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Old 24th October 2008, 02:25 PM   #1
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Default lm3886 PCB

Hi can anyone pls have a look at a preliminary PCB design for lm3886. It is a the bog standard design from the datasheet minus mute cap and switch, and no input pot. Its still rough but i want to sort issues out now!

I am unsure whether the supply and speaker ground should be connected to chip and signal ground as it is now. ie should there be only one ground.

The intention of this board is for application in a tri-amped system ie. 3-way active crossover with separate amps for tweeter, mid and woofer.

I am also unsure of decoupling cap sizes-planned is for 220uF in parallel with 100uF (not whats on the PCB-will still sort that out when im sure). The PS (external) contains quite a staunch cap network already.

Please point out any obvious mistakes or bad layout practice. any suggestions or questions....?

your help would be appreciated
Thanks Chris
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Old 24th October 2008, 02:50 PM   #2
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Quote:
Originally posted by 13814044
Please point out any obvious mistakes
You haven't posted a schematic or image of the layout...

w
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Old 24th October 2008, 04:22 PM   #3
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sorry it didnt attach-I'll try again
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Old 24th October 2008, 04:38 PM   #4
AndrewT is offline AndrewT  Scotland
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same comments as http://www.diyaudio.com/forums/showt...30#post1640130
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Old 24th October 2008, 04:43 PM   #5
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Ouch-will have another look. Thought the basic schematic was recommended.

thanks for respons
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Old 24th October 2008, 06:11 PM   #6
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Andrew
I'v had another look.

Do you feel all the optional components are necessary, especially wondering about the L-R network on the speaker out-is this often a problem or only with exceptionally long speaker leads.

I'll definitely add Cf, Rf2, Rsn and Csn.

Do you think i should go to a 2-layer board, with the star ground on top.

Should i use the component values listed in fig 2 or can you recommend better?

Thanks
Chris
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Old 24th October 2008, 06:36 PM   #7
AndrewT is offline AndrewT  Scotland
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a few links can usually avoid the need to/complexity of going to 2layer.
Allow for all the components. Adding them after you've omitted them is at best untidy.

Long speaker leads are not the sole problem. It's a combination of inductance and capacitance and interference coming back in to the speaker terminals that must all be addressed and using a full Thiele network rather than just a Zobel or nothing is recommended.
I used to recommend fitting the Thiele at the speaker terminals, but having read a persuasive argument on the Forum, I am inclined to now try mounting the Zobel part close to the output stage, i.e. on the PCB.

Star ground the power side of the PCB.
Star ground the signal side of the PCB.
Allow for a link to interconnect them or to take them separately to a remote Audio Ground.
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Old 24th October 2008, 07:17 PM   #8
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2-layer is not too much of a problem-PCBs cost me almost nothing. So I think ill go that route, I feel i can come up with a design with much shorter tracks and a more logical layout this way.

Is the ground pin on the lm3886 considered part of the power or signal side.

will have a look at the Thiele network and include all optional components.

Should I use the indicated values or can you recommend other
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Old 24th October 2008, 07:28 PM   #9
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could you perhaps point me in the right direction on the Thiele network
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Old 25th October 2008, 09:02 AM   #10
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I'v fixed her.

Can anyone let me know if this design is any better. Single sided with all optional components (Zobel network will be placed at the speaker as recommended)

Should i keep the power and signal ground separate or as in the current layout-joined.( AndrewT recommended either)

as before I will sort out supply caps when this PCB gets a thumbs up- i just want you guys to approve it in principle.

Any input is appreciated as this is my first PCB

Thanks
Chris
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