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Old 13th February 2015, 10:40 PM   #1
40Reeds is offline 40Reeds  Canada
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Default LM3875 separate chassis ground

Hi,

First of all, this is my first post here. I am fairly new in amp building. The LM3875 from Peter (audiosector.com) will be my first project.

As background in electronics, I've done a few guitar fx pedals but nothing more.

So, I didn't receive my kit yet but I am trying to put all the pieces in the right place before I start. I haven't decided yet but I maybe planning to separate the power supply from the amp.

That is where my question goes... is it necessary to run the ground (protective earth) from the power supply to the amp chassis? see the picture

I've seen setup that this is not done but I've read somewhere that it should be there... I'm confuse.

Thanks all for your input,

yan
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Old 14th February 2015, 09:41 AM   #2
AndrewT is offline AndrewT  Scotland
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It depends on your risk assessment.

Is there a risk of mains getting on to some/any of the isolated wires?
What is the risk to operators, when that rare incident occurs?

There is a long Thread discussing this from about 6 to 8 years ago.
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Old 14th February 2015, 12:54 PM   #3
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Quote:
Originally Posted by 40Reeds View Post
...is it necessary to run the ground (protective earth) from the power supply to the amp chassis?
It sure is. For one thing, a metal enclosure is not a fully effective RF (radio frequency) shield unless it's grounded ("bonded to ground"). For another, electrical codes require that any metal accessible to a user must be grounded for safety. I don't know what the actual legal liability situation might be in Canada, but I can't think you'd want to be a test case. And then, kids do get into everything.

Posted below is my understanding of "the way you do it." The chassis bond located right next to the audio input might appear exactly wrong at first, but it's from Douglas Self's "Audio Power Amplifier Design Handbook," page 402. He says:

"Mains/chassis ground will need to be connected to the power amplifier at some point. Do not do this at the transformer centre-tap as this is spaced away from the input ground voltage by the return charging pulses, and will create severe groundloop hum when the input ground is connected to mains ground through another piece of equipment.

"Connecting mains ground to starpoint is better, as the charging pulses are excluded, but the track resistance between input ground and star will carry any ground-loop currents and induce a buzz.

"Connecting mains ground to the input ground gives maximal immunity against groundloops."

(Also see his related comments on page 405, discussing so -called "ground lift" methods.)

If you don't know, Mr. Self is a well known and much-published guru in the audio world, so I'd say taking his advice can't steer you far wrong.
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Old 14th February 2015, 03:13 PM   #4
syklab is online now syklab  Hong Kong
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From page 411 of the Audio Power Amplifier Design Handbook "either a balnce input or ground cancelling output will be required", can nyone explain what is a"ground cancelling output" and how to implemen that ?
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Old 14th February 2015, 05:10 PM   #5
AndrewT is offline AndrewT  Scotland
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I'm fairly certain this topic (ground canceling) is in D.Self's website.

But this "ground" is audio.
That has nothing to do with Safety Earth.
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Old 14th February 2015, 07:04 PM   #6
40Reeds is offline 40Reeds  Canada
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Thanks for your response. This clears it up. As well as the handbook from Douglas Self. Great info in this book!

I will run that wire.

That brings me to my next question, will a 4 pin XLR connector do the job? I like the fact that it locks pretty well. And I guess that if it's labelled right no one will plug is headphone there. But who knows, might be better to go with a DIN connector? Something else?

And any suggestions about if I should use only 1 chassis mount connector with a non-removable cable or 2 chassis mount with a removable cable. My guess is less connection is better ?

Thanks again!

yan
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Old 14th February 2015, 08:15 PM   #7
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Quote:
Originally Posted by syklab View Post
From page 411 of the Audio Power Amplifier Design Handbook "either a balnce input or ground cancelling output will be required", can nyone explain what is a"ground cancelling output" and how to implemen that ?
There's some discussion in "Self on Audio," chapter 8.
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Old 14th February 2015, 08:48 PM   #8
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Originally Posted by 40Reeds View Post
...will a 4 pin XLR connector do the job? I like the fact that it locks pretty well. And I guess that if it's labelled right no one will plug is headphone there. But who knows, might be better to go with a DIN connector? Something else?

And any suggestions about if I should use only 1 chassis mount connector with a non-removable cable or 2 chassis mount with a removable cable. My guess is less connection is better ?
My 2 cents:

Before anything else, face reality: if it can be done wrong it will be, and that certainly includes plugging the wrong wire into the wrong hole. In fact, such an easy mistake is practically an engraved invitation.

In accordance with the same principle, it you have a 5 foot cable permanently attached, you'll immediately need a 6 foot cable.

XLR connectors scream what they are to the whole world, so mistakes are at least minimized. Sometimes there's wisdom in doing what's expected.

"DIN Connector" is a broad term, you have to specify: DIN connector - Wikipedia, the free encyclopedia

None of which is to suggest that there's anything wrong with better, and fewer and tighter connections are always better. But what's better enough to matter? XLR connectors, or just plain ol' 1/4-inch jacks (3.5mm consumer/computer), are used on amps costing as much as a car, so how bad can they be?
.

Last edited by bentsnake; 14th February 2015 at 08:52 PM.
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Old 14th February 2015, 10:28 PM   #9
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The way Peter builds the Patek Amp v2 is to create a star ground between the amp boards. This star ground has 5 wires, OG amp1/OG amp2/Chassis and from the PSU PG+/PG-. It is unclear how the mains ground/earth is connected. I advise an additional wire for this connection.

DC Cable V+/V-/PG+/PG-/chassis.
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Old 14th February 2015, 11:33 PM   #10
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OG = Output Ground.
PG = Power Ground.

As printed on the Audiosector PCB's.
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