[RFI] Noisy PC Speakers

[Jim Brown]

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