GaN-based Class D power amps

Hi Guys and greetings from Germany!
I'm new here this is my first post in this forum. While surfing around through the internet looking for news about Class-D amps I found this thread and somehow had the feeling to add a comment. Because luckily I'm one of the 1st owners of a pair of the AGD Vivace GaNTiube monos here in Germany. When I finally got a pair of the "ready to release" version of the GanTubes I was blown away and could not stop to listen these little things. I must say I was used to a Nelson Pass F5 Turbo (pure Class A) and I always were happy with the F5 but the GaNTubes have blown it away. In every aspect, especially the mid-highs they distribute so much... lets say 3D sound. I was just astonished about the difference to my F5. It feels like you can grap
every single instrument or singer in the room. Just fantastic! If someone has the chance to listen to the AGDs I can only recommend to take the time and do so, it will be a great experience.
I'm driving a 8Ohm broadbandspeaker-Expo horn with 93dB efficiency. And it perfectly fits together.
Of cource 15k $ for pair of the AGDs is a lot of money but for HiFi lovers in my point of view worth every single Cent.
Cheers,Mallard

OCD has 4 AGD Vivace monoblocks driving his Maggies and subs via an external custom 4-way crossover. 

So nice to see a GaN thead with such a wide range of comments. I will add a few factual items here for the benefit of the readers and non circuit designers.
First of all Class D has been around since the late 1950's. The concept of Class D could not be fully realized until the semiconductor technology caught up with the Theory.

The theoretical assumption of Class D design is that the switching device – the transistor or tube, can switch instantaneously on and off. I will refer to the device as transistor from here on. If the transistor does not switch on and off instantly, a lot heat and current will be required. There is heat generated in the transistor as it goes from theoretical zero ohms to the off state of the transistor. This transistor acts a resistor which generates heat. The 2nd item is that the conduction will start between the positive voltage rail and the negative voltage rail as one transistor turns on and the other turns off. This causes more heat and more current to run, with potential damage to the transistors or very large amounts of heat and current flow. What Class D designs have done is to delay the turn on of 1 transistor until the other is completely off. At this point there is no processing while waiting for the transistor to settle down. This is called dead time. This is what folks call “Class D sound”. Which is quite true. One way to get rid of this was to use heavy feedback or feedforward to reduce this dead time effect. The side effect of heavy feedback (or feedforward) was sluggishness and loss of detail as the feedback loops tried to correct for this dead time.

Transistors inherently have a lot of capacitance. This is the cause of the slow turn off. The capacitance in the transistors (MOSFETs in particular) keep the charge going even though the voltage has been removed. This causes running at the top and over shoot. Hence the hard edge of Class D also. Class D typically operates at 400kHz and so RF design methodology has to now take place on the circuit boards, wires, components and the whole design. A simple item like having a trace on the top of the board and the bottom of the board created a capacitor that could now affect the sound.

The overshoot is a common cause of the hard edge found in a lot of solid-state equipment. That is because the speed of the solid-state transistor led to all sorts of parasitic capacitance's to come alive. As mentioned prior, board capacitance, component capacitance, even the via’s on a circuit board make a difference now. The overshoot would suppose the frequency be in the 100kHz or higher. RF design is a key factor in Class D and in making good solid stage equipment.

What GaN (Gallium Nitride on silicon or Gallium Nitride on Silicon Carbide provide is
1. Ultra-fast switching speeds
2. Almost no overshoot and ring
3. Minuscule parasitic capacitance on the device.
4. Much greater efficiency of operation.

I can only speak for the Merrill Audio amplifiers at this point.
1. There is zero dead time. So it is smooth and you cannot tell or scope any switching.
2. There is zero feedback. Since we don’t have to correct for distortion or overshoot or deadtime. This results in pure detail and air (assuming the track has all of this recorded in it). The difference is quite stunning.
3. There is miniscule parasitic capacitance and inductance. There are probably have more capacitance in interconnects.
4. The wasted energy is minimized and the design is very efficient. Hence the amplifier operates at a stable low temperature, keeping all components around it operating at a low temperature also. This temperature stability means the operating amplifier approaches theoretical design. There is not thermal compensation required or degradation of components due to heat or thermal gradients of components to worry about. While green is a side benefit, the actual benefit is stable operation of components. This is big as inductors, capacitors and resistors have heat coefficients (positive and negative) and have different values at different temperatures.
From a sonics standpoint, the new Merrill Audio ELEMENT series are easily an order of magnitude better than the VERITAS. But don’t take my word for it, go listen to them yourself. Take your amplifier and do a direct A/B comparison.