Here are a few things. If you want a little more discussion on ITs I suggest looking for stuff Lynn Olson has written (some of his posts at AAdiytubes are very informative) as he has been working on P-P IT non feedback amps for quite a while and also Kevin at the the Lundahl forum. ( Im sure there are others too) Much of below is from them.
1) One design advantage is that you can reduce the voltage output of a stage and also get a reduction in driving impedance. The impedance (like OPTs) is reduced by the square of the voltage reduction, so it can be significant.
2) There has been lots of talk about caps and their sonic signature. One way of thinking is that any cap is a bad cap. Using IT coupling may allow you to get rid of a cap in the signal path and the sonic degradation some associate with it.
3) I have heard the sound of transformers (we are not talking output but IT) discussed but I have not seen the sound associated with any measurements. In fact, the sound seems to buck the measurements rather than follow any as far as Ive read. This is maybe one of those areas where measurements are not yet helpful or the improvement is imaginary(depending on your way of thinking about it).
4) ITs are gaining popularity. IT-transformer coupling largely disappeared at the same time that feedback became standard practice in the late Thirties as a way of increasing power at low cost. ITs do not do well with lots of non local negative-feedback.
5) As noted in above posts Trannies have their weaknesses. even the best transformers have at least a 12dB/octave roll-off at both ends of the frequency range - they are always bandpass filters. And a smooth rolloff (on both ends) is fairly unusual. Usually you see ripples in the time and frequency domains. When the transformer "sees" a high impedance on the primary, secondary, or much worse, both, the bandwidth decreases. This is part of the reason that IT's are notoriously harder to design ($$$$$$) than output transformers: at least the OPT sees a low impedance on the secondary ( the speaker load.) With an IT, the secondary is near-infinite, and the source Z on the primary is controlled by the Rp of the driver tube(s).
6) They are expensive to design make.
7.) Other things: They break the signal ground, isolate RF trash from the input stage, they conveniently filter off ultrasonic distortion components from preceding stages (preventing IM crossmodulation), they give very precise phase-splitting and re-summation (if designed for the task), and protect the speakers from nasty DC offsets that could destroy them.
Cheers
I remain
1) One design advantage is that you can reduce the voltage output of a stage and also get a reduction in driving impedance. The impedance (like OPTs) is reduced by the square of the voltage reduction, so it can be significant.
2) There has been lots of talk about caps and their sonic signature. One way of thinking is that any cap is a bad cap. Using IT coupling may allow you to get rid of a cap in the signal path and the sonic degradation some associate with it.
3) I have heard the sound of transformers (we are not talking output but IT) discussed but I have not seen the sound associated with any measurements. In fact, the sound seems to buck the measurements rather than follow any as far as Ive read. This is maybe one of those areas where measurements are not yet helpful or the improvement is imaginary(depending on your way of thinking about it).
4) ITs are gaining popularity. IT-transformer coupling largely disappeared at the same time that feedback became standard practice in the late Thirties as a way of increasing power at low cost. ITs do not do well with lots of non local negative-feedback.
5) As noted in above posts Trannies have their weaknesses. even the best transformers have at least a 12dB/octave roll-off at both ends of the frequency range - they are always bandpass filters. And a smooth rolloff (on both ends) is fairly unusual. Usually you see ripples in the time and frequency domains. When the transformer "sees" a high impedance on the primary, secondary, or much worse, both, the bandwidth decreases. This is part of the reason that IT's are notoriously harder to design ($$$$$$) than output transformers: at least the OPT sees a low impedance on the secondary ( the speaker load.) With an IT, the secondary is near-infinite, and the source Z on the primary is controlled by the Rp of the driver tube(s).
6) They are expensive to design make.
7.) Other things: They break the signal ground, isolate RF trash from the input stage, they conveniently filter off ultrasonic distortion components from preceding stages (preventing IM crossmodulation), they give very precise phase-splitting and re-summation (if designed for the task), and protect the speakers from nasty DC offsets that could destroy them.
Cheers
I remain