[Elecraft] ferrites for subwoofer: before or after isolation transformers?
Nicklas Johnson
nick at n6ol.us
Wed Jun 3 15:53:17 EDT 2020
Happy to report that the subwoofer problem got about 95% better with the
placement of ferrites immediately before the subwoofer on the power cord,
the LFE channel and the speaker channels. It still picks up just a tiny
bit of 20m racket even with nothing connected to it, which I suspect could
be a consequence of the speaker coil picking up the signal and getting
rectified in the subwoofer's amplifier. I think it would be difficult to
mitigate without more effort than the very small amount of sound is worth.
(I'd probably have to disassemble the subwoofer and add a new cable
between the amplifier and the speaker that took a bunch of turns through a
ferrite ring... something I'm not willing to do yet.)
(The question about the isolation transformers came up because the
subwoofer must necessarily plug into a different outlet than the receiver
due to its placement, and the answer is to add the ferrite as close to the
amplifier as possible, on both ends, so necessarily after the isolation
transformers, right up against the subwoofer is the place to go.)
Adding ferrite to all the speaker wires, the LFE channel, and the power
cord at the receiver as Dave (NK7Z) suggested got rid of the small
amount of noise that was being picked up on other channels, so I'd call
that piece a total success. Because of the way our AV cabinet is arranged,
I needed the ferrites to be a little bit closer to the wall than to the
receiver, so I gave the wire leading to the receiver several twists on its
way for those last few feet.
In all cases I was able to get at least 7 turns through the clamps I
bought... they were pretty good hefty sizes.
Incidentally, Mouser has very good prices and reasonably priced shipping on
Fair-Ride material 31 clamp-ons, and it pays to buy in bulk-- 10 costs the
same as 7. They also vacuum-seal and bubble-wrap the hell out of it so it
arrives intact.
Leaving this message so it's searchable by the next person. As always, I
appreciate all the help and suggestions from everyone.
Nick
On Sat, 23 May 2020 at 17:01, Nicklas Johnson <nick at n6ol.us> wrote:
> 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
>> > >>>
>> > >> ______________________________________________________________
>> > >> Elecraft mailing list
>> > >> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> > >> Help: http://mailman.qth.net/mmfaq.htm
>> > >> Post: mailto:Elecraft at mailman.qth.net
>> > <mailto:Elecraft at mailman.qth.net>
>> > >>
>> > >> This list hosted by: http://www.qsl.net
>> > >> Please help support this email list:
>> http://www.qsl.net/donate.html
>> > >> Message delivered to nick at n6ol.us <mailto:nick at n6ol.us>
>> > >>
>> > >
>> > >
>> > ______________________________________________________________
>> > Elecraft mailing list
>> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> > Help: http://mailman.qth.net/mmfaq.htm
>> > Post: mailto:Elecraft at mailman.qth.net <mailto:
>> Elecraft at mailman.qth.net>
>> >
>> > This list hosted by: http://www.qsl.net
>> > Please help support this email list: http://www.qsl.net/donate.html
>> > Message delivered to nick at n6ol.us <mailto:nick at n6ol.us>
>> >
>> >
>> >
>> > --
>> > *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.
>
--
*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.
More information about the Elecraft
mailing list