"If so much are the factors that make difficult the design of an amplifier, even more are those affecting the design of a preamplifier"
"In a HI – END chain, the preamplifier should be as much transparent as possible, to be able to interface every power amplifier, to be as quietest as possible, and so forth"
FLIGHT PRE combines the merits of AUDIA circuit design based on the local feedback and the transconductance amplification (see the "Design concept" page), the use of a constant impedance attenuator with 0.5 dB steps and a chassis designed to get an almost total separation among every stages, still keeping a single cabinet.
Cabinet
The cabinet plays a fundamental role in the realization of a preamplifier since its structure (and the corresponding placement of the boards) helps to fix the degree on immunity against external noises and of interaction among different internal stages.
In a preamplifier is also very important the shielding of audio boards with respect to transformers, since the stray flux could be "detected" and amplified by sensitive stages.
FLIGHT PRE is a single cabinet unit (the hypothesis of designing a "two chassis" unit has been left out for two main reasons: the problems connected with installation and encumbrance and the question of the alimentation cable that could have reached "worrying" length), however the internal volume is strategically divided to suppress interference, with the horizontal partition placed at half height. In the lower section are placed the three toroidal transformers and the main supply board, whereas in the upper section is placed the main audio board.
A massive sheet of damping material is stuck to the divider and another one is under the upper cover plate.
Since the FLIGHT PRE is microprocessor controlled, the control board is placed behind the front panel and it is closed in its proper volume in a metallic shell.
The cabinet is completely made of aluminum and has been made exclusively by means of Numerical Control (NC) units and laser machines; the front panel is realized starting from an aluminum ingot 25 mm – thick and worked out by means of NC micro millers.
The internal finish is realized with conductive chromium-plating, whereas the external finish is a black anodization.
The front panel, the buttons, the knob and the standing feet, after the chromium-plating, are covered with a particular "texturized" paint (containing glass micro-spheres) giving to the appliance elegant looking and, at the same time, warrants a unique resistance and endurance. The front labels, logo included, are cut by NC micro miller with a subsequent paint filling; the rear labels are not made with serigraphic techniques but with laser cutting.
Supply stage
The main supply stage of FLIGHT PRE is placed in the lower part of the cabinet. Here, in fact, there are two toroidal 50VA transformers (one for each channel) and a third 30 VA one to feed the control logic.
All transformer are explicitly made for audio applications, that is designed to have the least stray flux but, at the same time, with a great driving ability. The 10.000 Gauss core plate is autoclave resin sealed and dried in oven. The coils are realized with automatic machine-tools assuring the appropriate wire tension.
The high-current supply stages for the audio sections are realized exploiting, in each channel, four "Ultra Fast Recovery" diodes with bypass capacitance to eliminate possible commutation noises of the diodes. There are eight filtering capacitors ROE – EYS series by 3.300µF 63 V (26.400µF total) with high discharge velocity (low internal impedance) and discrete stages stabilizer, for a stabilized output tension of 50 + 50 V.
The third supply stage, the one for the control logic, uses EMI filters and an integrated stabilizer.
Moreover, on the amplification board are inserted four more dual discrete stages stabilized supply units per channel, that provide the alimentation of the various section of the amplification board, drastically reducing interference among different stages in the preamplifier.
In practice, FLIGHT PRE is endowed of a whole 18 stabilization circuits just for the audio stages supply, each designed as a discrete stage.
Input selector
The input selector allows to choose four unbalanced inputs and two balanced ones by means of sealed relays in inert atmosphere and with golden terminations.
An analogous system is used in the selection of the recorder rail, which is independent from the source and with a buffer output.
Attenuation control (volume)
Achilles' heel of many preamplifiers is represented by the volume-control since usually this introduces a variable impedance element as a function of the attenuation value (that is of the position of the potentiometer).
The result is that the frequency response, the slew-rate and even the internal impedance couplings among different stages are not fixed but are function of the attenuation value (volume).
This is a problem which affects above all the potentiometers; using instead integrated attenuators (I.C. dedicated to volume control or the use of D/A converters) solves the problem of variable impedance, but not that of the frequency response and of the absolute velocity response.
For this reason, AUDIA developed a constant impedance attenuator, realized with discrete components (low-noise metallic film resistors and relays), which provides a gain range as high as 127 dB, with constant steps of 0.5 dB.
The result obtained probably overcome the wall of standard performance, since FLIGHT PRE grants an extremely extended frequency response (3Hz / 1MHz –3 dB ) and the high value of slew rate > 250V/µs in all the gain range (-90 / +10 dB).
Gain stage
The gain stage of the preamplifier is realized using AUDIA technique, that is using a current-feedback circuitry, entirely A – class biased and realized with discrete components.
As sketched in the section about "alimentation" is fed by four stabilized independent stages per each channel: one for the input buffer, one for the input stages for the signal amplification, one for the current mirror and the last for the output stage.
The high-current low-noise output stage, made of audio-use transistors, A-class biased, is not closed in the feedback ring.
Control logical
The hearth of this section is based on the use of an 8 bit micro-controller endowed with a 4 Kbyte ROM.
The control of peripherals is given to two PIO devices, to get the control of the 40 characters fluorescent alphanumeric display, of the infrared receiver, of the HP optical encoder, the buttons and the relays.
The control logic allows the following operations:
ON / STAND-BY
VOLUME
BALANCE
INPUT SELECTION
SELECTION OF THE RECORDING RAIL
LUMINOSITY REGULATION
MUTE
PERSONALIZATIONOF THE LABEL OF THE INPUT
Remote control
The remote control too, as the front panel, is realized starting from an aluminum ingot later milled.
The surface is firstly completely brushed, then anodized with an "avio" color.
The command label are cut with laser technology.
Realization details
Obviously, every resistor used in this design is of the "metal sheet" kind with a 1% tolerance, the polystyrene capacitors are from Philips, polypropylene, electrolytic and tantalium ones are by Roedenstein; all transistors are by Motorola.
All printed boards are made with 70 µm – width copper with an additional 30 µm of metal layer for an overall of 100 µm.
The balanced input connectors are by Neutrik.
Selling package
FLIGHT PRE is delivered with a detailed instruction manual, screened alimentation cable 220 Vac with high current delivery, a remote control with battery and tool for their substitution. The crate is made of multi-sheeted ply-wood and the unit is contained in a tissue envelope and a second insulated envelope containing silica-gel.
Colour Options
The following colour options are available: Black, Silver
