One important point I forgot to mention. Just because you connect a 600:600 transformer to the output of your interface does not mean it is driving a 600 ohm load. As I has said many times before, transformers do what it says on the tin - they transform voltages, current and impedance according to their turns ratio. A transformer has no intrinsic impedance of its own. If you stick a 600 ohm load on the output of such a transformer it will reflect it (transform it) to look like a 600 ohm load at the input and hence only then will your interface see a 600 ohm load. If instead you connect it to a 10K input impedance line level input, the transformer will do its best to reflect that impedance to the primary and show it as a load to your interface.
A 600:600 transformer is only called 600:600 because it gives its best audio performance, particularly in terms of frequency and phase responses at the extremes of the audio range, when working at that impedance. If you drive it and load with with different impedances then its performance may not be optimal.
The bass end response is determined primarily by the inductance of the primary. This inductance appears directly in parallel with the primary.A good 600:600 transformer will have sufficient inductance such that its inductive reactance will be a lot higher than 600 ohms across the greater part of the audio band. At the bass end of the spectrum, as the inductive reactance decreases, it will eventually drop to 600 ohms and the response of the transformer will therefore be 3dB down at this frequency. Really good transformers have sufficient primary inductance that this happens well below 20Hz but to achieve this means more turns of wire, possibly a bigger core and hence more expense. Cheaper 600:600 transformers cut corners on both core size and number of turns so the primary inductance suffers. The inductive reactance will often be 600 ohms or less at 20Hz. How do they get away with it? The answer is simple. Most modern circuits have an output impedance much lower than 600 ohms - 100 ohms or less is typical so even if the transformer primary inductive reactance falls to 600 ohms at 20Hz, the response will be less than 1dB down.
The other thing to rememeber is that transformers do not care how heavily you load them; all that happens is you lose some bass but transformers do not distort when loaded heavily. They will reflect that heavy load back to your driving amplifier and it may distort but the transformer will not. The only external factor that directly affects a transformers distortion or colour is signal amplitude. High signal amplitudes cause the core to saturate and it will do this more easily at frequencies below 100Hz than at any other frequency. Inside the transformer, what determines the signal level at which saturation occurs is the core material, the core size and the number of turns. Good quality transformers have big cores made of steel with lots of turns so they can handle higher levels. The VTB2291 can easily handle +20dBu at 20Hz with very little distortion.
The more exotic core materials like radiometal and mumetal produce a lot less distortion than steel but saturation occurs at much lower signal levels (for a given core size and number of turns). This is why they are use a lot for mic input transformers where the signal levels are low. They still distort more below 100Hz though just like steel cores.
So if you want to produce transformer colour across the audio spectrum it might be an idea to use a low turns ratio mic input transformer, roll off the input below 100Hz to even out the distortion across the entire audio spectrum, then restore the bass at the output.
Cheers
Ian
