What does "transformer coupled" mean?

The one advantage that i can see about transformer coupling from component to component is that the circuit of within the device tends to see a relatively stable impedance and is somewhat "buffered" from reactance that the load component may see and try to pass back further up the chain. Other than that, it too has its' disadvantages. Sean
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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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Marakanetz stated: "If roll-off touches too much an audiable freequency we can apply a small feedback and make tranny's load responce much wider."

This sounds like you are taking one negative ( transformer induced frequency response errors ) and trying to correct it with another negative ( increased negative feedback ). In most instances, at least when it comes to audio, two negatives don't make a positive.

Audio Xpress had an interesting article about an amp that was somewhat similar in design to the Marantz 8 / 8B and the trade-offs that they had to make when building that amp. The unit built & described in this article showed the differences in bandwidth / linearity using various amount of negative feedback, etc.. while making use of what were supposed to be "high tech" wide-bandwidth toroidal output transformers. None of the results looked real good to me as each approach seemed to solve one problem and create another. As a side note, the measured distortion on this amp was pretty attrocious too.

Having said all of that, much of this does not really apply to interstage or "coupling" transformers due to the low current levels in use.

Clueless stated: " 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.

My thoughts about this are that you have simply traded one sonic signature for another. While every cap made will have its' own characteristics, so will a transformer. The ease of changing a cap within a circuit is FAR easier than doing the same with a transformer. This is not to mention that one literally has dozens upon dozens of caps to choose from / manipulate whereas the options with transformers are rather limited ( in most cases ).

Clueless stated: "4) ITs do not do well with lots of non local negative-feedback.

Personally, i don't know of any gear that uses a lot of global feedback that sounds real good. It might measure quite well in specific areas, but sonically, it is a mess. Can you say "sterile & lifeless" ???

Clueless also stated: "7.) Other things: They break the signal ground, isolate RF trash from the input stage,"

My thoughts about this are that any transformer that is not well designed / shielded becomes a source to pick up / pass on RFI. Transformers work by creating a magnetic field and varying the energy transferred within that field. RF based signals are nothing more than "floating fields" and can easily find their way into such designs. Obviously, a poor design combined with high levels of RF would become pretty obvious and HIGHLY annoying. Once again, it is not so much the design as it is the quality of parts used and how well the design is implimented.

Other than that, i thought that some of the points that he covered in parts 4, 5, 6 & 7 were very fair and even-handed. All designs / products have trade-offs and i thought he did a very nice job of highlighting both the pro's and con's of IT's. Sean
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Clueless: I think that we are on the same page and discussing semantics here. The only point that i would bring up is, while caps do have their sonic signature, i think that they are far less susceptible to picking up RFI. On top of that, the "fields" emanating from a cap ( and they DO produce a field ) is of far lower intensity than that of a transformer. As such, using a transformer would require placing it further away from sensitive circuitry in order to minimize potential "contamination". The end result would be a longer signal path with greater potential for degradation.

As such, it all boils down to what the engineer / circuit designer is trying to achieve with his product and what you like as an individual. I think that most of us here agree that everything in audio is a trade-off. As such, personal preference is the bottom line. Sean
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