If more than 25 dB of gain is required, an active preamplifier must be used instead of a transformer. Most of the differences involve the limitations stated above. For example, an expensive transformer will have a flatter and broader frequency response. Often, a hotter input signal can be put through an expensive transformer without saturating it. Expensive transformers are also shielded better.
Shielding reduces pickup of hum and interference from outside sources such as power supplies.
Not only does the shielding keep unwanted signals out of the transformer, it also keeps the desired signal within the transformer. Many inexpensive transformers have no shielding while expensive transformers may have multiple shields. Do use a transformer to match impedances. The following equations are used to determine the total gain or loss of a circuit when using a transformer. Calculate the load loss between the mic and the primary winding.