However, regardless of what you know or don't know, this thread
was started by someone who wanted to know whether he could
re-use the transformers in telephones for another purpose so
clearly he has some!
** That is entirely false.
What the OP *actually asked* was this:
" If I strip down some landline phones I 've got here, then
will there be a matching transformer in each one? "
Chances are high there are no iron core transformers ( hybrid or
other) in his phones at all - cos they are too modern.
Secondly, his purpose requires a transformer with high voltage
safety isolation from the phone line - which the common
600:600 ohms phone line types all have and others do not.
Thirdly, his purpose requires a transformer with very high CMRR
- cos that is why he is getting humming noises at the moment
when making recordings with his passive ( ie non-transformer )
adaptor device.
Have you ever done this ??
I can assure you it ain't as simple as you suspect to get a hum
free and distortion free result.
Not the least of the problems is that portable voice recorders
have high sensitivity mic inputs with automatic gain control
circuits - which cannot be defeated by the user. This results
in annoying gain pumping and regular bursts of overload
distortion PLUS increasing background noise & hum whenever
there is a pause in the conversation.
The trick is to use just the right amount of resistive divider
attenuation before and after the 1:1 transformer to eliminate
this - plus cap couple the input side to stop pulling the
line low.
..... Phil
[/QUOTE]
Hi Phil and others. I am the original poster. Thank you to everyone
who has kindly contributed to this discussion.
Please don't overlook some details I posted at:
http://groups.google.com/group/uk.telecom/msg/4513d4edcdc47c3e
You are quite right about high sensitivity mic inputs and automatic
gain controls. As you can imagine, when the phone's handset is
picked up and used, the mic input is faced with something approaching
line levels and I get severe overload of my recorder. Then when it
goes quiet, the AGC kicks in. It's as you describe.
I have tried to "listen through" any such automatic gain as best as I
could and I feel there is still detect much more hum than I would
expect on the line.
In truth, I really need a more suitable flash recorder but they are
harder to find than I expected.
Currently I use the Olympus WS-331M.
<
http://www.olympus.co.uk/consumer/2581_WS-331M.htm>
Cost is important to me so I don't want to fork out for a digital
field recorder like this low-end one especially as some reviews have
not been kind to it. <
http://www.dv247.com/invt/41901/>.
Prices in the flash-memory recorder market are tumbling such that now
personal video recorders (like the Archos AV400) are about the same
price and offer some additional very useful function. Although i
don't know if that sort of model has suitable input and output
sockets.
On the other hand, I don't want some MP3 player with a recorder
thrown in as an afterthought because ones I have seen can have
surprisingly limited bit rates and low quality. Anyway, many
probably don't have a line level input, so I will once again most
likely get problems with a mic input and AGC.
I take your (and other posters') points about a phone probably not
having a suitable transformer. My headache from trying to make sense
of some of the more technical aspects in this thread suggests it
might be easier in the end to go and buy some 600:600 transformers.
I'm not too clear about this "CMRR" you mention. I know next to
nothing about "common mode rejection ratio" although I did come
across the concept when looking at balanced inputs with chunky XLR
connectors. I googled it and got the following PDF but in the end I
couldn't really understand anything in it about CMRR. Do I really
need to consider CMRR if I am going to be making a recording from a
phone line?
<
http://www.jensentransformers.com/an/Audio Transformers%
20Chapter.pdf>
One new line of thinking I would like to bounce off you and the other
kind contributors to this thread is whether it is possible in my case
to use a simple mixer (like those a pop group might use). If I can
avoid those 3-wire balanced inputs on such a mixer and use just the
2-wire inputs then would the mixer's op amps and other electronics
have a suitable isolating effect? The price of such a mixer seems
very competitive. I am thinking of a unit similar to this one:
Behringer XENYX 502 (£32)
http://www.dv247.com/invt/31558/
In turn, that leads me to consider if I should attempt to get a
separate a analogue-to-digital converter which goes into the PC via
the USB. That way I hope to eliminate earth loop hum and other
spurious noises which I mentioned in my first post.
Maybe something like the Behringer UCA202 for £20
http://www.dv247.com/invt/32730/? To be honest I have no idea if
this would work but it seems to be good value.
I'm sorry to be darting about with various options but I didn't know
it was going to be so complicated!
Any views and useful info from anyone would be most welcome.
Andy.