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

[Dave Cole]

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
>