What does "transformer coupled" mean?

Actually, the transformer reflects the load impedance to the primary side (described in another post of mine), so there's no real isolation of reactance. The real benefit of transformer coupling is that it allows one to match almost any load impedance to almost any source impedance, and it provides excellent isolation between stages to boot. This allows more freedom in selecting circuit topology.

The problem is that it's hard (read expensive) to build a transformer that's truely flat across the entire audio range.

Ghostrider hit upon one negative aspect of transformer coupling i.e. the potential for non-linear frequency response errors to be introduced into the signal path.

As far as signal paths go, transformer coupling is equivalent to running the signal through an interconnect that is hundreds of feet long. This is due to the amount of wire that the signal must pass through on both the primary and secondary sides of the transformer.

The fact that the signal is NOT directly coupled and is only linked via the magnetic flux of the transformer also opens the door to signal degradation. I would also imagine that the potential for increased susceptability to RFI becomes more of a factor in heavily populated areas.

Transformers can also run into saturation, which is not much of a problem when dealing with line level signals. None the less, a transformer coupled design is both tougher to design and more likely to suffer bandwidth related distortions than a capacitor or direct coupled design.

Isolation can be improved with a transformer but the capacitance must be kept down to a bare minimum. In most cases, any fluctuation that the transformer runs into on the secondary side due to irregular loading conditions will be passed back into the primary side of the transformer. The effects of the loading irregularities may be reduced but they are still there and the circuit on the primary side still has to deal with them.

I can continue on but will some it up by saying that there is no "golden goose" when it comes to audio circuitry design. They all have their ups and downs. As such, it is not so much what topology or design that one uses, but more of how well that design is implimented and the quality of parts used. Sean
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Basically, one of the "big deals" is the ability to drive long interconnects with a high input resistance at the other end with minimal voltage (gain) loss. Also, the xmfr acts like a filter which does not amplify the tag-along garbage that the signal carries. As always, there are no free lunches, but a well-designed xmfr coupled output stage can have advantages over a cathode follower output - but the xfmr has intrinsic and application imperfections that have to be considered (such as it has to be sized for the average DC current and the harmonic distortions introduced). Not better, not worse - just another audio compromise

I think it's weird how sometimes you read ads about a product that describe a certain design as if it is the "golden goose." Audio Note talks about transformer coupling in this manner (only their best sounding pre-amps are transformer coupled):

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Why??

Quite simple, in the M5 (and all our other transformer coupled pre-amplifiers, the M3, M6 and M8), there are no impedance mismatches, there is more than sufficient gain to maintain the dynamic envelope of the signal intact combined with a natural (meaning not created artificially by feedback or other trickery!) drive impedance so low as to render the load (the power amplifier input) irrelevant.

The disadvantage is cost, you heard for yourself the sonic advantage.

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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 I’ve 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