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

[Dave Cole]

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> 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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> 
>