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

[Nicklas Johnson]

I've got a set of these on the way, as well as a handful of their next two
smaller siblings, just because I like to have a variety in my desk for
various applications:
https://www.fair-rite.com/product/round-cable-snap-its-2631181381/

Given the arrangement at the subwoofer of wall-connection-->isolation
transformers-->subwoofer, would you put the ferrite right before the
subwoofer then?

I didn't think about adding one at the amp; though I haven't had problems
with any common mode noise getting into the amp from the other speakers in
the room, I can't be sure about the LFE coaxial cable, so that wouldn't
hurt.

   Nick


On Sat, 23 May 2020 at 10:08, Dave Cole <dave at nk7z.net> wrote:

> Grab some FT-240/31 ferrites from Fair-Rite, (these are the large
> rings), and put seven or eight turns of speaker cable through each,
> tight wound.  Add one at the speaker, and one at the amp.
>
> 73, and thanks,
> Dave (NK7Z)
> https://www.nk7z.net
> ARRL Volunteer Examiner
> ARRL Technical Specialist
> ARRL Asst. Director, NW Division, Technical Resources
>
> On 5/23/20 9:37 AM, Nicklas Johnson wrote:
> > 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
> >
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