Ferrite on audio leads passing near PC?

I do understand (as John Larkin posted in this thread) that distance is my friend ~ to the power 4!

However, sometimes distance is hard for me to control. Even if I extend the length of the lead and try to reroute it the lead may still have to run close to potential sources of interference especially where it has to approach a device such as a PC to terminate its run.

On a related topic can I ask this ... if I have a stereo source then is it much better to use a twin cable in which each core has its own shielding than a cable in whch both cores are together and inside a single shielded?

Alas, I don't see that you have any practical alternative. No amount of filtering, shielding, or magic cable is half as effective.

Probably not. Many of us regularly use "twisted pair" shielded for L/R stereo. I just finished building eight 50-ft stereo mic cables with "star-quad" cable which has no shielding between the L/R pairs. I anticipate no significant issues with that practice.

Don wrote in news:Xns9BDC17F2C469E5D4AM2@69.16.185.247:

Yes. Small losses due to capacitance rarely matter in this case so close individual screens are good, and even over longer runs it's mainly an attenuation of higher frequencies well beyond those for audio except in runs of a hundred metres or more where it might best be considered differently.

You might lose signals to the other line causing crosstalk and reduced stereo width in much shorter runs if the channels are not screened from each other. It might not be significant but if for any reason you want one to be silent while the other is at full bore (studio effects and panning), it really pays to screen them separately.

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.

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.

sci.electr>

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.

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.

"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.

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?

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.

That's a great chart. I printed it out. I found my chosen suppliers didn't have several of the leads or even list Belden reference numbers in the same format as the table. Is the chart out of date?

After seeing your chart I went here.

The Belden range is bewildering. Even the guides to cables are bewildering! For example:

Cable Finder Guide (780k)

Residential Cables (1.2MB) It's too much!

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

That's the problem! Shielded audio cable is good for eliminating electrostatic interference, which is a common cause of audio noise. But, it does practically NOTHING to eliminate low-frequency magnetic (inductive) interference. And, a computer monitor is exactly a source of magnetic radiation at several frequencies. The horizontal sweep and the switching power supply frequencies can be shielded to some extent, but it is not really practical. The vertical sweep (50 - 100 Hz) can't be shielded without insane effort, so the only real way out is to eliminate its influence on the circuit. That's why professional audio types use balanced circuits for mikes and all sorts of other low-level signals.

in the presence of interference sources, clearly NOT true!

Forget this diameter stuff. Unless the cables were FEET in diameter, and almost all solid copper, they'd have NO effect on 60 Hz magnetic fields. These fields penetrate conductors.

If you can route long cables away from the source (the CRT is by FAR the worst offender) that is the simplest approach. If not, a pair of transformers and a balanced, shielded cable should also work. But, the TRANSFORMERS need to be shielded! Many cheap audio transformers are NOT magnetically shielded, just put in a pot-metal box. if they are near any power transformers or CRTs, they can make the problem WORSE! If you think a single wire stretched in space can pick up a signal, how bad would 10,000 turns of wire be?

Jon