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

[Jim Brown]

On 5/23/2020 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 

NOT "ground loop hum"  but "buzz," which is triplen harmonics of 60 Hz 
-- 180 Hz, 360, 540, etc..
(because of a tiny difference in potential

Yes, but it's a difference in potential of the green wires.

The solution -- run AC for the subwoofer from same outlet that powers 
the receiver.

> 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. 

Consumer electronics is notorious for this problem.

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.

What three connections are we talking about here? Is it speaker cable or 
unbalanced audio coax? If speaker cable, replace with twisted pair, 
which can be built from two colors of ordinary THHN house wire. Spool 
out lengths of the two colors, put one end in a vise (or equivalent), 
the other end in a drill, and slowly twist a lot. When you've twisted 
more that you think is needed, lay the drill down overnight to let the 
cable develop some memory. When you do remove it from the drill, it will 
untwist some. That's why it's best to twist it a lot.

Chokes as Dave recommended should be applied individually to each audio 
cable and the AC cables for both the receiver and the sub-woofer.
> 
> 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.

If you have run AC as I've recommended, the transformers are probably 
not needed.

73, Jim K9YC