[ProAudio] AES on the west coast, was Re: Ampex/ProAudio List Dinner
Steve.Lampen at belden.com
Mon Nov 4 16:57:59 EST 2019
I’ll go one further!
Every piece of metal is an antenna. This is why touching a rack frame, or air conditioning box or even re-bar, with a signal conductor can introduce EMI or RFI into a circuit. So when you make a star ground, where every point being grounded is at exactly the same potential, you can ruin all that work by accidentally touching something unintended that is also metal. Shielding is one of the least effective ways of getting rid of RFI and, as far as EMI and other low frequency stuff, any shield can be ineffective at best. A foil shield (common in audio cables) is clearly an RF shield – it’s 100% coverage (great at short wavelengths) but it is very thin. Low frequencies want a large conductor, low resistance, which is why you go to a braid shield. The best single braid is perhaps 95-96% coverage, but it’s full of holes! And even a double braid shield ($$$) is maybe 98% coverage, but it has all those random holes. That means that shield effectiveness starts to drop off around 400 MHz and by the time you’re at 1 GHz or higher, that braid is simply the drain wire for the foil shield which is (hopefully) underneath. At the other end of the spectrum, the braid shield isn’t beefy enough. Shield effectiveness begins to drop off below 1,000 Hz and, by the time you’re at 50/60 Hz, there is NO SHIELD that can be put on a cable that has any effect. Even solid steel conduit, perfectly installed, is around -30 dB at 60 Hz.
There are only two ways to effectively deal with EMI or RFI. The first way in distance. The inverse square law clearly tells you that twice the distance is one quarter the effect. You can get very low noise by just moving cables away from each other. And who can forget the old adage that crossing cables at 90% gives you the lowest induced noise between cables. Even audio snakes crossing power cords. I would love to see a demo of 120 volt single phase VS audio snakes, then 240 volts, heck, why not 480 volts 3 phase. (I have the symbol but I’m sure that it would not be translated correctly to y’all.)
And then we have the second way of reducing EMI or RFI, balanced lines. This does not prevent (or even reduce) the RFI and EMI from being induced on a conductor. It’s just that the conductor next to it has exactly the same noise but 180 degrees out of phase. To be a perfect balanced line, the two signals on the two wires should be the same level (intensity) exactly 180 degrees out of phase, and in time with each other (which is really saying the same thing). Getting those three things in a twisted pair of wires is NOT EASY. The ultimate version, so far, is Belden’s bonded pairs, where the two wires are extruded together (not glued!) and the conductors are exactly the same size (or as close as current manufacturing will allow). This allows an UNSHIELDED cable to get noise levels very low. I have seen test results running audio where the noise was below -110 dB which is the noise floor of the network analyzer being used. Is this perfect? No way. Bonded pairs were invented in 1993. Each year, especially with robotic factory conversion, these cables get better and better.
I’ve been retired from Belden for two years, but still have enough influence to get you a sample if you want to test it.
2310 Broderick St.
San Francisco CA 94115-1005
Cell 1-415-531-4668 worldwide
steve.lampen at belden.com<mailto:steve.lampen at belden.com>
Check out my web page www.stevelampen.com
From: ProAudio [mailto:proaudio-bounces at bach.pgm.com] On Behalf Of Corey Bailey Audio Engineering
Sent: Monday, November 04, 2019 4:05 PM
To: proaudio at bach.pgm.com
Subject: [EXTERNAL] Re: [ProAudio] AES on the west coast, was Re: Ampex/ProAudio List Dinner
Jim Brown, Bill Whitlock & David Josephson:
All great advice,
For equipment that I want to be truly ground isolated, I use a driven stake (Grounding Rod) and avoid AC ground if at all possible (local electrical codes be damned!). Then, follow the advice of someone who's knowledgeable on the subject of proper grounding. (Like, any of the three mentioned above.)
Corey Bailey Audio Engineering
On 11/4/2019 10:18 AM, Jim Brown wrote:
On 11/3/2019 7:38 PM, David Josephson wrote:
For instance as Jim Brown says, every conductor is an antenna.
My favorite quote on this topic is from Henry Ott, who in his lectures spoke of the hidden schematic lurking behind the ground symbol. Since my retirement, I've been devoting my energies to ham radio, teaching fundamentals to folks with a wide variety of technical backgrounds. No surprise, the Pin One Problem is a hot topic -- Neil Muncy repeatedly told me that it was the primary mechanism behind RFI, and work for a paper that David and I co-authored in 2003 proved it. Fast forward to today, Pin One in CATV and DSL systems is the cause of EGRESS of RF noise that pollutes the radio spectrum.
I do, however, strongly object to the phrase "ground loop," instead using Bill's excellent model showing the mechanism as a difference in potential chassis-to-chassis between interconnected equipment (or chassis to ground) t as the cause of shield current, whether that difference is created by IR drops in green wires or antenna action. The reason I so strongly object is the that solution, especially in small very local systems, is so often proper bonding, which visually creates a loop with signal conductor shields. I'm thinking here of a home entertainment system or a ham station. I use a drawing of Bill's model in my talks and on-sine tutorials.
I stopped teaching at trade shows several years ago because I was unwilling to participate in one held in Florida, in protest of the acquittal of that vigilante murder and the law that allowed it.
As to our conventions -- my interest has always been in the papers and workshops, and hanging out with colleagues. In retirement, I can no longer justify the cost of a convention that requires a hotel stay in an expensive city.
Henry Ott also did a great breakdown on the inductance of a wire as part of a loop.
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