How does OTL amp get its power?

You could also wait until I find this thread :)

I'm not sure what you mean by 'OTL amp get its power'... of course the power comes from the wall :) seriously though an OTL amplifier is able to make its power in a way that is
not unlike a transistor amplifier. A lot depends on the type of tube used, some tubes are vastly more suited than others, mostly due to a low plate resistance and high transconductance.

The primary advantage of an OTL is that without an output transformer, distortion is reduced and bandwidth is increased. A less obvious but still important advantage is that an OTL can be a simpler circuit, as with any output transformer the output voltage of the tubes has to be stepped down to loudspeaker voltage, whereas in an OTL this does not happen, so you don't need as many gain stages in the amp. In our case, that means there is only one stage of gain, making for a fairly simple signal path. The less stages of gain, the more bandwidth and lower distortion.

Anytime you reduce distortion, the result is a more detailed sound that is simultaneously smoother. Increasing bandwidth can have the effect of increased impact on the bottom with greater low frequency extension (although right here I will interject that so-called 'tight bass' does not exist in the real world and is a phenomena of excessive negative feedback in an amplifier design), and an obvious increase in speed on top.

The heat is a function of the the class of operation, just like with any other amplifier. A class A amplifier will run hotter, regardless of the technology.

Clipping is a function of the power of the amp. Some OTLs can be unstable at clipping, but that can be true of many conventional amplifiers too. We have built OTL guitar amps that are intended to be overdriven and they work quite well. I believe that any proper amplifier design, regardless of technology will have instantaneous overload recovery and will be unconditionally stable- that is to say it will be stable regardless of the input signal or output load.

'Reserve power' is a term that refers to the class of operation- by definition a class A amplifier will have 0 db of reserve power. IOW, the better the amplifier (regardless of whether it is an OTL or not) the lower the reserve power figure will be (FWIW this term is counter intuitive on purpose to make less expensive AB amplifiers look better).

OTLs can drive JM Labs speakers quite well. This has more to do with the power of the amplifier rather than its technology. JM Labs speakers, IMO, have traditionally been tube-friendly, but they do need some power.

Rafael, no I do not agree with all his comments, although at at an earlier time some were true.

For example, the comment

1. The transconductance characteristics of vacuum tubes operated in an OTL push-pull fashion is both inherently non-conjugate and non-complimentary - essentially similar to a the "all-NPN" solid-state amplifier designs of the early-1970s.

applies to Futterman amplifiers only. The Circlotron output circuit (we were the first to use this in a practical real-world OTL) eliminates this problem in both tube and transistor circuits, allowing one to use non-complementary pairs (and for the record, complementary transistor pairs, such as NPN and PNP are never exact matches, so the argument is really a red herring).

I also have to clarify something about this statement:

2. The plate resistance of virtually all vacuum tubes is WAY too high for efficient power transfer to a typical loudspeaker load. Paralleling a bunch of output tubes is the usual solution, and power-efficiency of OTLs is still very poor, even worse with all of those filaments to run. Now when direct-coupling to electrostatics, it's a whole different story . . .

There *are* tubes that have low plate resistances. The 6AS7G is an example, as is the 6C33. So in an OTL, you will not find anyone using 6550s or EL34s! It is true that you still have to parallel tubes, but of any higher power amp that is a fact of life. Kirkus is correct about the filament issue, although the filaments often get blamed for excess heat, which they are *not* responsible for. That comes from the class of operation.

Comment #3 applies to Futtermans OTLs only. Circlotrons don't use a split-voltage supply, nor is there any need for an output coupling capacitor. Many people think that if its an OTL, it has to be set up like the old Futtermans, which do have these 'design features' but that is not true.

Comment #4... not IME; many customers of ours have commented on the fact that they can set the DC Offset of the amplifier and it will be exactly right 6 months later. The trick (and you would think this is obvious) is to be able to control the power tubes.

Comment #5 was never true- even the old Futterman amplifiers from the early 60s had slew rates far in excess of their transformer-coupled counterparts. We've measured slew rates of 600V/micro-second in the output section of our amps. This translates to extremely wide bandwidth. Our early prototypes exhibited this trait right away- they made very capable RF booster amplifiers at frequencies as high as 50MHz without oscillation. In our production amps we limit the bandwidth in the driver circuit to minimize RF issues, but the output section retains its speed.