[Elecraft] ferrites for subwoofer: before or after isolation transformers?
Dave Cole
dave at nk7z.net
Sat May 23 19:38:58 EDT 2020
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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> >
> >
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