A Little Theory; Class A; Class Ab With Negative Feedback; Mixing Stage - Zahl HM1 Manual

Reference headphones mixing amplifier

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A little theory

Advantages: no crossover distortion at the output transistors and no artefacts which under certain circumstances

can be 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.

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.

Concerning the reference quality of the HM1, the idea of using it for critical A/B comparisons is most obvious -

be it for evaluating different sources like D/A converters or cartridge systems in the high-end range or comparing

different mixes or sound processing options in the professional range.

For this purpose, the HM1 has two inputs with independent level control and on/off buttons. Exact level matching

is essential for a critical A/B listening comparison. By the way, the A/B switching can also be operated via a cable

remote control, which facilitates a blind comparison (Future function, details will be published when available).

Toggle: After power-up, both A and B On/Off switches are in the off state. If you press A or B, the corresponding

channel is switched on. If you then press the other button, this channel is switched on while the one that was on

before is switched off. So you can toggle between A and B.

Individual: Pressing a button repeatedly will switch the respective channel on and off and on and so on.

Hold: If you hold down one button while pressing the other, both channels are switched on.

DETAILS | P.11

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