[RFI] Noisy PC Speakers

[Jim Brown]

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.