MATCHING TRANSFORMER

[John F. Regan]

Common misconception is that the phone line is 600 ohms.  Actually the
phone line varies in impedance all over the place if you are at the
residence end and looking into the phone line as a load with the Phone
Central Office on the far end.  The impedance can vary from a few hundred
ohms to well over 1000 ohms and can be very reactive.  Since equipment has
to be specified somehow, 600 ohms is the commonly used impedance for test
and measurement.  Used to use 900 ohms in the early days.

I gather you are simply trying to hook a speaker output up to the phone
line.  The main issue is that you must use a transformer to thus maintain
the longitudinal balance of the phone line since neither side of the phone
line is connected to ground.  Almost any transformer will pass audio up to
the telephone upper voice bandwidth of 3400 Hz and if the winding is full
floating (no ground reference) then you will not intruduce any hum on the
phone line due to impedance unbalance.  Since the average level on the
phone line is about 1 milliwatt (.7745V ACrms) across 600 ohms and most
speaker output things have at least a watt of power, you really don't need
a really close match to get the volume you need to the phone line.  If you
are trying to create a hybrid type phone patch with balance so that you can
transmit and recieve at the same time, that is another story all together
and frankly very difficult to achieve and in some cases impossible to
achieve.  If you are simply trying to couple a reciever speaker output to
the phone line and have a T/R switch to transfer the phone line to the mic
input on Recieve then don't worry about the impedance of the phone line
since the longitudinal balance is far more important than the impedance.  I
would simply put a 4 ohm power resistor across the speaker winding of the
rcvr to protect its output assuming it is a transformer and then use a 1:1
telephone coupling transformer to connect that to the phone line.  I watt
across 4 ohms is 2 volts and you only need .7745V or so to drive the phone
line so probably you are all set.  Call JP Marlin Surplus at 1-800-652-6733
and buy one of their 600-600 modem transformers or steal the one out of a
junk Modem that you can get at surplus stores.  They mostly all go down to
300 Hz.

If you want another option that will also work in a pinch (surprisingly
well too), just wire an ordinary 12V to 115V small power transformer using
the 12v winding as the 4 ohm and the 115 winding as the output (about 370
ohms - close enuff!).  I have used small filament transformers at audio
before and they are only a bit lossy but the receiver has ample power to
overcome the loss and drive the phone line easily to full volume.  OK they
are not HIFI but at 56 I can't hear the difference anyway.

John K9SWN

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