I started the thread because after reading a white paper the design didn't make sense to me and was looking for the reasoning behind it. The claimed benefits (for example higher swing and faster slew) can readily be obtained with more conventional designs. Kwkarth's explanation still makes the most sense to me. If I can find another reason for the design that couldn't just as easily be obtained through a more conventional circuit I would be happy to change my mind on this. So far I haven't seen any presented.
To continue I would like to try to define terms since some of these names can be used for different purposes. Correct me if any of this is in error. I will try to use terms as they are most commonly used in the industry.
Push-pull amplifier: By far the most common. At least two devices (or more in parallel) must be used in the output stage. One device handles the positive waveform, the other the negative. Class (A, B or AB) defines the degree of overlap in the two phases. Solid state can be complementary symmetry with N-type and P-type devices or quasi-complementary with two devices of the same type. Tubes must use a phase splitter. Inputs can be balanced or unbalanced. If it us a true balanced input the input stage should be configured as a differential pair which provides the advantage of common mode noise rejection. Balanced inputs are pretty much universal in pro audio and fairly common in high-end audio. Ground referenced output.
Single ended amplifier: A single device (or multiples in parallel) handles both positive and negative signal portions. Must be class A by definition. Again inputs can be balanced or unbalanced. Extremely inefficient but it has its adherents. Output is ground referenced.
Bridged amplifier (this is what we are referring to as a balanced amp in this thread): Two completely separate amplifier sections per channel. Since they are run with opposite signal polarities, when one amp is running positive the other will run negative. I believe that this can be described as source/sink. Input can be either balanced straight in or unbalanced in with a phase splitter (I have bridged amps by just building a dual op-amp circuit, one run inverting and one run non-inverting - crude but it works). Since the amps are not differential configured there is no common mode noise rejection. Ground must be floating. I imagine that the amplifier sections can be either push-pull or single ended, the white paper didn't state, but I'd suspect push pull as this is more common.
Now that terms are defined, lets find some OTL designs. I know there are many more out there, these popped up with a few minutes searching.
One the classic designs is the Futterman. Here is a schematic for the OTL-3. Push-pull, unbalanced input. Definitely ground referenced:
Headphone OTL amp with balanced input and push-pull output. Ground referenced so this is not a bridged amplifier:
Doesn't seem to be as much info on OTL electrostatic amps online as there used to be, but perhaps more germane to the discussion would be an electrostatic headphone amp:
I find this one especially fascinating in terms of this discussion. People have called moving coil headphones balanced devices. No they are not. An electrostatic headphone (or speaker) is. Note that here we have an actual balanced load, yet the input is unbalanced and the output is in effect push pull (one could also argue that the output is balanced since there is no common signal conductor - thoughts?). What we don't see here is a bridged amplifier design, even though we are driving an actual balanced load. As an aside my fantasy headphone rig would be a truly portable electrostatic setup (and not the in-ears once sold by Stax). This doesn't seem to be possible unfortunately.
And yes, you caught me out on the HV tube design, it was a signal generator and used a transmitter tube called a planar triode, an extremely nifty and compact device. While it is apples and oranges compared to audio design, my intended point was that the design should drive the parts selection and that an appropriate device can be found for the task. The devices should not drive the design.
Originally Posted by nikongod
If you are this easily convinced that you are right, why start the thread?
Balanced amplifiers are established design. This is nothing new or unique to headphones, just the fringe of popularity.
OTL electroststic amps are an interesting point, could you show me a single ended one?
There are plenty of balanced power amps. This includes amps that are *not* 4 channels slapped together in the fashion of most headphone amps. I mean 1 channel for both phases, 'cause thats fun. Its a shame the world went cheap with williamson, Blumlin was a better designer but williamson was cheaper.
On the very very high voltage swings front: radio? Although the voltage swing & slew rates are indeed impressive I think that is apples and oranges.