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Old 26th March 2012, 09:52 PM   #1
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Default Star ground and pcb

A while back I build a chipamp from chipamp.com. I build it on a wooden board.
I used a star ground mounted on a big heatsink.

Now I want to build a stainless steel casing. My question is when using the starground do I need to isolate the pcb's from the case?

I've don a lot of searching and there is a lot of info, but I can't find the answer.
Or this is a very stupid question or my knowledge isn't sufficient. I guessing it is the last bit so I hope to gather some knowledge.
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Old 27th March 2012, 10:16 PM   #2
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Quote:
Originally Posted by Xerxes75 View Post
A while back I build a chipamp from chipamp.com. I build it on a wooden board.
I used a star ground mounted on a big heatsink.

Now I want to build a stainless steel casing. My question is when using the starground do I need to isolate the pcb's from the case?

I've don a lot of searching and there is a lot of info, but I can't find the answer.
Or this is a very stupid question or my knowledge isn't sufficient. I guessing it is the last bit so I hope to gather some knowledge.
The typical configuration is to use a grounded (3-prong) plug. The earth ground from that is connected to the metal case using a single heavy gauge wire with a toothed washer that will bite into the case, and a lock washer to insure that the nut can not vibrate loose. This is to make sure that the case is grounded to prevent risk of shock. To form the star ground, I believe that you can either connect all the other grounds back to this point, to another bolt located nearby on the same metal chassis, or between the caps of the power supply. The "ground" return from the loudspeaker should always be connected between the caps of the power supply, however.

There is some useful guidance on wiring here:
Power Supply Wiring Guidelines

Specifically this figure is useful to answer these kind of questions:
Click the image to open in full size.

You can use the search feature in this forum to look up other threads on this topic that will go in to more detail.

-Charlie

Last edited by CharlieLaub; 27th March 2012 at 10:19 PM.
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Old 27th March 2012, 10:41 PM   #3
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Thanks Charlie!

And this drawing was what I was looking for. But the question stays..... On my pcb's there are groundpads on the sides wich if connected to the chassis with mounting screws make ground. So when I use the star ground (as in picture) do I have to isolate these with isolating pads or by using teflon screws?

I think I know the answer (isolate) but I want to be sure! And ground is proving to such a large discussion that it hard to filter out the right answer when you not quite sure of the question.....

But again much thanks for the info so far!
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Old 27th March 2012, 10:58 PM   #4
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Quote:
Originally Posted by Xerxes75 View Post
Thanks Charlie!
So when I use the star ground (as in picture) do I have to isolate these with isolating pads or by using teflon screws?
!
I would isolate.

It takes very little to cause an earth loop.
I have managed it with a 100mA supply !!!
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Old 27th March 2012, 11:50 PM   #5
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Quote:
Originally Posted by Xerxes75 View Post
Thanks Charlie!

And this drawing was what I was looking for. But the question stays..... On my pcb's there are groundpads on the sides wich if connected to the chassis with mounting screws make ground. So when I use the star ground (as in picture) do I have to isolate these with isolating pads or by using teflon screws?

I think I know the answer (isolate) but I want to be sure! And ground is proving to such a large discussion that it hard to filter out the right answer when you not quite sure of the question.....

But again much thanks for the info so far!
I would also isolate. You want to avoid making loops in the grounds, that is avoid multiple paths back to the ground entry/reference from any point in the circuit whenever possible. Any loop is an antenna, and can pick up noise, etc. This is one reason why the STAR ground reduces noise and hum pickup.

-Charlie
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Old 29th March 2012, 11:06 AM   #6
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Ah thanks. Having decided on a star ground the next question is where to get my ground from. There is a lot of writing about it and a lot of pictures but no article or picture shows in detail where I get my ground from the snubber power. I do have pg- and pg+ but I also need these for my amp. There are some contradictions on the Internet.

To be clear I am building a lm3886 from chipamp.com. I use separate housing for power and amps. Normally when I can't find a clear answer I do. The trail and error method but do to my illness I don't have the time and energy so after a lot of surfing and reading I turn to here...again.
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Old 29th March 2012, 12:29 PM   #7
gootee is offline gootee  United States
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The ground for your amp board is on the right-hand side, in the diagram, labeled GND. You can take +VE, GND, -VE, and SPK RET wires to your amp board (or wire SPK RET directly if there is no connection on your board for it).

If there is only one ground on the amp board, I would make it at least two, so that the input signal ground reference points come back separately to the GND point in the diagram above.

I would tightly twist together at least the +VE, GND, and -VE wires, all the way from the PSU to the amp board.

Also, in the diagram above, the blue and green AC wire pairs should each be tightly twisted, as should the AC pair from transformer to rectifier bridge, and the pair from rectifier bridge to caps. And keep all of the DC and small signal conductors away from the AC stuff.

Your input signal and input signal ground pairs should each be tightly twisted, all the way from the isolated RCA jacks to the amp board. Or, you could use shielded twisted pair, with the shield connected only to the chassis, only at the input end. Note that the shield should NOT be connected to the RCA jack ground.

Last edited by gootee; 29th March 2012 at 12:31 PM.
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Old 29th March 2012, 03:40 PM   #8
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Ah it is coming together, thanks for that. Only one question remane.

On my PSU board I have pg+ ve+ and pg- ve- but no GND. I have seen different solutions to this. But nobody just says this is the way to do it if you are building everything separate. So how do I create ground and go to the amp...

I have red the large article about ground, problem is that I can't find the solution for my problem in it.
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Old 30th March 2012, 06:34 PM   #9
wazzy is offline wazzy  Zimbabwe
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ground would come from the centre tap of your transformer, hence the 22v-0-22v, the zero is your ground. i would isolate this from the case though.
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Old 30th March 2012, 07:24 PM   #10
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Default star gnd.

hi,

first of all your star gnd should be located as close as possible to your amp.as you have said in your previous set up it is located at heatsink.theres no need to move or create another star gnd just put the power supply gnd together with the star gnd at the heatsink.
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