Perreaux Prisma 350 - Manual (Page 18)

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Special Design Features
Rugged Build
Quality
M echanical strength has been a hallmark of Perreaux products since the
company first started production back in 1974. The concept behind the physical
design and construction is that each structural member should contribute to both
rigidity and performance.
Ultra Stiff
Power Supply
The 350 incorporates a massive custom designed, toroidal power transformer,
employing unusually heavy gauge wire that reduces copper losses to a
minimum. An electrostatic shield prevents AC line borne interference from
entering the signal path. The power supply filter capacitors, totaling 60,000µF
(30,000µF per channel), have exceptionally low inductance and internal
resistance. They charge and discharge in response to load demand far more
rapidly than conventional storage capacitors and are capable of delivering the
instantaneous current required by the output stages, providing optimum dynamic
range and transient response. The wiring from the power supplies to the output
boards is designed for unimpeded transmission of the required current and
voltage and utilizes heavy gauge, tinned copper wire cables. Since power supply
leads radiate at signal frequencies, all wiring is carefully loomed to minimise
this effect.
Hybrid Class
A/AB
The bi-polar transistors used in the 350 are run in Class A mode. This avoids the
crossover notch distortion and the resulting odd-order harmonics present, to
some degree, in all other classes of operation. The devices used in the 350
output stage are M OSFETs, which with high quiescent current circuitry, are run
in the equivalent of Class A to 10 watts. Beyond this point the output class is
technically Class AB (hence the hybrid nomenclature), but with a major
difference. The combination of M OSFET characteristics and their application in
this circuitry, result in crossover distortion so minimal that it is virtually non-
existent.
MOSFET
Output Stage
The 350 output stage takes full advantage of the unique qualities of M OSFET
devices and in many ways they are superior to bi-polar transistors. A major
advantage is their tendency to draw less current over a large section of the power
bandwidth as their temperature rises (Negative Temperature Coefficient), hence
self stabilising thermally, whereas bi-polar transistors draw more current as their
temperature rises (Positive Temperature Coefficient) and protection circuits
become mandatory to prevent thermal runaway and eventual self destruction.
MOSFETs have the ability to swing fully across the amplifier’s internal DC
voltage and are therefore true "rail-to-rail" devices. Using M OSFETs
encourages the highest performance from the balance of the internal amplifier
circuitry.