Hybrid, tube, OTL ??

Basically:

OTL (output transformerless) means the tube is driving the HP, not the transformer. This design is great for high impedence phones like Byer 600ohm

Tube Amp with Transformer output (like my WA6SE) uses a driver tube to step up the preamp voltage and then uses a transformer to step up energy levels and is matched to lower impedence phones (like Grado and Denons)

Hybrid typically uses the same driver tube architecture but then has an OpAmp (or discreet transistors) to stpe up output and since it is not transofrmer based an opamp can match high or low impedence phones.

 

So think of hybrid as a non-dedicated amp that is flexible about low or high impedence, but it usually sacrifices energy (power) to have flexibility and hard to drive phones (HD800, T1, HE5) can rarely be driven from hybrid designs (there are more expensive hybrid designs which mimic speaker amps (Class T, Class D) and can run this cans requiring greater energy levels to drive them.

 

Some people pick and amp and HP combo with these things in mind so they may decide they want to have Denon D7000 cans and from there they have narrowed down their field to Hybrid or Tube amps with transofrmers as the OTL design does not work well with low impedence phones that require energy (I or current=milliwatts) to drive them, not voltage to act as the "pusher" of the drivers, as the high impendence ones do.

 

Now there are exceptions and certainly more detail to what I tried to paint for you. Some phones are high impendence and also hard to drive so they require higher voltages and higher energy (current) to drive them. Some are low impedence and also are not very efficient (HE5) and these require a high current solution which is one reason head-direct made the Ef5 which is a hybrid with more output than the EF2 for driving the low impedence inefficinet HPs.

 

There is also design considerations within the base architecture of OTL, transformer output, and hybrid which can help provide niche needs. You will see this in the design of the amp as being Class A, Class D, Push pull, Class T voltage swing, and many more including mixing of class and design to have even more hybrids of design within the base classes.

 

If it were easy and non-diversified, and not very detailed, I do not think Head-fi would be as popular as it is, but there are many designs and needs based on the many phones and amp designs, and many, many more tastes of sound which people enjoy. I am sitting here at my desk and have listened to AKG 702, Grado HF2, and Denon D7000 phones over the past 5 hours while working. I have alsow switched sources from PC digital music with optical out to Audio-GD NFB-2 to WA6SE, to my TT with RSA F117 Nighthawk to WA6SE. The HF2s I like with digital music but I like the D7000 with my TT as the dynamics are amazing with the closed cans but with the AKG and Grados it is a bit grainy and thin sounding.

 

So, I hope this helps, please realize this is how "I" understand the difference between amps and how some drive phones with voltage and others with current, and others are a combination, but in no way should this be considered accurate information by any means. I, like others here, have picked up this knowledge by reading posts and asking questions like you are doing and if there were a unanimous statement on here about blue wires being better to power your amp with, I am sure you would find a percentage of members repeating this all over the forums. As always this is IMHO & YMMV!!!

To fill in a little to DannyBuoy's great (though slightly generalized) post:

 

Pure tube can be either OTL (Output Transformer-Less with the tube driving headphone directly - something most tubes don't do very well with low impedance loads, and Transformer Coupled - as transformers can have windings quite low impedance loads).  Generally output transformers are expensive and simply aren't commonly used, so most tube amps are either OTL or hybrids.l

 

Hybrid amps simply mean a hybrid between tube and solid-state, usually (always?) with a tube front-end (voltage gain stage) and solid-state current buffer.  This is quite a great idea as tubes can run at very high voltages within their most linear regions and are most effective at driving high-impedance loads (like 1MOhm loads of solid-state devices).  Solid-state devices like opamps and discrete circuits generally have high input impedances and, while operating at considerably lower voltages (and generally limited to fairly low voltages), they can have very low output impedance and can output quite a bit of current into a low impedance load (in the case of unity gain current buffers, most devices/designs are stable outputting a lot of current while running quite low voltage).  This basically means a combination of the best parts of tubes and solid-state devices.

To pick a nit, you're going to find a variety of different tubes and circuits when you start looking deeper into tubes. These combinations make differences and make designs distinct from each other. I'll throw a few out that you'll probably run across.

One fine distinction is the Directly-Heated Triode (DHT). These tubes are noticeably different from others. Typically, a DHT lends a near-magical tone to the mids. Not necessarily accurate, but the lushness has a lot of fans. The highs and lows can be a little soft, but if you're listening to acoustic/vocal material, the DHT sound is special, to say the least.

There's also negative feedback (NFB). This is where part of the signal is fed back into the circuit. This can make the circuit more stable and linear. On the other hand, some think NFB is pure evil and that it ruins the sound.

You'll find another level of esoterica in the power supply. One major difference that turns up is whether the circuit is rectified with tubes or diodes. When the power is changed from AC to DC, you can have either a tube or diodes do it. They sound different. Some purists get upset when there's "sand" (diodes) in the tube circuit. Tube rectification typically costs more, as well. You'll also find chokes in some power supplies. These are sort of like a transformer, and they smooth out the power nicely. The more chokes the better (IMHO), but each one adds cost to the amp.

There's also a difference in construction methods. Some are built on PCBs, which keeps costs down and is easier to build. However, some are built point-to-point, where there are individual wires connecting the components. Point-to-point is labor-intensive, but is much easier to repair and has shorter circuit paths - a good thing. You'll sometimes find point-to-point on the higher end amps.

One last thing is the chassis. Some are built with two boxes, where the power supply is separate from the amp. This (of course) adds expense and complication. However, keeping the power supply away from the amp gives you a much lower noise level.

There are a few dozen other minor points, but this gives you a little bit of an idea what's going on when you start reading through the specs.