Originally Posted by RexAeterna
the power transformer in power amps help handle impedance matching in most cases. so when helping to match impedance it has to maintain a certain voltage and increase and decrease amperes/milliamps depending on the impedance given and demand of the source presented. that's why tube amps always has massive output power transformers(or dual transformers) cause tubes are high voltage sources but only operate in milliamps so the power transformer helps when it involves if the circuit needs amperes instead. i think in headphone amps it should be same formula maybe since they're basically just tiny low power, power-amps to drive headphones instead of speakers.
i'm sorry but you really need to get your terminology straight. a power transformer (which is quite logically; in the power supply) has absolutely NOTHING to do with impedance matching and it would be rare for a power amp to use output transformers either (a totally different device to power transformers) and the ones that do, the output transformer is a 100% passive device, it does not adjust to a load dynamically at all, it can be used for 2 main reasons:
• to transform a high voltage, low current signal (usually, but not exclusively from a tube amp) into a lower voltage but higher current signal that is more suitable for driving transducers (this can also happen in reverse for electrostatic speakers)
• a step up, or step down tx can be used to transform (in a completely passive and steady way) an amp with a high output impedance into one with a lower output impedance in order to drive the load more effectively. it does this to a set and unchanging ratio, based on the number of turns of wire on the core, it does not actively adjust anything and does not react to the changing load impedance (it changes over frequency too) at all.
as mentioned above, the vast majority of modern amps are either DC coupled (no caps) or AC coupled (with caps) its only really tube amps that use output or interstage transformers and even then the trend now, even for tube amps is towards OTL (output transformer less) designs. by in large most power amps (certainly ones that have any actual power, not pissie little 5W single ended, direct heated triode amps) are solid state, or chip amps and these do not as a rule use output transformers at all.
Edited by qusp - 10/20/11 at 4:47am