I tried a solenoid but not enough output and poor at sub 100Hz. Do you
know
what the response is like below 100Hz on your pass-thru Tx ?

Your findings are as I predicted, and no more or less than I would have
expected. I've never swept the CT that I'm using, but when I'm mending sub
amps, I normally test at 80 Hz and 40 Hz, and it produces plenty of output
at those frequencies. I would guess that it's probably specced for use at
mains frequencies anyway. I do actually use one for mains sensing on my
shower pump, to ensure that the shower room extractor fan comes on, and it
works just fine, producing plenty of output. When I designed that circuit, I
did a lot of trials with various loads, and it produced enough output to
switch a 4000 series CMOS gate, with less than 30 watts of load and one leg
of the cable passing straight through. If you take a single 'proper' turn
through the hole, then the sensitivity increases. The project is detailed on
Wiki somewhere. John Rumm on uk.d-i-y put it up there. I guess searching
'current operated switch' or some such would find it.

When I'm mending general amplifiers as opposed to subs, My 'standard' test
frequencies are 800 Hz and 8kHz, and again, the CT produces plenty of output
at those frequencies as well. If I get time over the weekend, I'll sweep it
for you, and see just how flat it is.


Dug out a couple of microphone Tx and checked they worked down to 40Hz ,
taking readings, less response but not too much. Then some more important
jobs came in.
One for the phones out line ( so same f response) and one with series 1K
or
so over the speaker, should work fed to a normal dual channel scope


Not sure how they are going to help you, though. Even the low Z side of
those is a proper winding, and will likely upset the output stage that you
hook it in series with. As I said, I tried all sorts of DIY 'solutions'
before settling on the current transformer. Some of these were attempts to
repurpose various transformers and coils, and as soon as you put a few
'primary' turns on, it interacts with the output stage, sometimes making it
oscillate, and sometimes giving you a misleadingly distorted waveform across
the secondary. Aside from this, I would have thought that the current
handling capability of a microphone transformer was very small, both from
burny - burny let go the magic smoke, and core saturation angles.

At the end of the day, I've told you what works well. I've been using it
daily for some years, and it just does the job, without issue. In the time
that's been spent discussing it and trying to come up with DIY alternatives,
you could have had one ordered, and ready to drop on your doormat early next
week ... :-)

Arfa