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jawbreak 27th November 2004 06:39 PM

Art of Properly Grounding
 
Ive been looking into build a gainclone amp. This will be my first experience with building an amp.

I was wondering if someone could explain or knows of a link where it explains how to properly ground.
I've read theres two grounds: one from the DC circuit and one from the AC power supply. These than are connected together at the end and to the ground. But where do you connect these too? All you do is connect it to a piece of metal? Or run it to the ground pin on the power supply? Thanks for any help.

jan.didden 27th November 2004 07:17 PM

Hi,

There are not really different "grounds". The key to understand it is to realise that the cable bringing your signal into the amp has a 'hot' signal wire, and a ground wire, and that the input signal is a voltage *with reference to the ground (or shield) wire!*.
In other words, the signal wire on itself is meaningless.

Now, what does that means for the ground? It means that if you in any way mess up that ground wire with some voltage or current, you immediately mess up the signal as well! The signal on itself may not change, but because it is referenced against that ground wire, the amp *thinks* it gets a modified input and will put out a modified output, and the output no longer follows the input.

How do we prevent this type of contamination? Suppose we have a piece of 5 inch wire connecting the supply capacitor "ground" to the transformer/rectifier "ground" wire. There will be a nasty current through that piece of wire, from the rectification process, and even with thick wires there will be a voltage between the ends of that wire. 10 Amps pulse through 1 milli ohms wire will still be 10mV!

If you inadvertently ground the shield of your incoming signal wire to one end of that piece of 5 inch "ground' wire, and the ground of the amp input circuit to the other end, you have effectively created a 100 cycles (or 120 if you're in the US) pulsating hum on top of your signal!
So, you want to create what is known as a "star ground" bringing all ground leads (especially those carrying appreciable current, like supply and speaker grounds) to a common point. The object is to avoid that any voltage across a current carrying wire can end up as "input" to the amp.

Jan Didden

jawbreak 27th November 2004 07:23 PM

Thanks for the reply I understand what you mean. So once you have all the ground leads connected, "star ground", does this need to be like connected to anything at that point, like a piece of metal?

Thanks again!

jan.didden 27th November 2004 07:35 PM

Well, when you have all your supply, input, speaker (or output in case of a preamp) connected in the proper way, you have a functioning piece of equipment.

The enclosure normally would be connected to mains protected ground. Then what you would do is connect the whole amp ground shebang to that enclosure as well.
Now, since there is (in theory) no current through this last connection, you could do it from the star ground, but sometimes it is done directly at the input plug shield or ground pin. This is all assuming that there is no other ground to enclosure in the system.

For instance, if you use metal supply capacitors screwed to the enclosure, and the metal has electrical connection to say the negative pole, you already have a ground to the metal and you can get the same problems as we discussed earlier.

To avoid any stray current (because you don't know what the ground or mains protected ground is upstream or downstream of your installation) sometimes the enclosure is "grounded" through 10 ohms from the input plug shield. That will force the current through any existing connection elsewere, but will not cause a loop.

Jan Didden

jawbreak 27th November 2004 07:39 PM

Wow, thank you so much for your quick replies think I got it now. Time to prepare to build this thing!


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