transformers

[Andy]

Hey guys, whats the difference between an audio transformer and a transformer you find in a power supply?  Is there any?  Is it just how the transformer is used?  What are some examples of an audio trans?

[Mike Burgundy]

a transformer is designed to a cpouple of criteria, one of which is cost. The amount of material is a big part in the cost of a transformer, so you want to keep that down.
What does a line-voltage transformer do?
-it supplies galvanic separation - it makes sure there's no *direct* electric contact between the AC wall-socket and the actual circuit. Safety is paramount. This is achieved by electrically separating the primary and secondary windings.
-it transforms high voltage (115 or 230VAC) to a lower voltage (12V, f'rexample), or even higher ones, as the circuit demands. This is achieved by winding less (or more) secondary windings (n) than primary. This follows the formula
Vprimary/Vsecondary=n primary/n secondary

what does an audio transformer do?
-mostly it transforms *impedances*. ideally a transformer does this the same way it transforms voltage.
This way it's possible to hook up an 8 Ohm speaker to an amp that liks to see a load of say 5kOhm.
-it provides electric separation from the high-voltage power supply. This is often applied to the center tap of the output transformer: the valves have their supply, and any ripple on the supply will be neatly cancelled out from the audio path because the two halves are reversed in phase as far as the B+ is concerned.

In designing a transformer, we look at the transformer ratio, which in turn directly influences the winding ratio (n prim/ n sec) that's needed, number of turns, wire thickness..., the power rating (wire thickness, core size), the frequencies involved (core type and size), the way of and amount of interweaving of the primary and secondary, etc etc etc.
A line transformer is made so it works *exactly at that point of operation* with minimal materials. Feeding a 115-12V transformer with 230V will possibly result in 24V secondary, but *very shortly*. Lots of pretty white smoke, too. A transformer like this has a core-size that's able to operate at 60Hz and XX W (or 50Hz in europe) without saturating, but *only just*. It's the smallest it can possibly be.
This, incidentally, is why switching power supplies are so small - the transformer core can be really small because the whol;e thing operates at waay higher frequencies.
An audio transformer has to work down to a certain frequency (say 20Hz for hifi), avoid core saturation in variable conditions, has rather different winding ratio, often center taps and several different impedance windings, and has to *sound good* to boot.
There's a LOT more to it, but this at least illustrates a transformer is not just any transformer, if you catch my drift. The requirements and construction of audio and voltage transformers are very different.