[ProAudio] Design mistakes, was Re: Understanding the grounding electrode system (NEC article 250)

David Josephson dlj at josephson.com
Tue Nov 5 23:12:01 EST 2019

> On Nov 5, 2019, at 10:22 PM, Bill Whitlock <engineer_bill at verizon.net> wrote:


> 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!

For years our mics only used phantom power, which has its own problems. I was rudely brought back to reality with the design of our hybrid tube mic a few years ago, and its power supply. Return current from the filter caps contains all the nasty stuff on the power line (really good detector of a refrigerator compressor turning on and off), and guess what gets in to the regulator opamp if common impedance coupling is created? It had nothing to do with earth or neutral return, but return to the center-tap of the power transformer secondary. It took a couple of iterations of the PCB layout to deal with that.

This is super basic analog circuit design, but has more to do with logical thinking than any whizbang engineering. Isolation transformers don’t help. Where does the current go? Lightning protection, EMC, all the same issue. Henry Ott taught about this, I wonder where else people would learn other than by discovering their own bad designs.

David Josephson

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