On Wed, 14 Jul 2004 11:00:09 GMT, "Highland Ham"
wrote:

Turns out on further investigation the circuit
specifies * two* 32 ohm insets in series for 64 ohms altogether. I've
still only got an 8 ohm earpiece, though. So that's an 8:1
transformation which adds up to.... a tap a quarter way along one
winding? Or is it three-quarters. Or of course a full, 4:1
transformer, I suppose. I always get confused with transformations,
for some reason. :-/
============================
For a (lossless) transformer V*V/Z = constant , hence the input to output
impedance ratio is proportional to the square of the voltage ratio.
For your 64 to 8 Ohms application ,hence 8 : 1 impedance ratio you need a
small transformer with a sqrt (8:1) = 2.8 :1 voltage ratio (equals winding
ratio). You possibly have a small transformer in your junkbox from a
wallwart , those low voltage DC power supplies you plug into a wall socket.
They often have a switch enabling different output voltages 3-5-6-9-12 V
The transformer inside has a single secondary winding ,which could be used
as an auto transformer for your application.
Perhaps you can also wind your own transformer on a toroid from an old
switch mode PSU or the like, say with 50 and 140 turns or a single winding
of 140 turns with a tap at 50 turns. The actual winding ratio is probably
not critical.
The impedance of your earpiece is only around 8 Ohms for a limited audio
freq range ; it is usually specified for 1000 Hz

Good luck with you endeavours

Okay, many thanks, Frank (& Tim). That should be plenty to get me up
and running. I've got some toroids that should do the trick. I assume
since you say it's okay to use ones from PSUs that loss isn't a
problem here. I've got some low-loss powdered iron RF types, but
they're probably too small for the number of turns required. Still, a
bit of experimenting is what it's all about!
Thanks again,

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.