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Re: [RFI] Noisy PC Speakers

To: rfi@contesting.com
Subject: Re: [RFI] Noisy PC Speakers
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 5 Oct 2020 10:48:47 -0700
List-post: <mailto:rfi@contesting.com>
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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