Leema-acoustics Tucana 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
TUCANA
12
Contrary to another common misconception, ‘timing’ cannot be compromised.
The square wave performance of an amplifier is the ultimate timing test. A look at the square wave performance of a Tucana is
testament to the time domain performance of the design. If multiple devices did cause errors in the time domain, the resultant
square wave would be significantly degraded. This is simply not the case. Apart from the gentle rounding of the corners,
caused by amplifier input bandwidth filtering, the square wave response is virtually perfect and has no overshoot or ringing.
On the subject of ringing, if the amplifier is tested in to a load including a capacitive element, inaccurately trying to simulate an
electrostatic loudspeaker, ringing will be observed at the output in response to transient signals. In fact, this ringing is not caused
by the amplifier. It is simply the resonant combination of the amplifier output inductor and load capacitance. No notice should
be taken of any test reports including the level and makeup of this ringing, as the results are totally dependant upon load type
and applied stimuli.
The power amplifiers incorporate no output stage protection circuits. Leema designers feel that even quite complicated
protection schemes remain audible. If the output stage fails due to a short circuit or component failure, power supply fuses offer
safety protection and an auto-detect and disconnect circuit protects the loudspeakers from damage.
The resulting amplifier has the following characteristics:
Low noise
Very low distortion, typically better than class leading class A designs
High output current delivery
Low output impedance
Fast and detailed yet silky smooth audio presentation
Very robust and reliable if used correctly
Topology
TUCANA
13
Tucana 1KHz Square Wave