May I have some help regarding..Electrical safety,Star Grounding and Suggested Layout - diyAudio
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Old 22nd March 2009, 11:05 PM   #1
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Default May I have some help regarding..Electrical safety,Star Grounding and Suggested Layout

I have recently built a pair of LM3886 chipamp PCBs along with a couple of Pedja Rodic's Discrete Regulated power supplies.

I intend to house the amps, power supplies and a preamp in one case, and the transformers and bridge rectifiers in another.

Click the image to open in full size.

Click the image to open in full size.


Here are some of the components I have used so far, but before I go any further I have a few questions...


Umbilicals, Earth Connection and Spade Connectors

The +&- DC output from each rectifier is fed to two, four pole Speakon sockets mounted on the rear panel of the transformer case. I have a separate Speakon socket that carries the safety earth to the amplifier case. I am using 4 Core mains cable for the umbilical.

Click the image to open in full size.

Ive used crimped spade connectors for the IEC mains inlet connections and DC output sockets (Speakon).

Are these spade connectors suitable for AC?

http://www.maplin.co.uk/module.aspx?...19181#features

The earth connection is as per Nick's diagram

Click the image to open in full size.

http://myweb.tiscali.co.uk/nuukspot/...clone_psu.html

Click the image to open in full size.

Pre-amp

Towards the end of last year I built a SOAP pre-amp

SOAP MKIII (Simple OpAmp Pre) With onboard PSU

I plan to position the pre-amp between the two regulated power supplies, with a 50k Alps pot coupled to a shaft extension.

Do you think it will pick up any interference here, or would I be better off putting it in another case with a separate ground connection?

Star Grounding

I am right in thinking I need to connect both ground connections from each regulated ps along with both chipamp pcb (CHG) chassis grounds to the safety earth terminal?

Sorry to bombard you with so many questions!

Thanks

Richard
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Old 22nd March 2009, 11:25 PM   #2
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Hi Richard,
Earth ground really has nothing to do with PS ground, The power supply ground is actually a neutral bus and not a ground. Whether you directly connect it to earth ground or through an isolation scheme such as a thermister or resistor is entirely up to you. Just dont use the earth connection for your star ground point, use the PS ground and bring everything to it.
Bill
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Old 22nd March 2009, 11:29 PM   #3
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Bill is correct.

From the pictures it looks like everything is OK.
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Old 22nd March 2009, 11:43 PM   #4
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Pot location shouldn't be a problem, everything is DC in there.
Nice lookin build for a chip amp, try Mauro Penasa's MyRef C next, sounds way better than it has a right to.
Bill
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Old 22nd March 2009, 11:45 PM   #5
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the ground looks fine. It may have been a better idea to use powercon connectors. They are almost identicle to speakon connectors but there are a few subtle differences.

I am using powercon for my power umbilical and as it is a 3 pole connector its is going to carry + - and gnd back to the psu.
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Old 22nd March 2009, 11:51 PM   #6
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Quote:
I have a separate Speakon socket that carries the safety earth to the amplifier case
I think this would trouble some members here. You, or someone else could disconnect the amplifier safety ground by accident leavind the case ungrounded. I think the recomendation is that the power cable ground pin should be the first to connect and the last to disconnect.

Unlikely I know, but worth bearing in mind.

John
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Old 23rd March 2009, 12:45 PM   #7
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Hi Tripmaster

Looks really nice! Just a quick off topic question: where did you get your heat sinks from? I am looking for some exactly like that.

Thanks
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Old 23rd March 2009, 01:21 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,

both metal chassis will/must have a Safety Earth. OK, your layout complies. Take note of John B's caution.

All exposed conductive parts MUST be connected to Safety Earth.

My interpretation of this rule is that "fault current" must be able to pass from any exposed terminal or socket to the Safety Earth. This can be a direct wire connection or a disconnecting network.

The main Audio Star Ground (reference) can be "floating".
I recommend it be located, as near as possible, between the amplifier output, the amplifier input, the main smoothing bank and the speaker return terminal, not "at" the PSU. Ultimately this "floating" Ground Must be connected back to Safety Earth.

On a different tack,
I reckon there should be pre-smoothing done at the transformer/rectifier location to send near DC to the amp box.
Then good quality final smoothing at the amplifier.
This creates an RCRC type power supply. If the power umbilical cable has some inductance, the supply becomes RCLrC which filters the HF artifacts even better.
This separation of the smoothing demands that the amp smoothing supplies all the transient current to the amplifiers and these local caps determine the audio quality that goes to the output.
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Old 23rd March 2009, 02:10 PM   #9
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Andrew,
Good point on the separate PS scheme, using smoothing in the PS chassis.
As for the audio star ground point, we have both oversimplified the choice of location. The optimum location is highly dependent on the particular circuit topology it is supplying, such as power amps- SE- PP- fully balanced, whether the circuit uses current sensing, and whether the circuit needs a ground reference at all. Experience and trial and error are sometimes the only solution.
Best, Bill
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Old 23rd March 2009, 02:32 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Bill Fuss
As for the audio star ground point, .........................and whether the circuit needs a ground reference at all.
I could be wrong, but as I see it, all amplifier require a signal return. All Speakers require a speaker return.
All amplifiers will in some way connect signal return to speaker return. To me, this seems the same as saying, all amplifiers will have a reference, but maybe not.

But more importantly,
all exposed conductive parts MUST be connected to Safety Earth.
That rule can never be oversimplified.
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