ABACUS Amplifiers...

…are transconductance amplifiers.

Common audio amplifiers are based on a common-collector output stage, i.e. the loudspeaker is connected to the emitter of the output transistors. This technique in combination with the complex impedance of the loudspeaker and the phase distortions stemming from the crossover network cause the output transistors to work in an incorrect manner; an error which increases with increasing power.

By contrast, ABACUS amplifiers drive the loudspeaker via the collector of the output transistors (c.f. anodes in tube amplifiers). Using a common-emitter output stage the output voltage is solely dependent on the input voltage and the phase distortions caused by the loudspeaker itself do not affect the output transistors. In other words: The ABACUS amplifier is independent of the load.

Typically, loudspeakers are considered a pure current consumer. However, whenever the diaphragm is moving, a current is induced into the voice coil; the diaphragm also acts as a current source. The ABACUS amplifier uses this induced current to control the loudspeaker at any time, i.e. the loudspeaker becomes a part of the negative feedback, which controls the amplification. In this way the current induced by the voice coil is eliminated, resulting in an output voltage signal which is perfectly proportional to the input voltage signal.

Using ABACUS amplifiers, the loudspeaker becomes an integral part of the amplifier. Common loudspeaker properties become obsolete and problems due to thermal changes of the material are overcome. Even the resonance frequency does no longer limit the usable frequency range of the loudspeaker system as the ABACUS amplifier is capable of driving the loudspeaker below the resonance, too.

The protection circuit

The protection circuit in the ABACUS 60-120C and ABACUS Ampino switches off the channel, whenever the permissible power dissipation of the output transistors is exceeded.

In common audio amplifiers the typical protection circuit cannot protect the output transistors, as only the voltage, the current or the temperature is measured. The protection circuit provided in ABACUS amplifiers allows the loudspeaker to be connected to the amplifier independent of the power rating, thus protecting even small loudspeakers. This protection mechanism is realized by the detection of error currents, which are caused by any mechanical or electrical limit of the loudspeaker. With conventional audio amplifier, this is not possible.