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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