After 10 years of design, projects and manufactured audio units were our intention and goal to build up a reference line and after the award winning Line preamplifier we developed a Reference Phono Preamplifier.
As Audio Alto mostly uses vacuum tube amplification the decision was to design and develop a Cost No Object phono preamplifier based on vacuum tubes. Our idea is realized in a preamplifier that takes all the benefits vacuum tube triode amplification can offer but supported with surrounding circuits to minimize and overcome the triode shortcomings.
The audio paths consist just of strictly necessary elements. All used elements are high grade state of the art components. The circuitry is true balanced concept. Balanced concept in a way actually doubles the audio path length and reduces few decibels to the noise signal ratio, but we disregarded this drawback to take advantage of the much more important feature that balanced topology offers – eliminating ground loops, that are the mayor case of sound degradation of most systems and much better common mode noise rejection. One of the benefits is also lower 2nd harmonic distortion. All audio circuits are designed to consume constant power (current). With no change in power supply current consumption we also eliminated power supply voltage fluctuation related to signal and with this the possible unwanted influence between different circuits. With this approach and without using any global negative feedback the resulting circuit hum and noise is very low.
The two inputs for two phono cartridges are balanced and single ended (RCA). All RCA inputs are differentially processed so the sources are not connected to common ground. Inputs which are not selected are all electrically disconnected from the preamplifier, so they have no influence on reproduction while not in use. The connectors that interface the outside world are WBT and Neutric connectors.
Inputs and outputs can also be electrically isolated with a high grade output transformer (as an option). This arrangement also lowers output section impedance to less than 600 ohms. The transformer is made on amorphous core and wound with high grade Cardas wire.
Outputs are balanced and RCA and both as mentioned before can be galvanic isolated. ANP provides also a not so common function – the inversion of the phase.
All the switching functionality in the preamplifier is managed with hermetically closed signal relays with Au/Ag contacts. The relays are “latching” (even after the control voltage is interrupted, this relay maintains its set or reset condition until it receives the next inverting input). With this feature while the relay is in use it radiates no magnetic or any other annoying signal to the reproduced music. The relays are driven with impulses only when they need to change state and managed with special software logic.
The amplification circuits consist of six JFET (parallel conncetion to lower the noise) that a connected in a cascade mode (to lower distortion) to two balanced connected triodes (per channel), that are matched and have golden pins. The triodes are supported by FET current sources with cascade configuration for maximum current source output resistance. The resistance is higher than 30Mohm so the current sources are practically excluded from audio path. RIAA equalization is passive and uses low tolerance selected and matched polypropylene capacitors.
The power supply is regulated with very fast shunt regulators (with Zero-Temperature Coefficient reference Zener diodes). All shunt current regulators use FETs.
All the supporting components are high quality audio dedicated and selected components (Kiwame, Tackman resistors, golden pins sockets, polypropylene capacitors…). The printed circuit boards have gold plated copper connections. The boards are without protection solder layers lowering the dielectrically influence. Printed circuit boards are mechanically damped to lower vibration and stress for components and vacuum tubes. The first amplifying tube is additionally dumped to lower the potential microphonic effect.
The internal audio path connections are realized with silver wire in Teflon sleeve. As a volume control we use top grade 48 position golden contacts switch from Elma connected in shunt mode. In this configuration the balanced attenuator consists of just 4 precision and developed for audio purposes resistors (Tackman). The attenuator is truly balanced.
The whole enclosure, printed circuit boards and all components in the preamplifier follow mechanical symmetry design.
