[RFI] Noisy PC Speakers

[Jim Brown]

Not surprising. Twisting primarily affects differential mode coupling. 
Audio power amps use a lot of feedback around the output stage, so any 
RF present on the speaker cable will be fed back and amplified. This is 
a common cause of RFI TO audio power amps, but so is common mode current 
on the speaker cables.

Another point: the vast majority of audio transducers (speakers, mics, 
headphones) are not even close to resistive. Rather, their impedance is 
complex at all frequencies, and is the result of resonances in each of 
the transducers and variations in the inductive reactance of transducers 
at the high end of the range of each. Resonances are both mechanical and 
electrical, partly the result of interaction with the enclosure; low 
frequency transducers have both, higher frequency transducers are mostly 
electrical.

The nominal impedance of an audio transducer is defined as it's minimum 
impedance in the mid-range; an 8 ohm loudspeaker typically varies from 
it's nominal value to several hundred ohms at one or more resonances and 
at the top end of its operating range. Loudspeakers are designed so that 
their acoustic response will be smooth when driven from a constant 
voltage source, and the best loudspeakers will have a well-controlled 
polar response in three dimensions.

It is a very bad idea to tie any audio power amp output terminal to 
reference ground -- some output stages can self-destruct when this 
happens, and it can cause stability problems.

My perspective on these issues is as a Fellow of the Audio Engineering 
Society, 50 years in the industry, and membership in AES Standards 
Committee Working Groups on Microphones, Loudspeakers, and Connectors, 
and Vice-Chair of the WG on EMC.

73, Jim K9YC

On 10/6/2020 8:37 AM, Cianciolo, Paul, W1VLF wrote:
> In addition to Ed's comments "twisting" the wire had little if any effect.  Connecting one 1 of 2 speaker leads lead to a slight decrease in emissions. A single wire added to either of the speaker outputs has little effect.
> Basically as soon as you connect anything to the speaker terminals that had actual length..  greater than a foot or so emissions started increasing.