< Second, every wire/cable connected to a noise source (in this case, the Class
D amplifier, but also including switch-mode power supplies) forms a
transmitting antenna for common mode current on those wires. So nothing in your
lab tests is surprising.>
Actually, it is very surprising, at least to me, because when we changed the
nature of the speaker loads, the conducted emissions on the AC mains increased.
-----Original Message-----
From: RFI <rfi-bounces+w1rfi=arrl.org@contesting.com> On Behalf Of Jim Brown
Sent: Monday, October 5, 2020 1:49 PM
To: rfi@contesting.com
Subject: Re: [RFI] Noisy PC Speakers
On 10/5/2020 8:59 AM, Hare, Ed W1RFI wrote:
> We then hooked up 16 feet of wire and real speakers. Bingo! The noise level
> went up dramatically. It was my guess that the speaker leads were radiating,
> so we tried a common-mode choke and bypass capacitor. It made a bit of a
> difference, but it still was not nearly as quiet as it was when we terminated
> the speaker connections with resistors. We tried a better filter on the
> speaker leads and the noise actually went up slightly.
Great lab work! Several observations. First, the zip-cord, glorified or
otherwise, commonly sold as "speaker wire" is a recipe for radiation of
differential current on that line. The smarter sound contractors, especially
those run by hams, have long used un-shielded twisted pair for loudspeaker
runs. Twisted pair provides at least 20-30 dB of crosstalk rejection, depending
on the twist ratio and the uniformity of the twist.
Second, every wire/cable connected to a noise source (in this case, the Class D
amplifier, but also including switch-mode power supplies) forms a transmitting
antenna for common mode current on those wires. So nothing in your lab tests is
surprising.
A major problem we face with EMC is that both laws and test methods fail to
realize that every cable is an antenna, whether its a power cable, an audio
signal cable, a control cable, or even a grounding/bonding cable.
If good engineering practice is not applied to prevent the presence of RF
current on any of these cables, that RF WILL radiate. Prevention can be
anything from proper bonding and bypassing to circuit/PCB layout to shaping of
waveforms to minimize the RF content.
I documented a grounding cable radiating power line noise at a decommissioned
AT&T Long Lines site owned by another ham, and where I had set up an HF
station. Just outside the building, the ground lead coming down the wood pole
was radiating noise generated elsewhere on that line (I detected it using the
HF RX section of my Kenwood TH-F6A VHF/UHF talkie).
73, Jim K9YC
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