"Another textbook design uses a single IC opamp; often preceded by a transformer. Chips such as the venerable 5534A exhibit good gain-bandwidth, low noise, and excellent THD numbers. Better front end specifications can be achieved on these designs by the inclusion of an input coupling transformer in which the microphone is correctly bridged. If the secondary impedance of the transformer is well matched to the noise resistance of the IC, acceptable technical performance can be achieved; especially if a superior transformer is used.
Our tests with mid-line transformers ($20-$40) proved disappointing. Low frequencies, such as found in bass drum or large organ (60 Hz and below) especially suffered. Even the finest transformers we tested nevertheless lent a certain character which was immediately recognizable, though the better transformers were much better behaved than their budget counterparts. Such transformer coloration was immediately apparent on larger dynamic excursions and deep frequency extensions.
Ironically, transformer coloration, whether slight or drastic, is often artistically desirable. In Neve Retro (article, R-E-P Magazine), the process of rebuilding old Neve console strips was explained. Rupert Neve has said that the harmonic distortion produced by those 1960’s era "class-A biased" designs contributed to his console’s unmistakable sonic character. Old Neve’s, as well as modern transformer-coupled designs, are sought out continually by top engineers for this reason.
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Transformers again revealed their subtle colorations and were rejected in favor of capacitive and servo coupling.
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While transformers offer better circuit protection and isolation than transformerless designs, they nevertheless present one additional layer of sonic personality. The best transformers are quite subtle at low to medium levels. Yet there is no transformer we've used which can rival the transparency of well-designed transformerless inputs & especially under higher dynamic range. Of course, sometimes transformers can offer a very desirable coloration. Some good examples are found in Rupert Neve's original Class-A designs (models 1073, 1272...) and a number of vintage and modern tube preamps. However, a transformer-coupled mic preamp, EQ, or any other signal path claiming "accuracy" should be approached with caution."
http://www.mil-media.com/preamps.html