I have done lol



I think you should change C6 back to at least 47uF. 22uF and 1k would be -3dB at ~7Hz I think, and from what I remember AndrewT was telling me, this would mean phase distortion (I think) all the way up to about 70Hz.
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Old 22nd December 2006, 11:59 AM   #6
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Yea Andrew said me the same but for another reason; to have about 1,5-2Hz lower cutoff frequency.. If i use 100F or so i can get 1,6Hz (-3dB) cutoff with 4,7F/27K input RC.. Then will turn to my old design with 100F
Thanks a lot guy.
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Old 22nd December 2006, 04:04 PM   #7
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Yeah 100uF would be better if you can fit one there. I'd have to use 1,000uF to achieve the same as my resistor values are 10x lower, and 1000uF is kinda hard to fit in the space I had left, so I had to settle for 470uF (would be 47uF for you). I'm getting along just fine with it

Let us know how you get on with it! I like mine, the sound quality is quite impressive for how small the chips are and how easy they are to build!

I can't quite explain the difference. With one speaker on the old amp and the other on the new amp, fading between the two (very unscientific I know), I really can't tell the difference (though even simple sinewaves do look slightly different on the scope). But with both speakers on the GC... it's just like there's a whole new depth to the music.

Might be imagining it, might be real. I don't know if I want to build a speaker switching (ABX?) test box to find out. I'm happy
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Old 23rd December 2006, 02:48 PM   #8
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Default a beginners pcb

i designed this pcb layout in autocad and got it etched also, i tried to include star grounding. any comments please
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Old 25th December 2006, 07:46 AM   #9
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I guess i cannot use that PCB.. Because Nico Ras gave me another idea on three LM3886s. Its a 2+1 as i tried with 2xLM1876s in the past. But it includes no low pass filter before amp. It boosts under 100Hz on power stage.. It looks like interesting and will apply it with some modifications.
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