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.
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|