Special Design Features | Perreaux 750 specs

6	Special Design Features

Rugged Build Quality

Ultra Stiff Power Supply

Hybrid Class A/AB

MOSFET Output Stage

Multiple

Protection

Systems

Mechanical 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.

The 750 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, 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.

The bi-polar transistors used in the 750 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 750 output stage are MOSFETs, 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 MOSFET characteristics and their application in this circuitry, result in crossover distortion so minimal that it is virtually non- existent.

The 750 output stage takes full advantage of the unique qualities of MOSFET 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 MOSFETs encourages the highest performance from the balance of the internal amplifier circuitry.

The 750 is equipped with a number of protection systems, isolated from all signal carrying circuits via opto-couplers, to protect both itself and ancillary equipment.