On Tue,6/30/2015 5:53 PM, Paul Christensen wrote:
If RFI ingress is occurring on the input
to a PC *power-amp* speaker system, then probably the most effective and
least invasive way to mitigate the common mode current is to wrap several
turns of the input cable through #31 mix ferrite cores. Wrap as close as
possible to the PC speaker's amplifier input. It's the *turns count*
that
adds series common mode impedance. Have a look at K9YC's materials in
the
link below. For several mix types, he shows the number of turns
required to
achieve a target choking impedance for a given frequency.
It is major mistake to think only of the input -- it is VERY common for
RF to be coupled onto loudspeaker wiring and from there to the output
circuit of the power amp driving the speakers, and from there, via the
feedback network, to some part of the circuitry where it is detected.
ALSO -- virtually every piece of modern gear comes with Pin One
Problems, which is a very common cause of RFI.
Also -- ALWAYS use TWISTED PAIR for loudspeaker wiring, NEVER zip cord,
glorified or not. Twisted pair is VERY effective at reducing
differential mode coupling.
Note also that when I talk about a loudspeaker, I'm talking about a
passive element -- the loudspeaker itself. In the pro audio world, a
loudspeaker enclosure with a built-in power amp would be called an
"active" or "powered" loudspeaker. Most powered loudspeakers are
notorious for RF susceptibility, even the most expensive pro units. I
used to go through the aisles at AES shows with a Kenwood VHF/UHF talkie
and exposing the bad actors.
The common means of coupling to these active boxes are 1) Pin One
Problems on audio leads AND on mains power leads; 2) obvious circuit
layout mistakes, like breaking a "ground" layer under a trace to add a
trace on the ground layer that first layout guy forgot; and 3) the
obvious ones, differential coupling onto signal leads and 4) inadequate
shielding. Capacitors across inputs and outputs affect ONLY #3.
Single turns through almost any core are useless below VHF.
73, Jim K9YC
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