[Elecraft] ferrites for subwoofer: before or after isolation transformers?
Nicklas Johnson
nick at n6ol.us
Sat May 23 20:01:10 EDT 2020
I certainly agree. And of course there's no "one size fits all" for RFI
problems. I think it's somewhat likely I have multiple problems going on
here too; I noticed, for example, that even when it's completely
disconnected from all its audio inputs, I can still hear a little racket on
the sub; it may need a hefty ferrite on the power cord, or I suppose it's
always possible it's picking up 20m from the speaker coil, then rectifying
that. I won't know until I get some appropriate ferrites delivered to test
out (mid-week).
Sadly, it seems like most consumer-grade audio equipment is optimized more
for cost than for design quality and RFI suppression, even equipment that's
supposed to be "high-end." And then sometimes you can buy a piece of cheap
gear and it rejects RF better than high-end stuff. Kind of a crap shoot.
I appreciate all the suggestions and pointers. In the absolute worst case,
I can always play on another band when my better half wants to watch a
movie or play a game, too.
Nick
On Sat, 23 May 2020 at 16:39, Dave Cole <dave at nk7z.net> wrote:
> All of Jim's material is like gold for RFI suppression.
>
> 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 2:02 PM, Nicklas Johnson wrote:
> > Thanks, Dave. A very good point about the amp picking up stray RF off
> > the cable and returning it as audio; I'll be sure to clamp down on both
> > ends.
> >
> > It's definitely not a new problem, and I've used Jim's recommendations
> > to much success in the past. In fact, I referenced it again today
> > because I couldn't remember which mix of Fair-Rite was the right one.
> >
> > Nick
> >
> > On Sat, 23 May 2020 at 10:58, Dave Cole <dave at nk7z.net
> > <mailto:dave at nk7z.net>> wrote:
> >
> > I would put the ferrite material as close to the speaker as possible,
> > and as close as possible to the amp...
> >
> > It is important you also protect the amp from stray RF. If the
> speaker
> > cable is picking up RF, and feeding it back into the audio amp output
> > stage, you can get rectification within that stage in the amp, thus
> > feeding actual audio, (not RF), back down the speaker cable into the
> > speaker(s), and then you start hearing things on the speaker(s).
> >
> > I had a ham friend living 700 or 800 feet from me-- when he lit off
> his
> > KW, I would hear SSB in the speakers, even with the amp off, and
> > unplugged. This was happening via the method above.
> >
> > See Jim's paper on quieting things down:
> >
> > http://www.audiosystemsgroup.com/RFI-Ham.pdf
> >
> > 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 10:19 AM, Nicklas Johnson wrote:
> > > 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
> > <mailto: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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> >
> > --
> > *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.
>
--
*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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