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Re: [RFI] Noisy PC Speakers

To: Rfi List <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 13:50:09 -0700
List-post: <mailto:rfi@contesting.com>
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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