[RFI] Noisy PC Speakers

Tue Oct 6 11:37:01 EDT 2020

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.

Also I tried 2 power supplies. The first was one of the original supplies used at the housing development to power multiple devices,  and then a wall wart supply.  Very similar results. 

73,

Paul Cianciolo, W1VLF
ARRL EMC Engineer

ARRL - The national association for Amateur RadioT
225 Main Street, Newington, CT 06111-1400 USA 
Telephone: (860) 594-0392 
FAX: (860) 594-0259
Email: w1vlf at arrl.org
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-----Original Message-----
From: RFI <rfi-bounces+pcianciolo=arrl.org at contesting.com> On Behalf Of Hare, Ed W1RFI
Sent: Tuesday, October 6, 2020 11:27 AM
To: jim at audiosystemsgroup.com; Rfi List <rfi at contesting.com>
Subject: Re: [RFI] Noisy PC Speakers

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 at contesting.com> On Behalf Of Jim Brown
Sent: Monday, October 5, 2020 4:50 PM
To: Rfi List <rfi at 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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