[Elecraft] ferrites for subwoofer: before or after isolation transformers?

[Peter Dougherty]

I'm experiencing something similar. I have my sub-2 output connected to bass
shakers on my sofa, and the wiring picks up an incredible amount of RF on
20. As my wife found out when she was taking a nap in the room while I was
contesting. As soon as I switched to 20 she was getting rumbled to the
pattern of CQ TEST. That was an interesting conversation, and something I
will look at this summer.

As for your low frequencies getting into your left and right main, why not
switch them to "small" in the AVR menu, and let the sub do all the work
below 80 Hz?

I have had some luck putting ferrites just before the speaker terminals in
the past. I sadly cut all my speaker cable to length so there's no room to
wind around a core, so these will need to be clamp-ons for me. I get into my
center and surrounds on 40 and 20.

Or I can tell my XYL to nap in the living room, since there's just the two
of us in the house.

 - pjd

-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
Behalf Of Nicklas Johnson
Sent: Saturday, May 23, 2020 12:38 PM
To: elecraft <elecraft at mailman.qth.net>
Subject: [Elecraft] ferrites for subwoofer: before or after isolation
transformers?

The backstory as briefly as I can make it: I wanted to place my home theater
subwoofer in the corner of our living room; doing so required running two
speaker wires and a coaxial cable under the house and plugging the subwoofer
into a different outlet than the AV receiver; this in turn resulted in
ground-loop hum (because of a tiny difference in potential between the two
outlets) which I worked around with a set of 1:1 low-frequency audio
isolation transformers.  The subwoofer is of a type that produces a signal
based not only on the LFE channel, but also on the left and right speaker
channels, thus the two speaker wires along with the coaxial cable.

Now the subwoofer is picking up common mode noise on 20m, which isn't
terribly surprising, as this happens a good bit with consumer-grade
electronics. I'm hoping to mitigate this with some substantial ferrite
clamps for all three connections and as many turns as I can get through
them.

My hunch is that the best place in the path to clamp them on will be
immediately before the connection to the speaker itself, on the speaker side
of the isolation transformer, but I wanted to get the opinions of folks who
have solved this problem in the past to see if there's any reason the
ferrites should come before the isolation transformers.

Thoughts?

   Nick

--
*N6OL*
Saying something doesn't make it true.  Belief in something doesn't make it
real. And if you have to lie to support a position, that position is not
worth supporting.
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