Pin 1 Problem

[n1ps]

I'm sort of surprised that I cannot find any link to this subject within this site. 

I have been looking into why I seem to be having some hum and noise problems (both common mode and differential), including some RF feedback when I use my "E" transmitter.  I have the problems with some but not all pro audio equipment.  I recently stumbled onto a few articles written about something called the "Pin 1 Problem" which was first discovered back in the mid nineties by a fellow by the name of Muncy.  I believe this is the root cause of some of the noise I have been experiencing (not limited to the E rig).  I plan to go through all my audio pieces and test them.  Should be interesting when I get finished.

Go to this link:        http://pin1problem.com/   

and a more in depth look at noise and the related Pin 1 Problem at:
   
http://www.audiosystemsgroup.com/RFI-Ham.pdf

Apparently many electronics gadgets, including pro audio and ham radios, have this problem. It refers to pin 1 of the XLR connector.  Normally this pin SHOULD go to chassis ground but frequently gets tied directly to the signalling processors within the chassis, delivering hum and noise directly to the processors.  I have not read Muncy's work, but I have read several other papers that confirm that this is indeed a common problem and also a root cause of much of the hum and noise we frequently encounter when using "professional" audio equipment including microphoniums.

I also have been using foil based shielded cables (recommended by a broadcast engineer a few years back) and discovered that they too may be a problem.  It seems that foil type shielded cables are OK around VHF frequencies but within the near field of HF, can add something in the order of 30dB more noise versus braided copper cables for say an audio patch cable (length not specified).  I think several people have posted papers on this...google it to review.  I think Jim Brown, K9YC, amongst others, has done some research on this.

I would like to find a good shielded twisted pair (single) cable that is only copper braid (no foil), hopefully with at least 70% braid.  Any recommendations?

73,
Peter
n1ps

[WBear2GCR]

Ummm... I am unaware that foil shield is ineffective at frequencies lower than VHF. To the contrary, afaik it is superior as it is 100% shielded. Otoh, maybe there is a reason, which might relate to some sort of capactive relationship with the wires??

I'll try to read the citations... see what they're on about...

maybe there is something new to learn here...

        _-_-Wombat bear two gorilla cheetah rabbit

[WBear2GCR]

Ok, so the solution turns out to be a cap to the chassis ground outside the box and then a choke (ferrite bead) in series with the ground wire that goes into the box...

Mmmm... 

Of course the XLR connector doesn't actually support such a connection unless one puts the XLR into its own enclosure first... which in some cases might make sense in the form of an adapter of sorts... but he's talking about VHF and up freqs being a problem, for hams doing HF, not an issue.


                   _-_-bear

[n1ps]

Hi Bear...I think we have talked once or twice on 75.  Yes, maybe the 1/4" connector is better in this regard? 

In regard to frequencies, the issue is more pronounced with longer cables as they are a larger percentage of the wavelength we work like 75.  The good news is most of the noise is of the common mode type, so ferrites can help in such cases.

~ps

[WA1GFZ]

I don't know about the audio world but the mil world we never bring a shield into a chassis. It is terminated to the connector backshell. When you bring the shield into the chassis everything floating on the shield is now coupled into the device. Also any crud generated inside the chassis appears on the shield. Terminating a shield inside a chassis is just like putting a loop pick up in a cavity. The only safe way to bring a shield into a chassis is if there is an outer shield isolated from the inner shield. The outer shield goes to the connector back shell while the inner shield goes into the chassis for say impedance control. IF the audio shield has to go inside the unit I would add an outer shield braid to the cable and clamp it to the connector backshells. ( small SS hose clamps work well) Sometimes ground loops can be broken if only terminated at the source end. Then there is lightning where you want to keep induced currents out of the unit and on the surface of the chassis outside the electronics.

[WBear2GCR]

Oy!

It gives me a headache thinking about it!

So are your saying that the braid between the two backshells are or are not connecting the two chassis together??

If the shield floats on one end, is it really shielding?

 

                    _-_-

[Steve - WB3HUZ]

If the shield floats on one end, is it really shielding?

Sure. Why not?

[WBear2GCR]

It's been said that it is called an "antenna" in that case...?


         _-_-bear

[Ian VK3KRI]

It's been said that it is called an "antenna" in that case...?


         _-_-bear

No , its an electrostatic shield , If its long in terms of RF wavelength and the unterminated shield is 'aboveground'  it doesn't matter because it couples equally into each side of the balanced pair.  That is assuming the pair and its source and load are balanced.

XLRs with phantom powered mics are a bit different as the shield is the earth return for power feed, but if the balance is correct, the power feed and the signal shouldn't interact.

                                         Ian VK3KRI

[WBear2GCR]

Except that the articles are talking VHF and UHF freqs being a problem!
So it's long in terms of wavelength! 

               _-_-bear

[Steve - WB3HUZ]

Not really. Plain wave shielding is almost 100% from reflection. A ground is not even needed. The reason the foil is provides mimimal shielding at the very low frequencies, and especially low frequency magnetic fields, is because here effective shielding is mostly from absorbtion. The thin foil does not absorb very well.

It's been said that it is called an "antenna" in that case...?


         _-_-bear

[n1ps]

Here is another interesting article by the same author (K9YC) on foil cables: 

http://www.rane.com/note166.html

I copied this paragragh:
My research shows that braid-shielded cables maintain this advantage to at least 2 MHz, but gradually begin to lose it above that frequency. My more recent work shows that the braid cables are still better to at least 7 MHz and about the same as foil/drain construction at 14 MHz. By 28 MHz, the foil/drain cables are better.

~ps

[n1ps]

TNX to all who posted on this subject.  I looked over my "pro audio" gear and concluded that they all have pin 1 problems, meaning the connector shells are insulated from the chassis and connect them directly (without passing GO) to the internal circuit boards. This includes the mixer, equalizer, and of course the compressor/limiter, all various brands.   I'm not certain I can rewire the connectors and shields, but I can add some ferrites to choke out the noise in hopes that this works.  Frank (GFZ), I like your idea on the hose clamps and shield.  I may try that on some of the pieces.  I'm thinking of bringing over some braid from the chassis of each piece to the connector body, but it just seems easier to wrap a big 31 grade or 43 grade ferrite. 

On the foil wiring, I'm afraid I'm an idiot (yes, the cat's out of the bag .  I mistakenly believed that foil shields were the latest and greatest but in fact are inferior to the old and trusty copper braid.  No wonder the foil crap is so cheap. 

I posted this because I don't believe for a minute that the problems that plague other fields, such as audio and broadcasting, are unrelated to what we do.  But what do I know?    Don't answer that actually...

73,

Pete
n1ps