[ProAudio] Understanding the grounding electrode system (NEC article 250)

Bill Whitlock engineer_bill at verizon.net
Tue Nov 5 22:22:08 EST 2019

Hi Fred,

Thanks for your earlier "confession" ... I always wondered how that myth got started. As I recounted earlier, it was still alive around 2005 (not sure of the year) when an AC power seminar at CEDIA (the home theater trade show) presented by an "engineer" from Cinepro recommended it.  He actually argued with me regarding it's safety!

You're absolutely right that a properly designed power supply is essentially bullet-proof from both differential and common-mode noise. But it's apparently not trivial for many designers.  Rejecting power-line noise it depends critically on how the designer ties the power supply common (often the power transformer center-tap) to power safety ground. An inch of wire or PCB trace in the wrong place allows primary-side noise to contaminate the internal ground system of the device.  A few years back, AES had a product design track and I presented a seminar intended for circuit designers ... and this was one of the main topics, under the heading "design mistakes."  Rane came out with a powered loudspeaker a few years ago that suffered badly from this flaw. With no input connection at all, the speaker would faithfully reproduce the noise of a brush-motor drill plugged into the same AC outlet.

Convincing manufacturers to do it right has been an uphill battle!

Bill Whitlock

-----Original Message-----
From: Adolph Thal <fred at ataudioeng.com>
To: proaudio <proaudio at bach.pgm.com>
Sent: Tue, Nov 5, 2019 8:07 pm
Subject: Re: [ProAudio] Understanding the grounding electrode system (NEC article 250)

> if you're doing that you might as well include an electrostatic shield too.

Yes, like the old Topaz Ultra-Isolators?

I have a "little" one, just 10 kVA.

(Little, but I certainly can't lift it!)

But my personal philosophy (and for certain I'm not an EE!) is that
this should not be needed for any audio gear.

Power supply inlet noise rejection problems, if any, ought to be
addressed by the equipment designer.

Linear or switch mode, I'm told that addressing this properly, ought
to be trivial for a competent designer.

Adolph (Fred) Thal
Director of Engineering, ATAE

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