Transformers have two primary (pardon the pun) issues if used in the signal path, which can be mitigated but not entirely eliminated:
bandwidth limitations
additional distortion
The bigger the transformer the more limited the bandwidth. In preamps where the transformer is fairly small this usually can mean a loss of bass impact. If you consider that you have to have bandwidth to 2Hz in order to reproduce a tone at 20Hz correctly, then the issue is clear. Most good signal coupling transformers are specced to 10Hz; the best are set 5Hz. That means that in the best cases you will have artifacts up to 50Hz.
Distortion is the other issue. Transformers will 'ring' (distort) like any inductor if not properly loaded at the output. There is a point called 'critical damping' where in the inductor is loaded at its optimal point. This is the point of least 'overshoot', but to say there is no overshoot (distortion) would be inaccurate. Anytime distortion is added, you can count on it to obscure detail/transparency.
This is why transformers have usually been avoided in preamps, although cost has something to do with it too. However there seems to be a cult of sorts associated with transformers in the Asian rim and we are seeing products appear here that are influenced by that. IMO the slight overshoot, which is adding a 2nd harmonic, is adding some lushness. That is nice for CDs which are often dry sounding, but not that great if you play analog and like all the detail that if offers.
bandwidth limitations
additional distortion
The bigger the transformer the more limited the bandwidth. In preamps where the transformer is fairly small this usually can mean a loss of bass impact. If you consider that you have to have bandwidth to 2Hz in order to reproduce a tone at 20Hz correctly, then the issue is clear. Most good signal coupling transformers are specced to 10Hz; the best are set 5Hz. That means that in the best cases you will have artifacts up to 50Hz.
Distortion is the other issue. Transformers will 'ring' (distort) like any inductor if not properly loaded at the output. There is a point called 'critical damping' where in the inductor is loaded at its optimal point. This is the point of least 'overshoot', but to say there is no overshoot (distortion) would be inaccurate. Anytime distortion is added, you can count on it to obscure detail/transparency.
This is why transformers have usually been avoided in preamps, although cost has something to do with it too. However there seems to be a cult of sorts associated with transformers in the Asian rim and we are seeing products appear here that are influenced by that. IMO the slight overshoot, which is adding a 2nd harmonic, is adding some lushness. That is nice for CDs which are often dry sounding, but not that great if you play analog and like all the detail that if offers.