2 external PSU ground question

[guitvinny]

Hi everyone,

Short intro : I`m making a tri-amp stereo system, so 6 channels with LM3886 BrianGT's kit. At first I've made 2 nice cases with 3xLM3886 + PSU each... but I soon realize that I haven`t planed enough space . So I've made a third case with all the 6xLM3886, and I have 'recycle' the 2 small enclosure to put a PSU inside each of them.

Question : I want to know how to properly ground everything, mostly for safety reasons. At the moment everything is working very fine and I got each PSU chassis grounded to the main, but the amp case is `floating`(input connectors, output connectors, LM3886s and power connectors are all isolated from the amp chassis). Is the actual set-up enough ? Should I run a wire from one PSU chassis to the amp chassis ? If so, should I run a wire from both PSU to the amp.. and will it create a ground loop?

Thanks for you knowledge and your wisdom.

[Sheldon]

Is your supply output voltage around 25V or so? And is that a shielded cable, and do the connectors have a solid connection and continuity with the chassis and shield at either end? If so, it should work fine for earth. If it were a high voltage application, I would make a separate earth connection with a separate wire from chassis to chassis.

If you are using a separate earth wire, it's only necessary to connect it to one power supply chassis. You may not have ground loop problems if you have both power supplies plugged into the same outlet. If you do experience ground loop problems, you can locate the power supplies side by side. Then lift the earth at the plug one one chassis, and connect a separate wire (good solid connection) to the other chassis, which remains earthed. This provides a single path to earth.

Sheldon

[guitvinny]

Yes, my power supply is ±28V, but it`s not shielded power cable and yes, there is good connection between power connector and chassis. So, I should only run a wire from one PSU`s connector to the power amp connector, is that it ?

[abraxalito]

Grounding is not only an issue for safety, it also relates to sound quality. Looking at your second picture, how are you connecting the input ground - it looks like that is running through the amp pcb? If so, I've found that to compromise sound quality - to get the best transparency the input grounds need to be grounded individually back to the star point. I'm not totally sure of the mechanism as to why this is so but I believe its due to RF contamination of the ground.

[infinia]

Your problem is the 2 star grounds are located remotely in the PS's. But it looks likes you made it at the PS connector of your amp due to low pin count. The chassis of the amp should have a single connection to the star ground usually through a small value resistor.
To make things less complicated and keep the power loop smaller you might consider adding 4 large PS caps (sized appropriate to your speaker loads) thus moving the star ground back to the amp chassis. Then you could work out your grounding and power distribution much easier and better. There are threads about chipamp grounding basics ie moving speaker returns back to star ground instead of the chip amp board.

[infinia]

I am assuming a permanent safety earth connection is made to the metal bits under the XFMR> Earth connections to the metal amp chassis is not required because of SELV low voltages. Your partially wooden case of the PS is another safety issue of the fire kind. Is the XFMR thermally protected is a primary concern now. UL markings on the XFMR? Cooling?

[guitvinny]

The power supply section is not completed, as you can see they are still in experimental phase. The main earth connection was not permanent (but will be) and you cannot see it in the pictures. The transformers are Antek AN-3222, which I don't think they are UL rated, but CE rated instead. I intend to put a thermistor and a 2 or 3 amp fuse in serie with the transformer, is it enough to minimize the risks ?

[AndrewT]

if there is absolutely no risk of the mains inside the PSU chassis ever coming loose and touching the isolated side of the PSU output, then you have created a virtual double insulated ClassII device.

We do not give any advice on trying to design, build, test or guarantee a ClassII device.

I have contributed to various threads on separate PSU from amp chassis and from memory all eventually agreed that all the chassis must be earthed back to the mains protective earth.

[gootee]

guitvinny,

All of your questions are answered in this article:

Audio Component Grounding and Interconnection

Cheers,

Tom

[Ted205]

i have a separate duel mono PSU for my BPA. Both power supplies connect to star earth in the PSU enclosure. Then my amplifers are connect to the reservoir caps (not directly to the star).

My amplifer is only grounded at the PSU but I have a separate safety earth connected to the amp enclosure.

This works well for me.

Your project looks good btw.