AUDISON THESIS – HV VENTI, THE AMPLIFIER BEYOND THE ABSOLUTE

The Thesis project, a perfect synergy between technique and emotion, is dedicated to the most demanding audiophiles. Distinguished by the absolute lack of compromise and the very high technological content, it reserves parameters of excellence and absolute quality to each component. HV venti, defined by experts all over the world, “The Amplifier”, represents a universal reference. A project of high engineering focused on extraordinary musicality, transparency and naturalness. TH amplifiers are born from the know-how obtained with HV venti with the integration of digital technology. Transparency, a direct inheritance of the HV venti, is the principle that inspired the creation of Thesis speakers, born with the ambitious goal of giving voice to the pure musical message of Thesis electronics.

Like its predecessors, HV venti is an absolute reference product for the years to come, indelibly marking the history of Hi-End mobile electronics.

Absolute lack of compromise in achieving maximum performance, this is the mantra followed by the Audison R&D for the HV venti, a project with the following unique features. Output power stage based on an innovative topology similar to IGBT (Insulated Gate Bipolar Transistor) but employing discrete devices. Total void of integrated operational circuitry that, due to their constructive technology and the low voltage level can deteriorate the signal. Their elimination allowed the engineers to work at higher voltages and with a Class biasing of the intermediate stages. All signal commutations were treated using special relays: too complex of a circuit would have led to longer tracks and parasitic inductances, causing signal deterioration and loss of musical information.

The whole input circuit is balanced up to the final stage “Front End”, while the amplifier’s configuration is “Dual Mono”. In order to avoid electro-magnetic interferences, no high current is passed on the main board.

The output power stage is based on an innovative topology similar to IGBT (Insulated Gate Bipolar Transistor) but employing discrete devices. The circuit is made from a complementary pair of Hitachi DMOS drivers, usually used as output stage devices (such as in HR100), which drive two pairs of SANKEN power BJT transistors providing each with 30 A peak current capability and 200 W dissipation.
This solution allowed the use of only two pairs of output devices, thus permitting bias currents which necessarily have to be shared according to the number of output devices in parallel to be reduced; therefore, increasing the bias current value on every output device is possible, keeping the total current absorbed by the amplifier stable when in idling status. All this leads to the increase of the output stage to A class value. The percentage of A Class and consequently the idling current can be adjusted via the BIAS current diverter. As a result of the configuration of the HV venti in Hi Power or Hi Current mode, there will be a variation of the A Class percentage and therefore of the idling current also.
To guarantee maximum sound quality, the crossover was made on a separate module supplied with the product. Available adjustments are Hi Pass/Lo Pass 12 dB and Lo Pass 24 dB mono. Four cut-off frequencies are selectable for the filter (45; 55; 65; 80 Hz). Although this solution is limiting compared to a continuously adjustable cut-off, filter linearity is drastically improved and long signal paths are avoided.
PREAMPLIFIER AND DRIVER: It is made of two groups, Front End pre-amplifier and control unit. The whole stage is balanced up to the driver outputs. The power supply voltage is 90 V and is specific for this stage and dedicated to the two separate channels. The separate 150 V power supply is thermally stabilized and A Class biasing. Input is balanced with the first stage, complementary JFET differential and the second stage, complementary BJT differential. The voltage amplifier is Cascode type with a buffer to drive the output stage.

The product features two Hi power supplies for each channel providing power exclusively to the Dual-Mono output stage. Each channel power supply pair works in Synchro-PMW mode: an independent circuit controls each of them. However, they are synchronized with one another and are designed to supply one single power source, one supplies the positive and the other the negative power. This solution allows engineers to increase the impulsive current transient response necessary to the amplifier. Furthermore, the two pairs of power supplies were synchronized to reduce radio frequency electro-magnetic interference. The layout featuring four separate power supplies has allowed the use of a single powerful MOSFET for each power supply rail, thus avoiding the paralleled configuration that is perhaps the main cause of amplifiers failures. Each transformer and related filter inductor were enclosed in nonmagnetic metallic shells and then filled with resin. This procedure further increases reliability for two reasons: first of all, coils are blocked and therefore made immune to vibrations; secondly, heat is homogeneously distributed, thus preventing any possible overheated areas. The resin-filled groups were then mounted onto the aluminium thick bottom plate to allow heat to be dissipated through side heatsinks. In order to avoid this kind of situation, you are often forced to use convoluted wiring which compel designers to accept compromises. Compromise is not accepted in the HV venti. Inside the HV venti a six layer thick copper board was used to gather all output stage return currents; the layout was created so that each current path comes close with the return current flowing backwards. This way the electro-magnetic fields are out of phase and nullify each other: this solution could be defined as a “balanced power supply”.

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