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Author Topic: Grounding and RFI  (Read 1567 times)

Tim Padrick

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Re: Grounding and RFI
« Reply #10 on: February 17, 2014, 12:53:30 am »

Make sure that Pin 1 is connected to the XLR case ONLY where two cables connect together (e.g. mic cable to snake), never where a cable connects to a piece of gear.  Check all cables and snakes on both ends.  And make sure that none of the cases that connect to Pin1 touch anything metal (e.g. truss).
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Jordan Wolf

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Re: Grounding and RFI
« Reply #11 on: February 17, 2014, 09:00:12 am »

Make sure that Pin 1 is connected to the XLR case ONLY where two cables connect together (e.g. mic cable to snake), never where a cable connects to a piece of gear.  Check all cables and snakes on both ends.  And make sure that none of the cases that connect to Pin1 touch anything metal (e.g. truss).
+1

If Pin 1 is connected to the shell, it's usually only on the male end.  However, if your cables haven't been checked out in a while, you might as well look at both ends to ensure everything is proper.
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Jordan Wolf
<><

"We want our sound to go into the soul of the audience, and see if it can awaken some little thing in their minds... Cause there are so many sleeping people." - Jimi Hendrix

Kevin Graf

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Re: Grounding and RFI
« Reply #12 on: February 17, 2014, 11:31:20 am »

As I wrote in reply #3: (and in many other threads)
Jim Brown, being both AES past committee chair on EMI/RFI and a Ham radio operator, often covers interference in his many papers.
http://www.audiosystemsgroup.com/publish.htm

*********************************************

This paper may be the best place to start, although there are several others on interference.
Don't let the title fool you, this started as an audio system paper, the a lot of Ham information was add:

"A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing"

by Jim Brown K9YC
Audio Systems Group, Inc.
http://audiosystemsgroup.com

The basis of this tutorial is a combination of my engineering education, 55 years in ham radio, my
work as vice-chair of the AES Standards Committee working group on EMC, and extensive research
on RFI in the pro audio world where Iíve made my living.

http://www.audiosystemsgroup.com/RFI-Ham.pdf
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Frank DeWitt

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Re: Grounding and RFI
« Reply #13 on: February 17, 2014, 08:11:54 pm »

Part of the reason I asked this is that I have fought RFI on our church choir mics for years. 


Here is a link to a thread on the topic.  That link has links to other threads.  Bottom line, ferrite beads.
http://forums.prosoundweb.com/index.php/topic,142374.msg1319364.html#msg1319364
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Kevin Graf

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Re: Grounding and RFI
« Reply #14 on: February 17, 2014, 09:20:52 pm »

There is a lot more to ferrites than just popping one on the cable. See the links in my reply #12.
Jim Brown's Ham group sometimes makes bulk purchases of up to 1000 chokes.
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Lyle Williams

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Re: Grounding and RFI
« Reply #15 on: February 18, 2014, 08:42:04 am »

Series inductance (eg small beads on each lead, not the clamp over the cable jacket style) to keep the RF out and shunt capacitance to give it somewhere else to go...
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Frank Koenig

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Re: Grounding and RFI
« Reply #16 on: February 18, 2014, 02:31:47 pm »

Series inductance (eg small beads on each lead, not the clamp over the cable jacket style) to keep the RF out and shunt capacitance to give it somewhere else to go...

The beauty of ferrite beads is that they're so lossy that, as circuit elements, they act more like frequency-dependent resistors than inductors, so that the Q (ratio of total energy stored to energy dissipated per cycle) of the lowpass stays low, which is generally good.

Without getting deep into the art of RFI filter design, some considerations include selecting a ferrite material appropriate for the frequency range, and selecting a shunt capacitor big enough to attenuate low RF without dragging down the signal while having low enough series inductance to remain effective for high RF. The physical layout with respect to the shield must be such that the RF doesn't leak around the whole shebang, and it has to fit.

--Frank
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Mike Sokol

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Re: Grounding and RFI
« Reply #17 on: February 18, 2014, 07:05:39 pm »

Just to throw another bone in the soup, it's also possible for RFI to sneak into speaker wires. You wouldn't think there would be enough RF energy to hear on a speaker, and that's true. However, if you have an amplifier which suffers from TIM (Transient Inter-Modulation Distortion) then any RF that creeps in the backdoor through the speaker wires can get into the negative feedback loop of the amplifier output stage. TIM was caused mainly by poor amplifier designs with too low of a slew rate in the negative feedback loop. Audio transients would overload the negative feedback loop's slew rate, which caused a splattering of distortion only during these transients, but which could not be detected by steady state tone testing. I figured this out about 30-years ago when my Dynaco Power amps of the day would pick-up CB radios in the street and "broadcast" them into my speakers during those tender love songs. The fix was adding small disc capacitors on the speaker wires where they connected to the amp outputs, effectively shorting out the RF before it got into the amp where it overloaded the negative feedback circuits. 

RFI will make you crazy since it can sneak into so many places and cause a lot of trouble.
« Last Edit: February 18, 2014, 07:16:02 pm by Mike Sokol »
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John Roberts {JR}

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Re: Grounding and RFI
« Reply #18 on: February 18, 2014, 08:49:56 pm »

Just to throw another bone in the soup, it's also possible for RFI to sneak into speaker wires. You wouldn't think there would be enough RF energy to hear on a speaker, and that's true. However, if you have an amplifier which suffers from TIM (Transient Inter-Modulation Distortion) then any RF that creeps in the backdoor through the speaker wires can get into the negative feedback loop of the amplifier output stage. TIM was caused mainly by poor amplifier designs with too low of a slew rate in the negative feedback loop. Audio transients would overload the negative feedback loop's slew rate, which caused a splattering of distortion only during these transients, but which could not be detected by steady state tone testing. I figured this out about 30-years ago when my Dynaco Power amps of the day would pick-up CB radios in the street and "broadcast" them into my speakers during those tender love songs. The fix was adding small disc capacitors on the speaker wires where they connected to the amp outputs, effectively shorting out the RF before it got into the amp where it overloaded the negative feedback circuits. 

RFI will make you crazy since it can sneak into so many places and cause a lot of trouble.

Not to quibble with teacher but TIM is an overly esoteric explanation for a simple and fairly well understood mechanism by many (perhaps not the young David Hafler of Dynaco) audio engineers. It's about the slew rate of the active amplifier (not the negative feedback loop which is passive and doesn't have a slew rate.) When the active amplifier is not fast enough to keep up with the too-fast input signal the output slews as fast as it can but still distorts the output. When that too-fast input is audio modulated RF, that distortion decodes (aka rectifies ) the RF into radio station audio.

In the case of power amplifiers that often have small inductors in series with their final output stage to isolate capacitive loads (for stability), RF can snake backwards into the should-be low Z output that is now high Z at radio frequency and back down the (passive) feedback network to the amplifier input. The amplifier does not know or care how the too fast signal got into it's input, but tries to keep up and can't.

This is not very common, but does happen. Sometimes the speaker run is just the magic length for some strong local radio station. Sometimes the amp is not very fast. 

Note: It is possible to design an amp that harmlessly rolls off too-fast input signals, but not every amp designer got that memo, either.

JR
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Mike Sokol

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Re: Grounding and RFI
« Reply #19 on: February 18, 2014, 09:01:31 pm »

Not to quibble with teacher...

JR, I'm not really a teacher... just a student trying to figure it all out. Lot's of fun, isn't it?  8)
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