Originally Posted by
Uncle Erik 
Well, this gets kind of technical. OTLs are not necessarily lush - the Zana almost sounds like a solid state amp, for example. On the other hand, the PPX3 (also OTL) I had was warmer and had more of a "tubey" sound. But neither is as lush and classically tubey as the Si2A3, which is transformer coupled.
It's like what my tax professor said, "the only correct answer is 'it depends.'"
If you want to get technical, tubes tend to have a higher output impedance. This comes into play with what headphones you're using. The output impedance of the amp has a lot of interplay with the impedance of the headphone. In short, it tells you how much power transfers from the amp to the headphones. The closer the impedance match is, the better the power transfer. If there's a mismatch, you can use a formula to show how much power is lost.
You then take the amount of power that gets through and the sensitivity (measured in Decibels (dB)) to show how loud the headphones will get. There's more to this, but you need to understand how impedance plays between an amp and headphones.
Again, tubes tend to have a higher output impedance. That means that if you use something like Grados (32 ohms) or an AKG K-701 (62 ohms) with an OTL you're going to have a lot of power lost in the transfer from the amp to the headphones. That means the headphones won't perform very well. As a side note, solid state tends to have a good low output impedance, which is why you almost never see a solid state amp with output transformers. I believe that McIntosh built some solid state power amps with output transformers - those have much, much more of a "tubey" sound than most solid state amps have.
One way to overcome high impedance output is to use an output transformer. The transformer steps the output down to get a lower output impedance.
Transformers add their own sound to the output. So the quality of output transformers is critical. I only trust output transformers from a very few companies - I like Electra-Print, Lundahl and Sowter. There are others, but Electra-Print will wind anything you need and their prices are more than fair. So I don't really see a need to go with others, with some exceptions. I recently bought a pair of headphone specific output transformers from Sowter - they're tapped so you can use a selector switch to match output to four common headphone impedances. There's nothing else like it on the market, and it'll go into an amp using the 45 tube.
As for the sound, a triode outputting to a transformer is a major part of the "tubey" sound everyone talks about. There are exceptions, but that's a big part of it. So if that's what you're going for, output transformers are a good way to go. As well as if you're running AKG, Grado or other low impedance headphones.
Another factor is cost. A pair of quality output transformers will run you anywhere from $250-$500, possibly more if you want silver windings or other custom touches. That's just for the transformers, you also have to pay for the rest of the amp.
On the other hand, an OTL uses a capacitor to couple the tubes to the output. Again, the sound of the amp depends greatly on the quality of the output caps. This is why Craig sourced the special soybean oil caps for the Zana - he went through a lot of different ones and only that particular cap had the sound that he was looking for.
Also important in the final sound are the tubes you use. The 6C33C has an entirely different sound from a 2A3. I don't want to get too much into various tubes, but the 2A3 is what's called a directly heated triode; they do things differently from indirectly heated tubes. The rectification of the amp also matters. Some have solid state diodes turning AC into DC, and some have vacuum tubes doing this. The final sound changes on how you do this. Same with whether the power supply is regulated or not. And if you want to be further confused, there are differences between single ended and push-pull circuits, as well as whether the amp runs in class A or AB. There are other classes, but most headphone amps are A or AB. I'm not going to go into all these things here, but be aware that there are differences with all these things and they're good for further reading if you want to learn more.
There's more to this, a lot more, but I hope that answers some of the questions you have. Make your choice based on the headphones you have and the sound that you're looking for. But beware of inexpensive output transformers. You truly get what you pay for. There's a lot of art and science in winding transformers correctly and even more on the core laminates, what they're made of, how they're stacked, and much else.
