On Tue, 2 Feb 2010 09:28:03 -0000, "David Looser"
wrote:

You are absolutely right,

blushes

stepping up the voltage results in a decrease in
*available* current. However the current you require to drive an amplifier
or your FM little transmitter is only a small fraction of that required to
drive headphones. So only a fraction of the available current from your mp3
player was being used anyway.

I getcha


To take an example. If the mp3 player can manage 0.5V output into 32 ohm
headphones, that means it can deliver 0.5/32 = 15.6 mA. However a typical
small amplifier might have an input impedance of 10kohm, so the current
flowing at 0.5V output is only 0.5/10 = 0.05 mA. Put a transformer with a
1:4 step-up in place and the voltage is stepped up by a factor of 4 to 2V.
The input current into the amp is now 2/10 = 0.2 mA. Because of the
transformer the current required from the player is 4 times that = 0.8mA,
still well below the 15.6mA that it is capable of. Note that since the power
in an electrical signal is proportional to the square of the voltage the
power delivered to the load is increased by a factor of 4 squared = 16
times.

Right. Makes sense.

--

blackbat /\x/\