Class A - H1 realisation

The H1 power stage provides massive power reserves, which is already evident from the fact that it consumes almost 40W in idle mode. The output power is more than sufficient for any application, and in the millisecond range it is increased substantially. The output resistance is remarkably low. The optimum operating point of the power stage was first specified by theory, then by measurements on the sample units and finally fine-tuned during numerous listening tests with experts, especially mastering engineers. The same applies to the selection of components. This meticulous way of design largely compensates for the possible disadvantages of Class A described in the theory paragraph below.

Class A & Servo - H1 realisation

To take Class A one step further, we implement an idea which is as simple as it is ingenious: We take the best of Class A and the best of negative feedback amplifiers and put it together: Class A operation is supplemented by negative feedback ("Servo"). Since there is no crossover distortion, there is nothing for the feedback circuitry to correct. Rather, it only corrects the errors which result from the interaction between the power amplifier and the headphones. The feedback path is designed in such a way that no artefacts are created by the correction. The output resistance drops to a value which is smaller than the sum of headphones cables and connectors. In this way, headphone systems can be controlled precisely. One can say that with the H1, any headphones sound as they should.

Why Class A and Class A & Servo?

The previous paragraph may lead to the conclusion that the optimum is achieved with Class A & Servo operation. So why do we still have the switching option? Quite simply because we judge our circuits by ear. Our Class A amplifier was developed with the highest musical demands in mind. We experienced that is does not sound the same as the servo version, but it sounds equally outstanding. Depending on headphones, music and personal preference, the user can make his choice. Even we were surprised to find that despite clearly different characteristics and measurement results, the hearing difference is rather subtle.

A LITTLE THEORY

Class A

Advantages: no crossover distortion at the output transistors and no artefacts caused by negative feedback.
Disadvantages: The internal resistance of the output transistors and usually also the emitter or collector resistors are in series with the load. Simplified, one can say that the material properties of the output stage components substantially determine the sound. And since both the resulting internal resistance of the output stage and the resistance of the load, i.e. the headphones, are complex items, the result is also complex: it is quite common that certain combinations of even very high-quality amplifiers and headphones do not produce optimal results. Experienced audiophiles know this experience.

Class AB with negative feedback

Advantage: Any error at the output caused by the effects described above is "corrected" by a negative feedback circuit. And furthermore, the output resistance of the power amplifier, as long as there is no overload condition, theoretically approaches zero; practically it is in the 2-digit milli-ohm range. The complex internal resistance of a headphone system is almost equalised by the low impedance output. A headphone system is therefore tightly “guided” by the power amplifier.

Disadvantage: The crossover distortion occurs first, then it is corrected. Thus, negative feedback control (at least theoretically) always runs a little behind the action, which can lead to artefacts in the signal. However, modern negative feedback audio power amplifiers are usually not purely Class AB. Very effective techniques are used to keep crossover distortion low without having to use the costly and power-intensive Class A technique. Certain techniques are used in the negative feedback signal, so that the "lagging" of the correction almost no longer plays a role.

Stereo Base control, a little theory

A special feature is the possibility to adjust the stereo width. The perception of spatiality when listening with headphones differs from listening with loudspeakers. Instead of a cross-feed circuit, we use our stereo base width setting. It is based on the mid/side technique which is commonly used in professional studios. It provides additional benefits.

The stereo signal is converted into a mid signal (in simple terms, what L and R have in common) and a side signal (in simple terms, what distinguishes L and R). Summing mid and side signals in a 1:1 ratio will result in the original stereo signal. However, if you change the ratio between mid and side before summing them, you change the stereo base width. In our circuit, we leave the mid signal as it is. We only adjust the level of the side signal. If we lower the level, the stereo image becomes narrower, if we raise the level, the stereo image becomes wider.

Stereo Base control, what it does

You have direct access to the perception of spatiality. With a music programme which seems too "wide" in the headphones, one will turn the control one or two steps to the left. If you perceive a recording as too "dry" (centred), turn the control one or two steps to the right. The leftmost position is mono. This is the preferred setting for early stereo recordings, when individual instruments have been mixed to the extreme left or right - without spatial reference.

Professional users check the spatiality of their mix with this function and may discover possible problems, especially in the extreme positions.

Also important: With the DIR (Direct) button, this stage can be completely switched out of the signal path. In this way, the purist approach is taken into account.

Balance