Zahl HM1, Manual, Page: 10 / 20

ZÄHL HM1 MANUAL V 1.0
P. 10 | DETAILS
Power Stage
Class A - HM1 realisation
The HM1 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 - HM1 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 HM1, 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.
DETAILS