Re: Ferrite on audio leads passing near PC?

Don wrote:

On Sun 29-Mar-2009 15:11, Baron
<baron.nospam@xxxxxxxxxxxxxxxxxxxxxx> wrote:

Don wrote:

On Sat 28-Mar-2009 17:50, Salmon Egg
<SalmonEgg@xxxxxxxxxxxxx> wrote:

I have seen this thread for a while without reading it.
Because of it longevity I will throw in my 2¢ without even
knowing what has been written in the past.

All the ferrite beads do is add lossy inductance to the
common mode propagation along the vague transmission line
formed by all of the conductors in parallel against free
space and the surrounding environment. It is the equivalent
of winding the cable on a transformer core. Ordinarily
adding these beads will help primarily in two situations.

1. The receiving device is sensitive to common mode
signals.

2. There is regeneration or parasitic circuitry that could
cause oscillation to break out,

In the former, the inductance "chokes" the common mode.
There are other ways of handling the problem. For example,
coupling the audio signal, through an audio transformer to
the receiving device will prevent the common mode signal
from being transmitted.

For the latter, the beads introduce loss and change other
circuit parameter. That may be enough to suppress
oscillation.

If you can, understand what is going on rather than merely
try anything.

Bill


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?

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.


Thanks Baron. So my blobs represent situation #2 in Bill's posting:

"There is regeneration or parasitic circuitry that
could cause oscillation to break out"

"the beads introduce loss and change other circuit
parameter. That may be enough to suppress oscillation."

Seems the blobs probably attenuate the (digital) signal. I'm not
sure I want that based on the understanding that an attenuated
signal is more prone to interference. OTOH ISTR USB uses 5volts
which is a fairly high level.

Maybe this is not going to help me after all with my analogue
leads.

Er, not quite ! You commonly see them on leads coming from equipment
that uses HF energy, like switch mode PSU's, CRT monitors etc. They
are they to suppress any leakage of that HF energy via the connecting
cable.

If you look carefully most of these leads are of the Co-Axial variety
and the energy that is being suppressed is carried on the outside of
the screen. Signals flowing on the inside of the screen are
un-affected.

--
Best Regards:
Baron.
.

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