Power Amp PCB design, grounding

[prairiemystic]

All of the consumer audio gear I see has a two prong power cord, it's ungrounded. Turntables, CD players, AV receivers etc. (in Canada with CSA/UL approvals). Is it the "double-insulation" rating for that?

For power-line (mains) grounded gear, it has to withstand the two possible fault conditions: line to chassis, and power transformer pri-sec short which fails with the lifted ground or "loop breaker" that Elliot Sound shows, leaving the bridge-rectifier approach looking pretty good.

[AndrewT]

Gordy,
NO
is in response to your dangerous advice.

That statement will stay there until you remove your dangerous advice.

I don't want anyone being injured or killed because of your dangerous advice.

[JensRasmussen]

Originally Posted by prairiemystic

I know Jen Rusmussen put a lot of energy into updating the Low-TIM pcb and it looks like a great layout. But I don't agree with the pcb grounding scheme, being a mix of techniques.

That would be Jens Rasmussen....

The problem most often seen with ground planes are that the designer just pours the plane, and never takes the time to make a low impedance path for all points that refer to ground.

Another point that is often forgotten is the ability for the ground plane to shield incoming RF signals and prevent these from disturbing the amplifier (or other circuits) Using star ground may be better at suppressing hum and LF ground currents, but planes are (IMHO) better when considerations for EMC is added.

\\\Jens

[AndrewT]

Originally Posted by prairiemystic

Douglas Self advocates putting the AC mains safety ground at (1), on the input RCA jack. This looks good from a hum-loop point of view, integrating with other gear, but IMHO is totally impractical.

fig13.1 (second edition) does not show a 10r resistor between Safety Earth and RCA Ground.
Fig13.1 does show the screen connecting the RCA Ground to star point F of the main Audio Ground. Builders must appreciate that this route must be capable of surviving longer with a fault current than it takes for the fuse to rupture and the arc to extinguish.

If done properly with the builder thinking about what the diagram is telling him/her then D.Self's method works.
I don't like it either, but that does not detract from the fact it works.

[marce]

Conrad,
had a quick look through chap. 3 of "Electromagnetic Compatability Engineering", I belive 3.2.5 is the relevant part. Basicly the best place to connect to Chassis ground is in the IO area of the PCB (where any IO connectors are). This is a very basic summerisation of what is quite an involved chapter (3. Grounding).
Ground planes with slots to control current (if required and if you know where to put them) have been used for analogue design for years, and if done right provide the best of both worlds (Hi frequency EMC shielding, and providing a return path that is low impedence and as near an equipotential point as possible).

[Gordy]

Originally Posted by AndrewT

NO

Good afternoon Andrew,

Factually I have provided references to Bryston and Pass Labs, both of which are professional, successful, well-regarded companies, and both of which have some form of resistance between mains ground and audio ground documented in their service literature. Further, I (post#12) and other contributors (posts #20 and #21) have referenced a similar solution that has been internet published by the experienced Elliott Sound Products.

I believe that they are right.
You believe that they are dangerous.

If you have the strength of your commitments you will telephone them immediately and express your concern that they are endangering the lives of their customers, and then you will telephone the authorities and advise them accordingly. If you really believe that those items of equipment are dangerous then morally you have no alternative, for failing to make the call will in effect mean that you are wilfully allowing people to be placed in danger.

Shouting ‘No’ here and then just sitting behind a computer when you feel that lives are at risk will bring judgement upon you. Andrew there is no option but to ‘call you out’ on this one: telephone them and put your money where your mouth is, or shut up.

You believe that people are in danger: until you act upon your beliefs I have no more to say to you.

Sincerely,
Gordon

PS, Mr. Moderator, while you may consider editing this post please take a moment to reflect upon the position of Andrew. If he really feels that lives are at risk then morally he should act upon it. Hence this post is straight and reasonable, and should stand unedited.

[AndrewT]

I have gone to PM in the hope I can argue the point with Gordy to a sensible conclusion.

[Conrad Hoffman]

Marce- agree completely on ground planes but the problem is knowing where to put them and how to slot them. The HF EMC advice today is never to slot them, as it can create big radiation compliance problems, but done intelligently it can improve analog performance tremendously. Certainly you and I know how to do this- everybody else is suspect.

Gordy- I'm no policeman. Companies have liability insurance and make their own determinations of how to go about things. That's their business. They also (hopefully) have the time and resources to test things to failure and explore various line fault scenarios. I agree with Andrew. All ground connections need to be able to handle large currents, enough to trip the breaker plus a huge safety margin. To me, that means solid wire of sufficient gage, properly bonded. I wouldn't take a product to market any other way, but that's just me. Have you ever used a ground bond tester? Resistance in a connection shows up mighty quick! Shortly thereafter there may not be a connection. In the US, it's isn't required to have a lab do a UL compliance test, in fact it isn't required to be UL compliant at all. Still, best practice is to wire things per their requirements. CE is a requirement depending on where things are shipped, and I doubt a compliance lab, if they were really looking at it, would allow anything other than a solid connection of sufficient gage.

CH

[AndrewT]

I have tested the Disconnecting Network. It works.
I connected L to Earth and switched on at the wall socket outlet.
It exploded the fuse in the IEC socket and blew out the contents. One fuse was substantially intact the other, I never found the bits. Both Live and Neutral were fused in this particular IEC filter + switch + fuses.

The diode bridge and the attached cap and 10r were completely unaffected by the Fault current that flowed through the Network.

If a similar test is recognised by CE, I would on my limited experience, guess that they would approve a similar Safety device.

[Gordy]

Hello Andrew,

I see that you have tested the configuration and found it satisfactory. You will now understand that it is not dangerous.

However the underlying tone of your second Private Message to me was unacceptable. As such I hereby re-state the facts:

Fact 1
Post#3 mentioned a 5.6 Ohm resistor. The drawing showed the initial position of the resistor, and also very clearly showed the position of the star point and the chassis ground.


Fact 2
In post #10 I said:

“Ref. the scheme in post #3; I've never been a fan of input / output separation via the resistor, and would echo the view of jcx on this point. Better (in my humbly grumbly opinion) to consider placing it between the 'star point' and the chassis ground.”

[ Clearly I was suggesting a 5.6 Ohm resistor between 'star point' and the chassis ground, as an initial idea for consideration in respect of ground loop defeat ].


Fact 3
In post #12 I added:

“It is normally bypassed by a small capacitor and often with large anti-parallel diodes.”

[ Clearly I was advising that additional components are normally used along with the small resistor, and that they are a capacitor and diodes ].


Fact 4
I then provided references:

Two references (Bryston and Elliott Sound Products) demonstrate the exact configuration that I had noted between the 'star point' and the chassis ground.

A third reference, Pass Labs, demonstrate a variation by way of a thermistor in that circuit location.

[ Clearly these references prove my point ].



From these facts any reasonable person should be able to conclude…

1.
I recommended a small resistor bypassed by a capacitor and anti-parallel diodes to be placed between the 'star point' and the chassis ground.

2.
This is normal practice with at least some professional companies.



From your test you have concluded that the configuration is safe, and I agree.



However I reiterate that the underlying tone of your second Private Message was unacceptable, hence there will be no further dialogue between you and I either publicly or privately.

I request that you never contact me again.

I will now withdraw from this thread.

Thank you,
Gordon