LM3886 tri-amp pcb ground question - diyAudio
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Old 25th May 2005, 06:24 AM   #1
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Default LM3886 tri-amp pcb ground question

I've searched the site a few times, but am still struggling with this one.

I'm designing a pcb that will hold 4 LM3886's. It's a mono tri-amp board: high, mid + 2 parallel chips for the woofer channel. I was using the schematics linkwitz has on his site as a starting point, but i'm still not certain about the grounding arrangement.

In one design, he suggests tying the speaker ground to a 0v point and then tying it to common ground with a low R resistor.

But in his pluto design, he shows tying power ground (from the filter caps) and speaker ground to 0v together.

Is the latter arrangement the better way to do it?

Also, for a single chip either grounding arrangement seems pretty straightforward. But if there are four chips on a board, does anything have to be taken into consideration? For example, can i tie all power grounds and speaker grounds together at a point or on a plane?

Should this be a separate from signal ground? If I tie the grounds together with a resistor (as in his LM3886 schematic), should this be done at a single point on the board or multiple (like one at each chip)?

I know there are a thousand threads here on LMxxxx pcb grounding, but I can't find one that seems to tackle the multiple-chip aspect in a way i can get my head around.

Any insight would be greatly appreciated.

(My last multi-channel LMxxxx p2p amp ended up in smoke. The gf is now demanding I use boards.)
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Old 25th May 2005, 07:56 AM   #2
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I will tie all power grounds together with speaker grounds,then connect to the chassis through low value resistor.
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Old 25th May 2005, 01:01 PM   #3
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Thanks for the response, Leolabs.

I think i get it. How would you handle signal ground then?
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Old 27th May 2005, 03:32 AM   #4
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I forgot about small signal ground,sorry pal I usually:

-connect all small signal grounds together,
-connect all power grounds together with speaker grounds,
-connect("isolate") them with a 10 ohms resistor,
-then connect chassis ground together with power+speaker grounds

U may notice the differeces here with my previous configuration.What i m trying to say is:U should try every possible grounding configuration for the lowest noise result.Hope this help
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Old 27th May 2005, 06:14 AM   #5
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Hey Leolabs, thanks again.

Where are you in Malaysia? I've been in the region since 2000, but i'm still not that familiar with Malaysia. (when i have time to travel i tend to head for the nearest beach.)

Maybe you can help me out. I'm about to get a board made and of course I'd like it to work reasonably well the first time. I've seen many chip amp designs and can get my head around the current requirements and star grounding.

But i'm not sure what happens when you put 4 chips on a board. Is there any conflict if 4 chips are sharing the same 0v rail for grounding? Is there any conflict on the power rails? I was thinking about sharing the rails and the power ground (0v) but then having separate signal grounds on each chip (tied in with a resistor). Do you think that's ok? Will there be potential problems?

Thanks again for you help.

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Old 27th June 2005, 05:47 AM   #6
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Default here is my 4 chip LM3886 mono pcb w ps

Here is my first attempt at a 4 chip pcb with a psu. It's mono, for a tri-amp setup (2 bass drivers). It's based more or less on the linkwitz schematics.

Can anyone comment?

The file is express pcb.
Attached Files
File Type: zip new.zip (35.6 KB, 155 views)
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Old 28th June 2005, 01:52 AM   #7
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Not bad.But try to rearrange all grounds.
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Old 28th June 2005, 06:25 AM   #8
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I've made several designs with several LM3886 and the result was very low noise and 0V DC output.

For any audio design, the key is to use a star ground, i.e. every ground return should have a separate path to the central star ground. The star ground itself lies a short distance away from the main decoupling capacitors.

When a star ground is not possible, a compromise can be made. Have separate returns for input ground, speaker ground, feedback ground, decoupling ground. If the power supply is not on the same pcb, then yes, have all these ground wires return to the star ground on the PSU PCB. An alternate option is to use a secondary star ground on the amp PCB.

If your layout is good and your traces are generally short, you may want to try a copper flood (i.e. ground plane). It also provides the extra benefit of reducing crosstalk. Ground planes work best for more than 1 layer count PCB's.

For a project like yours, I'd use star ground & secondary star grounds. Speakers return to main star, power amp related grounds return to secondary star. Preamp related grounds return to another secondary star. Both secondary stars connect to main star via separate short, thick traces. A single wire then connects star ground to chassis ground.

I usually don't use ground lift resistors unless there's a problem. Sorry I can't comment on your layout. My CAD can't open it. Best of luck with it.
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Old 28th June 2005, 09:26 AM   #9
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Thanks for the comments, everyone. I've attached a jpeg of the board for those who don't have exp. pcb. It's a shot with top and bottom layers and one with bottom layer only.

Click the image to open in full size.
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