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Old 17th August 2013, 06:45 PM   #1
iggy111 is offline iggy111  Croatia
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Default LM3886 grounding details

I have a question about properly grounding my point-to-point LM3886 amp, which I initially sent to AndrewT on PM, but he asked me to copy it here together with his answer, and I agree, so here it is:

...here's what I applied already (from one of your (AndrewT's) earlier posts):
Connect the HF decoupling together.
Connect the MF decoupling together.
Connect these two and call them Power Ground.
Connect the output Zobel to Power Ground.
Connect Power Ground to Main Audio Ground.
Connect Speaker Return to Main Audio Ground.
So the questions that remain are these:

1. Where do I connect the IN-? Do I connect it directly to the Main Audio Ground, or connect it together with the bottom of Ci and R2, and then to Main Audio Ground? (there was a post on this and ground lift, but I can't find it)

2. Where to connect the chip's ground pin (pin 7)? Do I connect it to the Power Ground?

Here's a schematic of what I think is right:

Click the image to open in full size.


************************************************** *********

And here is the answer:

Complete the INPUT circuit.
Signal In and Signal Return MUST form a complete circuit.
This circuit MUST have very low loop area.

The amplifier measures the voltage applied to the INPUT SOCKET, i.e. between Signal Hot and Signal Return.
The amplifier then processes that difference in voltage and sends a scaled copy to the Output.
To enable that processing the NFB MUST also be taken to the Signal Return.
Again this loop area must be minimised in this extra circuit that touches the INPUT circuit.

Finally reference the Input circuit to the Main Audio Ground. You can do this at the PCB input terminals or at the Chassis mounted input terminals or for a stereo amp at the common link joining all the input terminals together.

REMEMBER
every circuit allows current to flow around a loop. That loop ALWAYS comes back to where you started your analysis of the current from.
Every loop must be completed. Often these individual loops need to be referenced to each other. This should be at individual points. You select the points of contact between loops to minimise the interference caused by the link.
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Old 17th August 2013, 07:05 PM   #2
iggy111 is offline iggy111  Croatia
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The schematic in the first post shows how I actually connected everything in the end. I did have a problem with LOTS of hum and buzzing, thought I did something wrong, but it was down to separating the MAG from the Chassis Ground, which was polluted by my PC and who knows what. Now there's virtually no hum in the speakers

Unfortunately my camera died so I can't post pics, but I'm very proud on the whole P2P adventure - my HF decoupling is the length of 2 small Wima 100nF caps, MF decoupling less than 4cm per side, both connected with a AWG20 thick copper wire only a few mm apart. Soldered the Zobel (length of R and C) to that same wire, and pin7 (chip gnd) was soldered to this Power Ground, and in the end I connected the middle of this wire to the MAG.

I didn't go for symmetry, as it was clear to me that the shortest possible traces could be done if V+/V- and output are kept on the left of the chip (pins 1, 3 and 4), and the input on the right (pins 9 and 10).

Speaker returns are taken directly to MAG, and I soldered the signal returns with a piece of wire and from the middle of it a wire to the MAG. Tried to keep all the wires going to MAG the same length, so only PSU grounding wires are a bit longer.

So, any better ideas for grounding?
Also - should I lift the signal ground with a small value resistor or should that be considered only when humming is a problem?
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Old 17th August 2013, 09:55 PM   #3
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It took me awhile:
MAG = Main Audio Ground

What is circuit board and what is chassis?
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Old 17th August 2013, 10:10 PM   #4
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If you connect the input ground to the 22k and the feedback ground, you'll apply noise from common-mode currents emanating from your source (DAC, CD player, preamp). To avoid this, give the input ground its own dedicated wire back to MAG. This is likely to induce a little more hum though from stray mag-field so keep the trafo distant.
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Old 18th August 2013, 09:55 AM   #5
iggy111 is offline iggy111  Croatia
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@Speedskater - there is no circuit board, I soldered it point-to-point. I actually started with a chipamp.com boards, but after lots of changes I had problems with lifted traces, so I went for P2P.

@abraxalito - I might try that just to see if there is change, but right now I have just some slight hum, heard only when I put my ear to the speaker. But, as AndrewT wrote, shouldn't signal return and NFB loops be as short as possible? This way I would add more than 30cm (2x15cm) wire from signal ground > MAG > nfb.
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Old 18th August 2013, 11:23 AM   #6
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iggy111, please show us your PSU for both channels before any comment about grounding.
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Old 18th August 2013, 12:31 PM   #7
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Quote:
Originally Posted by iggy111 View Post
But, as AndrewT wrote, shouldn't signal return and NFB loops be as short as possible? This way I would add more than 30cm (2x15cm) wire from signal ground > MAG > nfb.
They should be as short as reasonably possible, nevertheless still constrained by other important aspects including not coupling ground noise into sensitive nodes of the circuit. I suggest having a listen - if you don't hear the effects of moving the input ground to a 'safe' place then leave it as is.
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Old 18th August 2013, 06:37 PM   #8
iggy111 is offline iggy111  Croatia
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@abraxalito - let me get this right, which of these are you saying I should do:

1. connect signal gnd to bottom of R2 and then wire to MAG, also give nfb gnd a separate wire to MAG

OR
2. connect signal gnd directly to MAG, and connect bottom of R2 to nfb and then wire to MAG?

@aparatusonitus - here's my PSU - notice the two separate ground wires going to MAG and no ground wires going to amp

Click the image to open in full size.
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Old 18th August 2013, 06:55 PM   #9
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Ok, so you are using single PSU to power both channels? If that's the case do it like in the images below ;-)
Edit: Take output (speaker) ground for left channel from power ground point at left channel pcb, do the same for right channel.
Attached Images
File Type: gif Proper_grounding_a.gif (16.4 KB, 511 views)
File Type: gif Proper_grounding_b.gif (9.9 KB, 502 views)
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Last edited by aparatusonitus; 18th August 2013 at 07:19 PM.
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Old 18th August 2013, 07:18 PM   #10
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Hi,
Just in case it make give you an idea how to ground your amplifier or as a reference. Attached it is a schematic in the way I wired the start ground in my LM3886 amplifier.
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File Type: doc Start ground.doc (168.0 KB, 168 views)
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