The subtle signals of the extremely low output moving coil cartridges pose real challenges to the analogue circuitry in order to achieve low noise, low distortion, high channel separation and maximum linearity while providing more than 80 dB gain – equivalent to raising the signal level by 10,000 – yet must exhibit level and phase accurate behaviour while adhering strictly to RIAA-IEC equalisation standards.
The 755 Phono preamplifier easily complies with this challenging specification: Now analogue records really can sound their best. Their most captivating ever.
Unbalanced circuit design ensures having as few components in the signal path as possible. Dual-mono layout with discrete left and right channels maximise audio purity and virtually eliminates crosstalk. Input selection (including the ground connection) is done by high-grade relay. And input buffering ensures the 755 presents a benign load to the cartridges. For all three inputs (2 × MC, 1 × MM) the gain factor and the terminating impedance are freely selectable. To suppress rumble from warped records there is an IEC subsonic filter (< 20 Hz) which will protect loudspeakers against hazardous cone excursions. Mono operation is selectable as well.
The analog, passive volume control uses high-precision, low-noise metal foil resistors, and controls both volume and balance. To block switching peaks during volume changes which may be hazardous to power amplifiers a second volume control with PGA (Programmable Gain Amplifier) is used. Active only when the volume is changed, it is similar to a potentiometer and enables the volume to be adjusted in smooth steps without clicks. As soon as the new desired level has been selected, the unit returns control to the precision resistors.
In case the 755 Phono preamplifier is not connected directly to the power amplifier but to a line preamplifier, the volume control may be switched out of the signal path completely.
Phono module (equaliser)
The challenges to the ideal phono amplifier are immense and represent the «pièce de réstistance» for every designer of high-class audio amplifiers. With a gain of up to 80 dB (which equals an amplification factor of 10,000) the amplifier is expected to show a level and phase true behaviour, i.e. an ultra wideband quality (internal amplifier bandwidth 1 MHz/– 3 dB). At the same time it is to compensate the equalisation curve in accordance with RIAA-IEC standards. This can only be realised by means of a multi-stage amplifier concept using super-grade components.
No deletions at the source of the music reproduction are key, therefore a top grade output stage is mandatory. We use the identical circuit as for the 725 preamplifier. The wideband output stage is based on an optimised circuitry with special power transistors which are further linearized by an analogue computing network and has a bandwidth of 40 MHz (– 3 dB). With such speed of response all musical details are reproduced true to life creating a three-dimensional, spatial sound to bring real listening pleasure.
Key to any good sounding audio source component is a stable, noise-free power supply – especially for the audio stages. The 755 is equipped with two dedicated power supplies: one for the audio stages and one for the digital circuits. For powering the audio stages an innovative, amplifier-like circuit design is used, providing an extremely stable supply voltage. With more than 500,000 µFarad of storage capacitance it provides almost unlimited impulse current for the analog circuits. Bus bars supply this power throughout the 755 phono preamplifier without any losses. A unique concept in the audio world!
The 755 phono preamplifier is operated from front-panel buttons and a rotary control. Further functions can be used to optimise and match an entire audio system around different components.