Unique: Class A power and a step beyond

• switchable negative feedback [„servo“] while maintaining Class A operation
• Class A power amplifier in its purest form

Stereo Base Control

Stereo base control for adjusting the sound stage to your preferences. Precision tool for widening or narrowing the stereo image or switching to mono.

Three output connectors

XLR 4-pole – ¼” Jack – Pentaconn

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 them 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 the 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 it 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 their choice. We were surprised to find that despite clearly different characteristics and measurement results, the hearing difference is rather subtle.

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.

In the centre position, the stage is completely bypassed by relays.

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

Dual Mono vs. Balanced amplifier design

With its dual mono design, the H1 features the same advantages as a balanced output stage but avoids the drawbacks.

The drawbacks of a balanced output stage are the significantly increased number of electronic components – a balanced output contains two output stages per channel, which work in reverse phase. Two output stages result in doubling the output impedance. More components result in an increased risk of non-linearities.

¼ inch jack compatible

Another advantage of the H1 architecture is that unbalanced headphones with ¼ inch jack plugs can also be operated without any problems.

Use as a standard:
XLR in Bal Mode: Pin 1=Ground, Pin 2=hot,

Pin 3=cold
Cinch in Unbal Mode: Case=Ground, Centre contact=hot

Use in special cases:
XLR Unbal Mode: Pin 1+3=Ground, Pin 2=hot
Cinch Bal Mode: Case=cold, Center Contact=hot (Ground Lift mode)

The idea

The idea behind strictly separating left and right channels is to keep any interference away from the circuits. A term like “crosstalk” does not get to the core of the matter. Music signals usually have left/right references, so that an extremely low crosstalk value, moreover, usually measured with sine waves, does not mean so much. But with music, highly energetic signal pulses occur, and then high currents are individually demanded by the output stages. It is extremely important that in these cases there is no coupling via a common power supply or via ground lines carrying signal current. Only this way can an amplifier produce absolutely undistorted output signals.

Realisation inside the H1

In the H1, the power supplies for the left and right channels are separate, for preamplifiers and power amplifiers. The mains transformer has independent, completely potential-separated windings for each channel. Rectification, filtering and voltage regulation for each channel are individual. For logic circuitry and front-panel indicators, there is another winding and separate rectification, filtering and voltage regulation.

The ground potentials of the two channels are connected at one point only, but this is done purely as potential equalisation; signal currents can’t be mixed.

Headphones in the professional field

Today, top-quality products allow applications in the professional field which were inconceivable before. While in the past headphones were mainly used during recording, today they are also used by sound engineers for reference monitoring during the final mix. It is essential that the reproduction is pure and does not mask or brighten up anything.

Headphones in the high-end range

Demanding high-end enthusiasts – in general – primarily have in mind to enjoy the music. So why shouldn‘t their headphones or amplifiers shape the sound a little more pleasingly than the recording actually is? Unfortunately, such „sweetening“ cannot be relied upon. What is advantageous for one recording can be disadvantageous for the next, which can also apply to instruments within a recording. If a recording is really good, true-to-life reproduction is always the best option.