What does "transformer coupled" mean?

Sean;Such as what disadvantanges?I know of none.

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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