Ferrite on audio leads passing near PC?

On Thu, 26 Mar 2009 20:01:00 -0700, John Larkin wrote:


Reminds me of something that happened in the late '60s. I got dragged into a problem with a newly-commissioned ink drying oven on a web offset printing line. Temperature sensed with a type T thermocouple, driving a controller consisting of one of those "new-fangled" 709 op amp things driving a panel meter with another 709 as a comparator giving on/off set point control. Yes, they did things like that, back then.
The controller burst fired a 3-phase thyristor bridge, using, wait for it, unijunction oscillators. Whoever designed it (no names) must have felt mighty proud to be right at the cutting edge :-)
Problem was, the input 709 kept dying. It would run maybe half an hour, then poof. Maker's guy had been tearing his hair out for a week.
On the bench, the thing would run forever. Back on the machine, instant death. Voltages all within limits. It wasn't until I hefted a scope onto site to look at the supply waveforms that I found the tens of volts of common mode HF on the thermocouple every time the thyristors fired. Not only on the TC, on every bit of metal nearby. Ironically, the sales blurb claimed that it used zero-crossing switching and was RF immune, an absolute lie. (30 kilowatts, burst fired.)
Fifty cents worth of ferrite and ceramic caps cured it in half an hour.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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wrote:

Hi Bill, thanks for the detailed info. You're right that I was guessing about the idea of using a ferrite without really knowing the theory! :-)
I was making my assumption based on my observation that all the USB leads which have come with my dictation machines or MP3 players have a plastic "blob" on them.
I was told this blob is a ferrite and that it's purpose is to prevent trouble from interference. I didn't get told if the blob prevented "incoming" intereference affecting the signals on its lead or, alternatively, if the "blob" limited "outgoing" interference being generated by using the lead. From what you say, I guess the idea of the blob containing ferrite is incorrect.
Perhaps the blob does not contain a ferrite but something else? So I dug around and got this interesting web page. http://www.bitpim.org/help /
<QUOTE> "This cable has a blob half way along the cable that converts from USB to serial and then connects to the serial interface in the phone. The chip inside the cable is a Prolific 2303 and this is how your operating system will report it. It is marked as for the LG VX1/10 but works fine on the VX4400. </QUOTE>
There's a chip in their blob? WOW. Well I never use my blob leads! I use just ordinary USB leads and there seems to be no problem.
Admittedly in this case the lead carries *digital* signals which go to the USB port. By contrast, I had been asking about *analogue* audio on a shielded lead going into the line-in socket of a PC (or perhaps going to some other device).
(a) So what is really in the blobs in my leads? (EG on my Olympus WS-331M) The above link talks about conversion to serial but I don't think my blobs would need to do that.
(b) Whatever is in the blob, would one of those help reduce interference on my intended longer leads because the leads will run near equipment and will carry analogue audio signals?
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Don wrote:

Yes the blob located near to one end of the lead is there to stop radiated interference from the lead. Yes it is a ferrite tube. Note "From" not to.
--
Best Regards:
Baron.
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You probably don't need them. Computer equipment has many localized switching power supplies running in the 10A range. Even a very solid ground plane will have some RF noise on it. Better computers feed peripheral connectors through ferrite blocks to fix this. Cheap or compact computers may rely on a ferrite cylinder over the cable.
There should be no advantage to putting ferrite beads on cables passing near the computer. If it does, there's probably something broken.

Get the specification for the shield coverage. Musical instrument cables are about 95% copper coverage plus 100% conductive PVC coverage.
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wrote:

Your info about shield coverage reminds me that I once heard someone guess that shielded twisted pair would make good audio cable. Any views on using that?
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"Don" wrote ...

Twisted pair (whether shielded or not, as the situation dictates) is a great way of reducing noise *IF* you have equipment (and especially at the destination/input end) that has a *balanced* input circuit. If your (still unidentified) equipment has only un- balanced connections, then using twisted pair over such a short distance is likely not worth the effort to try it. Moving the cable away from the noise source is a *much more likely* method of remedying your problem.
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In fact good quality balanced audio cable usually is shielded twisted pair. But not exactly the same as similar computer cable since the requirements are a little different.
MrT.
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The shielding is often better. If you're running fixed balanced audio lines it might be worth using. As far as I know, the main thing with audio lines is they're made to meet a wide demand for strong and flexible cables with soft coverings.
Given that audio uses a low output impedance into a high input impedance, it doesn't matter what the impedance of the cable is, if it's a balanced line in a twisted pair inside a foil sheath, it's ideal, so long as it's not flexed to damage point. What is NOT a good idea is using audio cables for S/PDIF and other high speed digital transfers.
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Don wrote:

I usually use one at each end of the cable.

Something with good shielding.Be it some kinda of pro audio cable,instrument cable,or a good old fashion coaxial cable.
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In sci.electronics.components,alt.engineering.electrical,sci.electronics.design,rec.audio.tech,

I agree with (most) the other posters, ferrites are unlikely to help with a direct baseband audio signal being injected, and perhaps the easiest solution is use a longer cable that doesn't go as close to the CRT.
I'm thinking you could use audio transormers (which itself may be sensitive to the CRT's deflection coils from several feet away) at each end and run a balanced cable between them. "Star Quad" type configuration would be best to minimize/cancel interference. But all that gets expensive.
Another "expensive" idea - replace the CRT with one of the new-fangled flat-panel display things, surely they generate less interference, and they also take up less space.
Or move the CRT. Get a few phone books and/or encyclopedias to put under it and lift it up, just a few inches may help.
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Ben Bradley wrote:

sci.electronics.components,alt.engineering.electrical,sci.electronics.design,rec.audio.tech,
Recalling an old trick/cure with low mv level magnetic pickups for record players - use two conductors inside a shield and ground the shield at one end only to avoid a ground loop. It works well for 60/120Hz and I see no reason for it not to work at audio frequencies. No guarantees but it is worth a try.
--
Don Kelly
snipped-for-privacy@shawcross.ca
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Don wrote:

If the interference is magnetic radiation from the CRT deflection and (at startup) the automatic degaussing coil, ferrite will not help at all. Wires in the presence of an AC magnetic field will pick up a voltage. The only hope is to lay the wires parallel to the magnetic flux lines, or get them out of the area. Only MASSIVE shielding with Mu-Metal shields will help, by conducting the flux lines in the shield. I suppose in theory totally enclosing the wires in a string of ferrite rings would be going in this direction, but separate ferrite rings are no comparison to a solid Mu-metal shield. Usually these are used to protect CRTs, photomultipliers and vidicons from stray magnetic fields, and they certainly do work. I've never seen it used to protect a cable.
Putting a balanced-unbalanced transformer at each end of the cable would be another time-honored fix. Then you can have a truly balanced signal even with equipment that is unbalanced.
Jon
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