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Old 30th December 2011, 06:20 PM   #1
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Default Is my grounding correct?

Hi, i'm planning on making a stereo AMP, for the first time, based on 2 parallel LM3886 chips for each channel.

I read some explonations about grounding and i really got confused about how everything i read are reflected when using PCBs.
My first AMP unit PCB design included a main ground plain and a line-level ground plain, that were connected whith a thin lead.
however i read this may cause a potential difference between the left channel ground plane and the right one.

So i came up with a new design and i really need your advise about it.

Click the image to open in full size.

Notice each board has 3 stars that are connected only through the main star ground.

I would like to know if this design is correct and if there is any way i can improve it, or what should i do insted.

thanks alot
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Old 31st December 2011, 01:32 PM   #2
djoffe is offline djoffe  United States
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Your grounding looks fine. There is one subtlety that you may want to deal with...You may want to have a 10 Ohm resistor between your Big green star and your little blue star...one for the right, and one for the left. This link explains why...

http://www.updatemydynaco.com/docume...lemsRev1p4.pdf

You'll find more interesting stuff here:

Update My Dynaco
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Old 31st December 2011, 05:39 PM   #3
AndrewT is offline AndrewT  Scotland
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The 3 grounding "stars" on each paralleled chipamp PCB:
Are they connected in the PCB?
What currents flow from, or to them?
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Old 31st December 2011, 06:20 PM   #4
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Thanks for your replies!
The 3 stars are not connected in the PCB.
I don't know what currents flow from, or to them, I just know that in theory it should be 0V but practicaly it wouldn't be.

So do you think this design insures there won't be any ground loops?
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Old 31st December 2011, 06:39 PM   #5
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One other thing: what defines the current distribution between the paralleled LM3886's? Don't you have the risk that there will be large currents flowing from one LM3886 to the other when they have different offset voltages? For example, when one output tries to force the output to +10 mV and the other to -10 mV while there is only 0.05 ohm of track resistance between the outputs, you will get 400 mA flowing from one output into the other.
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Old 31st December 2011, 07:24 PM   #6
AndrewT is offline AndrewT  Scotland
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OK, let's start you off by looking at the currents flowing in the input leads.

The Left Signal arrives at the RCA and makes it's way to the Left PCB input PIN,.
It then flows around the input circuit and passes to the signal star (labeled Line Level Ground). It has only one choice, it returns along that line level ground back to the input RCA return terminal (barrel) and there goes to the source output ground. The signal return current ignores the line level ground route that links back to the green Main star ground.

Now repeat the exercise for the Right Signal.
Are there any loop areas in either of those signal flow and return routes?
Are there any ground loops in the shared L&R signal grounding?
Does the signal ground loop have any significant loop area?

Will the ground loop aerial pick up interference and superimpose it on the voltage drop along any part of the Signal flow and return loop?

You need to repeat this exercise for the speaker route
and for the Decoupling route
and for the Zobel route
and for the Power route.

You must find all the loops and eliminate the loop areas that will pick up or transmit interference.
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Last edited by AndrewT; 31st December 2011 at 07:28 PM.
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Old 31st December 2011, 09:10 PM   #7
gootee is offline gootee  United States
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The two RCA (line-level) grounds will be connected together somewhere in the source system. So the line-level ground conductors could potentially enclose a lot of loop area, making a big antenna.

So should the line-level "ground-voltage input reference" wires shown, which go from the boards to the big star ground, each be tightly twisted with their respective line-level signal input and signal input ground wires, all the way back to the RCA jacks, and then twisted with each other on their way to the big star ground?

That should eliminate almost all of the enclosed loop area in the line-level grounding, which would be one of the worst places to have any enclosed loop area.

Cheers,

Tom

Last edited by gootee; 31st December 2011 at 09:24 PM.
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Old 16th January 2012, 07:53 PM   #8
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Hi, I finaly have some time to deal with my dream.

MarcelvdG i have a 0.1ohm resistor on each chip output.

AndrewT i don't know how to answer the qustions, thats why i opened this thread.

gootee i will take your advice, but this should be it?
i mean, beside that, the wiring itself is good? is their an ultimate wiring scheme i can work according to?

Guys i really appreciate your help, but my knowledge in electronics is minimal, so if you can, I'd like you to make things more simple for me.

tnks again
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Old 16th January 2012, 09:06 PM   #9
gootee is offline gootee  United States
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You should already have looked at AN-1192, at National Semiconductor | High-performance Analog .

So you should know about the necessity of having low-value resistors on each of the outputs, and about the necessity of close matching of the gain-setting resistors in the chipamp circuits, and should have calculated whether or not your power supply will be sufficient.

There is an amateur wiring diagram also at the link below (another thread about paralleling). But it might not be entirely correct:
Help with Lm3886 paraller configuration

Where did you get the 3886 PCBs?
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Old 17th January 2012, 07:06 AM   #10
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Hi gootee,

I am already after this.
Currently I have a PCB I designed myself, and in order to continue to the manufactoring level, I just need to solve the grounding issue.
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