When there is a radiated or conducted problem with a piece of equipment,
there are two VERY GENERAL methods to reduce problematic emissions:
A) Reduce the level of emission at the source (source suppression), or
B) Reign in the radiating structure. a.k.a., antenna.
In this case, a length of conductor attached to the problem source becomes
an antenna. It's that simple!!! Given and antenna, radiation of energy
happens. Even dressing a long conductor in the near vicinity of a magnetic
source like an inductor within a SMPS can become an efficient radiator of
the switching frequency and harmonics.
Dave - WØLEV
On Tue, Oct 6, 2020 at 3:27 PM Hare, Ed W1RFI <w1rfi@arrl.org> wrote:
> No, again, this was not a change in impedance, at least not much of one.
> We initially terminated the speaker outputs with 8 ohm resistors. We
> measured conducted emissions. We then put real speakers on the amplifier
> at the end of 16 foot speaker cables. Emissions conducted on the ac mains
> increased. We replaced the speakers with 8 ohm resistors at the end of the
> cables. The conducted emissions stayed about the same. I also connected a
> piece of wire to the ground on the audio amplifier PC board and the
> conducted emissions went up, telling me that if I were to ground the
> chassis of an audio amplifier, conducted emissions onto the AC mains might
> increase.
>
> For power supplies, we saw similar effects. Putting the load for the
> power supply at the end 10+ foot wires caused emissions onto the AC mains
> to increase over what we measured when the power supply was loaded with a
> resistor with short leads. Connecting a piece of wire to the chassis of
> some supplies caused conducted emissions onto the AC mains to increase, in
> a way that seems related to the resonance of the wire. Touching the power
> supply case did the same thing.
>
> This is an anomaly I haven't seen documented anywhere.
>
>
> -----Original Message-----
> From: RFI <rfi-bounces+w1rfi=arrl.org@contesting.com> On Behalf Of Jim
> Brown
> Sent: Monday, October 5, 2020 4:50 PM
> To: Rfi List <rfi@contesting.com>
> Subject: Re: [RFI] Noisy PC Speakers
>
> That's probably because virtually all modern audio power amps are constant
> voltage devices -- VERY low output Z, typically 1/100 of the nominal load
> impedance. Up to the current limits of their power supply, current (and
> power to the load) will increase inversely with load impedance. It's not
> uncommon for solid state power amps to be specified for any load from 16
> ohms down to 2 ohms.
>
> That 1/100 ratio is called the "damping factor," because the source Z of
> the power amp prevents woofers from getting sloppy.
>
> Likewise, mic level and line level audio output stages have much lower
> output Z than the circuits they are designed to drive. There, a ratio
> between 1:10 and 1:50 is typical. Hams seem to have missed the fact that
> 600 ohm circuits haven't been used in pro audio or broadcasting solid
> state took over from tubes. The only exception seems to be those engineers
> working ONLY in radio systems -- several years ago, I had a conversation
> with a designer of big BC XMTRS who hadn't gotten the word, and the ham
> community has totally slept through it.
>
> In pro audio and broadcast studios, 600 ohms is used ONLY to specify the
> minimum value of load Z that the output stage can drive, thus its current
> capability. Pro line level outputs are typically ~100 ohms and inputs are
> typically ~10Kohms.
>
> 73, Jim K9YC
>
> , On 10/5/2020 11:00 AM, Hare, Ed W1RFI wrote:
> > 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.
>
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--
*Dave - WØLEV*
*Just Let Darwin Work*
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