I/C with shield lifted one end

I think you mean The RFI Problem (click the link)

Try also reading this EMC awareness-shielding of cables

If taken out of context, the 3rd paragraph looks like it's recommending lifting one end of the shield, and in fact it contradicts the first line of paragraph 2. Such is the difficulty of explaining the problem. Unfortunately there is a flood of misinformation from IC makers that correct explanations will go unheeded.

In prestigious professional installations where RFI has to be avoided at all costs, such as "west-end" theatre productions, signal cabling now has shields connected both ends.

The thing the hi-fi people latched onto was the excess currents flowing in shields, even though professionally we're talking about very long cable runs where cable burn-out can result.

Admittedly, lifting one end can resolve ground loops, but this is a crutch because so many pieces of equipment fall foul of proper internal grounding. Once you lift one end the cable can no longer reject electromagnetic fields (radio frequencies).

The practice of lifting the shield one end should be done with caution, and the RF spectrum in the vicinity can change over time, and therefore today's solution may become tomorrow's headache.

Considering the information I've presented here - good solid scientific information - the deliberate lifting of the shield at one end on a production basis seems the height of stupidity.

Edit: if you search the web long enough you can uncover lots of information about shielding for EMC - the reason why cable manufacturers have always made shielded cable ever since radio was conceived. Somehow proper cable shielding makes sense to me.


Edited by Graham Slee - 20 Mar 2008 at 4:19pm

Hi tg,

You note correctly. I hadn't got around to that.

The good news is you may get away with it.

But as you note, the direction has an effect, and therefore there has to be some RFI magnetic field straying onto it for you to notice this subtle difference. Either that or it could be the directional aspect of the cable structure which unfortunately I can't explain, but I have experienced.

A customer reported that he'd used a Nordost Solar Winds IC (a reportedly non-shielded cable) between his Reflex and his amp and on upgrading to MC by purchasing the EXP, he got nothing but hum. He initially thought about complaining about it to me, but he actually decided to buy a Cusat50 online instead (divine intervention?). After receiving it and replacing the Nordost Solar Winds with it, the entire problem went away. He then decided to contact me and tell me of his findings.

This started me off on my investigations and that's how I discovered the Ofcom page linked to above, and many more covering the subject.

Although I put output resistors in the Reflex in a bid to attenuate RFI that may enter via the output, this may not effect a total cure depending on RFI signal strength in the location of use. Once RFI penetrates the output it is connected to the Reflex negative feedback input via the negative feedback path (all amps are like this with the exception of a minority of zero NFB amps whose performance is debated I'm sure in other circles). This input cannot be filtered on the Reflex nor any such amplifier stage. As such, the RFI is amplified by the Reflex op-amp, which although capable of passing video signals, will probably rectify the signal (turn it into something audible) because the frequency of TV transmission is in the hundreds of MHz which is beyond the op-amp's capabilities.

What makes for the easy detection of hum - the TV framing signal - is that this is amplitude modulated, and AM transmission is very easy to detect. However in some circumstances the frequency modulated audio is also detected, but in all reports, this is much quieter as FM is not as easily detected.

The addition of the EXP upstream can exacerbate the problem by it adding some 23dB of gain in the "system". It is possible that this stage can pass undetected RF but with some slight oscillation (like a booster), and that is all that is required to trigger the Reflex into effective detection.

Consider that many years ago RFI was less of a problem, but still manufacturers went to great lengths to include elements to prevent detection. Today the increase in the number of mobile phone transmitters is relentless as the networks compete for land to complete the cellular structure of the system. What have mobile phone transmitters to do with TV? They're a mirror! Just as you can get the sun to shine where it usually doesn't by use of a real mirror, a mobile phone transmitter does the same for RFI. In fact a steel chimney does the same as any TV aerial installer will tell you. Transmitters can also reinforce the weak transmissions from other transmitters - something I can see from my own window from the number of TV aerials down in the valley pointing at the local FM radio Barnsley repeater transmitter. My aerial being at a higher altitude and on a long pole, points the opposite way, at the real TV transmitter.

Proper shielding of cables is of tantamount importance in high gain systems and by that I mean vinyl systems. The silly practice of not shielding at all or not shielding properly, may be "OK" for CD which has unity or near unity voltage gain, but it will not work for vinyl.

Unfortunately the manufacturers of such ICs don't voice that, which helps them to a larger slice of the market, but leaves the likes of me to pick up their mess, and a quite costly mess not only for the vinyl user, but for me the cost in lost time is beyond all reason.

edit: doub;e post


Edited by Charley Phogg - 23 Mar 2008 at 8:36pm