Leema-acoustics Hydra Handbook - Technical Discussion
Technical Discussion
Topology
Starting with the pre-amp and control stage, input selection is performed by gold over silver contact relays under control of the
microprocessor. The audio is then buffered by audiophile quality OP275 amplifiers in order to present the Burr Brown precision
attenuators with a low impedance drive signal. Next, the controlled signal passes to the dual-mono class AB power amplifiers
and also to the buffered preamp outputs.
The left and right power amplifiers are totally separate and have their own power transformers, rectifiers and reservoir capacitors.
A third transformer powers the control electronics, thus completely separating the microprocessor from the audio electronics.
The power amplifiers use a well established topology comprising differential input stage, class A voltage amplifier stage with
constant current load and class B output stage. Each stage is highly optimised following the general teachings of design guru
Doug Self, with notable exceptions drawn from our own design research.
Each output stage uses a class A driver stage and has three pairs of very high performance, hand matched output transistors.
Recently, some reviewers have been critical of the use of multiple output devices. This is completely unfounded as multiple
devices offer many benefits over single pairs of devices:
Distortion is greatly reduced at lower load impedances due to reduced beta droop.
Output impedance is reduced due to multiple parallel-connected emitter resistors.
Device electrical and thermal stresses are reduced offering increased reliability.
Output current capability is increased.
Safe operating area of the output stage is increased.
Topology
HYDRA
12