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Nagra Classic PreampDevelopment review 1.GENERAL OVERVIEW- Incremental evolutionThere have been some major technological developments in the area of hi-fi innovation over the years, such as the arrival of transistors or digital. But mainly, the industry evolves little by little as improvements to electronic components are successively introduced and as circuits and their implementation are constantly optimised.It is in the extreme attention to detail and mastery of manufacturing techniques that current engineering often finds its biggest potential for improvement. In order for a new generation of devices to do better than its predecessor, existing boundaries have to be rolled back on every front and often in tiny proportions.This incremental improvement is something that Nagra has always espoused from its very beginnings. It makes an essential contribution to the longevity of the brand’s equipment, as it means that new products are launched with parsimony: a new model only replaces an existing one when it can bring an appreciable benefit to the user.This is the path that has been followed by the new Nagra Classic Preamp preamplifier, which continues in the footsteps of the Nagra PL-P, the Nagra PL-L and more recently the Nagra Jazz models. Launched respectively in 1997, 2001 and 2012, these three valve preamplifiers became benchmarks for their time and are still highly rated in the world hierarchy of top hi-fi equipment. Having been revised from top to bottom, the Nagra Classic Preamp is now up to the task of taking over the mantle from its prestigious predecessors and takes another step nearer towards total audio fidelity. Its name Classic Preamp lets anyone know it is a line preamplifier as well as it is part of the Classic collection of Nagra products. 2.DEVELOPMENT OBJECTIVES2.1 - The Sound of SilenceThe silence of an electronic circuit is one of the main criteria in a listener’s evaluation of what he or she is hearing. The human ear calibrates itself, whether consciously or unconsciously, on the background noise generated by a unit in order to appreciate the dynamic range produced by the music. The lower the noise threshold, the greater the “space” in which this dynamic range can express itself which will correctly be perceived as open, clean and natural, increasing the impression of realistic sound.A large part of the work carried out by the engineers in the development of the Nagra Classic Preamp concentrated on this aspect. Nagra’s engineers have taken as a starting point the critically acclaimed Nagra Jazz preamplifier and they have incorporated elements of the design philosophy that characterizes the HD DAC. The Jazz preamplifier already displayed an excellent level of signal to noise ratio greater than 105 dB. But given current knowledge and state of the art technology, a noticeable gain in this area could still be expected. Taken together, the changes decided upon have borne fruit as the Nagra Classic Preamp, which is also valve based, now offers a signal to noise ratio greater than 125 dB.In this way, the electronic is able to match the performance of the quietest digital sources currently available. Using it with such sources is ideal as the preamplifier does not generate any additional noise that would also be amplified by the power amplifiers downstream.2.2 - Simplification and rationalisation wherever technology allowsThe progress achieved in electronics increases not only performance potential but also contributes to a general reduction in complexity and the size of the circuitry. The engineer makes the most of this to create solutions that are increasingly stripped down and simplified, in order to benefit from all the advantages that this implies, such as the reduction of the noise threshold, the generated heat, the power consumption and the risk of breakdowns.The evolution in design the Nagra Classic Preamp has directly benefitted allows it, for example, to do away with the Nagra PL-P battery power supply or with the Nagra Jazz compact external power supply that appeared at their time as the ultimate solution for achieving a very low noise threshold with valve circuitry. Better results are now obtainable with simplified designs.The circuits have been designed with the shortest possible paths that bolster their immunity to disturbances and radiation.As for the Nagra Jazz, all the connectors are mounted on the back plate rather than on the sides of the unit as it traditionally was with the Nagra PL-L and the Nagra PL-P. Even though the numbers of inputs and outputs are identical on the Jazz and the Classic Preamp, the latter gets one XLR output whereas the former only offered RCA outputs. Moreover the Classic Preamp embeds a headphone output inherited from the HD DAC.Last but not least, it can be powered either directly from the mains supply or with a Nagra ACPS II dedicated power box or even better with a Nagra MPS multiple power supply, so that it can fit any kind of configuration, existing or not. 2.3 - Suitability for multiple sourcesAll sorts of sources, both digital and analogical, which are likely to be hooked up to a preamplifier these days, deliver a signal which may vary notably. Too weak a signal and the preamplifier will not allow for exploiting all the power of the amplifier. Should the signal be too strong, the amplifier will be pushed into saturation before the volume even attains its maximum level.Like the Nagra Jazz, the Nagra Classic Preamp brings a solution to this problem by offering two distinct and toggle switchable gain levels. In the “0 dB” position, the electronics adds no gain at all and acts almost like a passive preamplifier. In the “12 dB” position, maximum gain is provided that is suitable for low level sources.2.4 - UpgradabilityUsing an external power supply has several advantages. The first advantage is to avoid bringing mains alternating current into the interior of the preamplifier that would introduce a potential source of disturbance and pollution to the circuits. As an additional advantage, it allows for power supply upgrades right now going with the Nagra MPS, and in the future to any new power supplies that Nagra may develop. This is a valuable plus as the company is constantly innovating in this area.The Nagra Classic Preamp has been designed to be adaptable to various different uses. The XLR input as well as the XLR output can be upgraded with optional input and output mu-metal shielded transformers boards that are available separately to create a fully balanced floating signal and to further enhance performances. The optional output transformers are different from the ones intended for the Jazz, they feature a sensibly more linear phase response. These transformers are hand-wound in-house by Nagra in order to meet the exacting standards of the design brief. A balanced input is especially recommended when the cable between the source and the preamplifier is particularly long. The upgrades can be factory supplied or fitted subsequently that means the equipment can be upgraded at any time.2.5 - Taking its place in the rangeThe Nagra Classic Preamp respects the aesthetics of the brand in terms of format, ergonomics and finish. They are designed by the same engineers and made by the same team as the professional recorders, according to the same peerless standards.3. DESCRIPTION OF THE ELECTRONICSThe Nagra Classic Preamp preamplifier is built around six printed circuits, as well as two optional ones for the input and output Nagra transformers. These four-layered, gold-plated, epoxy glass military grade circuits form the basis for the unit’s components that have been chosen according to the most rigorous criteria in terms of tolerances, durability, reliability and listening performances. A ground layer screens out disturbances and radiation while it avoids static loops then prevents from hum. They are linked together with ribbon cables that ensure that the paths between them are as short as possible. The motherboard is mounted on elastomer silent blocks that filter out vibrations.3.1 - Input/output selection circuitsThere is only one large board mounted on the back panel of the Classic Preamp with all the input and output connectors directly soldered to the board so as to avoid any wiring, versus four small circuits and their interconnection wiring inside the Jazz. It carries all the sealed relays dedicated to the switching process of input and output selection and because the topology of the schematic is single-ended, it also converts any balanced signal entering the XLR input connector to unbalanced. The possibility is offered to run balanced signals in and out the Classic Preamp by installing the optional input transformers and output transformers boards inside the case just behind the input/output board. The use of those boards is recommended only if the end user has to deal with balanced signals. The single-ended topology distributes all odd and even harmonics of the audio signal whereas a balanced circuit cancels all even harmonics and only works with odd harmonics, meaning you have to locally inject some negative feedback in order to get the signal back on its feet. So adding optional components even though their qualities are indisputable like to Nagra transformers boards have to be carefully analysed and understood otherwise it could spoil the overall final performances of the product. The Nagra Classic Preamp has six inputs. Five of them, one XLR and four RCA all unbalanced, are managed by all the internal circuitry and enable the preamp to be connected to a wide variety of sources. All those inputs are floating and allow the signal to be managed in a pseudo-balanced way until it reaches the amplification circuit. Each input is filtered to eliminate radio-magnetic interference (RMI) by a combination of inductances and capacitors. High quality sealed relays switch the inputs. Only the ground of the active input is connected, all the others being removed from the circuit so that no static loops are created with and between the inactive sources. The sixth, one XLR called “Bypass”, connects directly the input signal to the output XLR also called “Bypass” in a true balanced configuration. When the “Bypass” in and out connectors are used, the preamplifier is fully bypassed and can be hooked up to a high-end home cinema processor. The “Bypass” signal is linked even if the Preamp is not working or disconnected from the mains supply.The unit has three outputs. Two are line level, one XLR and one RCA all unbalanced, and one is a 6,35 mm jack for headphones. They are selected and switched by relays and only one output can be used at a time.3.2 - Inside view of the Nagra Classic Preamp (with optional output transformers)4819650222250123456700123456713970015081258910111213008910111213226695031553150027051003285490003683000212852000377190014839950021653501428115004000502432050003810002409825004000502343150002127250267335000277495028295600038608021050250041910036220400040005033642300038100043554650041211503757295004000501604645002724150313055001187450459105001. Input / output board, input and output selection with relay switching, balanced signal across XLR connectors converted into unbalanced2. Location for the optional input and output Nagra transformers boards(boards installed in the picture, output transformers to the left, input transformers to the right)3. Output coupling polypropylene custom made capacitors4. Low voltage symmetrical power supplies board, several outputs5. Stage coupling polypropylene custom made capacitors6. Input stage, hand selected ECC83 double triodes with mu-metal screening7. Headphones amplifier8. Mains power supply board with shielded transformer underneath9. Output stage, hand selected ECC81 double triode in mu-metal screening10. High voltage power supply bypassing polypropylene custom-made capacitors11. High voltage power supply board12. Motorized Alps stereo potentiometer13. CPU board, controls (front panel, remote, modulometer) and protection management3.3 - CPU management boardAll of the controls of the Jazz have been implemented in the Classic Preamp. However the Classic Preamp has been completely rethought in a more user-friendly way. A CPU board manages all the functions of the preamplifier and the unit can also be customized. Thanks to a LCD display and a rotary “controller” knob to be turned, pressed and held, the user can navigate through a menu. He can change the language of the display (English or French), rename every input, adjust the balance with a -3.5/+3.5 dB by 0.5 dB step, switch between stereo or mono and use the Classic Preamp with another Philips RC-5 formatted remote control. Last, a set of useful information is also supplied through the menu such as the software version, the tube operation time, the total operation time and the serial number. When tubes are changed, the tube operation time is reset, this is by the way the only information that can be reset.The CPU board is mounted behind the front panel. It controls all the management functions. The microprocessor chip links the controls and the display of the front panel to the matching circuits. It manages the usual functions such as input and output selection, toggle switched and remote controlled actions, muting of amplification stages and standby mode. It acts as a calculation interface with the Nagra modulometer displaying the output signal level and it opens the preamplifier to end user custom settings. It also enables the device to soft start by delaying the high voltage to the valves for 2.5 minutes after switching on in order to protect them from premature wear.The motorised Blue Velvet volume is manufactured by Alps. These components are renowned for their precision in pairing tracks and their reliability over time.3.4 MotherboardThe large board located at the bottom of the chassis has been designed specifically for the Classic Preamp. It houses all the amplification circuits and it holds two vertical boards, one to the back takes care of several symmetrical low voltage symmetrical power supplies (relays, tube heaters, etc…), one to the left creating and regulating the high voltages for the tubes.The Classic Preamp embeds much more components than the Jazz. A huge part of them have been made for Nagra under very strict specification. All the components along the signal path have been carefully and finally selected after intensive hours of listening sessions. Compared to the Jazz that was still employing some electrolytic capacitors at critical locations in the audio circuit such as stage coupling, high voltage bypassing and components decoupling, the Classic Preamp is only using polypropylene capacitors custom made by SCR. As far as the valves are concerned, they are subject to the most rigorous selection and testing. They are burn-in for 48 hours and then individually measured and sorted in accordance with multiple criteria: gain, hum and sensitivity to vibrations (microphonic effect). At the end of the test, more than 60% of them are rejected and the remaining valves are paired up.The Classic Preamp can be powered directly from the mains supply. The mains voltage passes through an EMI/RFI filtered IEC connector then is directed to a linear power supply board where a shielded potted transformer, rectifiers and a Pi filter capacitors smooth the power signal before dispatching it to the different power supply boards. As soon as the various voltages are issued from those boards, a lot of polypropylene decoupling capacitors are installed for improving the signal-to-noise ratio.The overall structure of the Classic Preamp with a three tubes complement has remained unchanged since the Nagra PL-P in 1997, however the specifications has been continuously improved. The first stage of the circuit is built around one 12AX7/ECC83 double triode per channel, the two triodes of each bulb are biased in a class-A and tied together in a differential anode follower topology. This configuration ensures the valves perform in a highly stable manner. The second stage where most of the gain of the preamplifier is performed is entrusted to a type 12AT7/ECC81 double triode, with one triode (one half of the tube) per channel in a class-A cathode follower arrangement and some local negative feedback for very low output impedance and very low distortion. The signal out of the cathode is then directed to the output through a custom made polypropylene capacitor. This valve receives the variable gain from the gain toggle switch on the front panel. According to the position 0 dB or 12 dB of the switch, a relay arranges a set of resistors that adjust the gain of the 12AT7/ECC81 stage accordingly. Mu-metal shields are installed around the tubes in order to improve immunity to external noises.3.5 - Headphone amplifierSince the Nagra PL-P, no Nagra high end preamplifier has featured a headphone output and in accordance with the end users feedback, Nagra was determined to equip the Classic Preamp with a very high quality headphone amplifier that can easily drive the most difficult loads including low impedance ones. The amplifier is based on ultra high-performance and high-speed circuits with noise and distortion reduced to extremely low levels that act as a solid-state current buffer behind the tubes. There are no capacitors along the signal path, and a servo-circuit that permanently maintains a precise 0V DC preserves from any constant voltage across the output.The circuit allows for choosing an output level by means of jumpers that enables it to achieve the best performance with whatever headphones are used.3.6 - Remote controlAs a convenience, the Nagra Classic Preamp is supplied with a remote control that offers various functions such as on/off switch, volume, mute, source selection. Using the widespread Philips RC-5 communication protocol, the unit can also be used with multifunction remote control.4. DESCRIPTION OF THE CHASSIS4.1 - CaseEntirely constructed in finely brushed, anodised aluminium, the Nagra Classic Preamp case is faithful to the look and ergonomic criteria that characterise the brand. However, compared to the Jazz, the Classic Preamp circuits have been significantly improved in terms of quality of audio and power supply components on one hand, and in terms of electromagnetic noise insulation on the other hand. As a consequence the engineers had to design an extended version of the Nagra historical chassis to provide extra space for implementing those improvements. Therefore, while the case base adopts the same 277 mm width and 76 mm height as the Jazz, the Classic Preamp overall depth increased from 254 mm to 379 mm.The 14 mm thick front plate is machined out of solid billet, as well as the top. The back and the chassis are constructed out of folded sheet aluminium.4.2 - Front panel35687001858645020000426085071374000454025020491451020000137719002051685302000034121150204660520200002277495010585450038925501217930003263900114935000426085013684250034226502045335402000043143250205105050200005354393513798550029337002047240602000063003550140970000272415020459707020000720256502050415802000082146300107505500132715020548609020000913970001158240009779002058670100200001010985501420495001. Infrared receiver for signal from the remote control, yellow LED indicator light below2. ?” jack headphones output3. Nagra rotary main selector, on/off, remote control and mute4. Line out selection, XLR, RCA or headphones positions5. Motorised volume potentiometer6. Gain selector, “0 dB” standard or “+12 dB” high 7. “Controller” knob for source selection and browsing through menu8. LCD display 9. Modulometer indicating the output signal level in d10. LED and modulometer light intensity switch (7 levels)4.3 - Rear panel44704001753870802000084889500127000000419100015278100039814509144000038417501434465003422650143954500265430014541500077152515506700041910014789150037020502984550200005391160027305602000064121150266707020000748196502730598009834925026035102000011327150279402020000225844502476530200003335280024765402000041. XLR input2. Cinch inputs3. “Bypass” XLR input with bridge to the “Bypass” XLR output4. XLR output (unbalanced)5. RCA output6. LEMO connector for external Nagra power supply (ACPS II or MPS)7. Gold-plated ground connector8. Shock-absorbing Nagra VFS L long double-plate support (option)9. IEC connector, mains switch and fuse holder4.4 - Shock-absorbing VFS L support (optional)The Nagra Classic Preamp preamplifier can be delivered with an optional shock-absorbing Nagra VFS L long double-plate support.This solid aluminium support equipped with special Alpha-gel? feet is designed to isolate the equipment from vibratory disturbances efficiently. It is especially appropriate for valve-based equipment that is more sensitive to this kind of vibration.The shock absorber is finished off with three little conical feet screwed into the case of the preamplifier itself. These feet are made out of Arcap with points in Delrin? resin which locate precisely in little holes that are machined into the upper plate.5. TECHNICAL SPECIFICATIONSClassic PreampJazzInput impedance50 Kohms75 KohmsOutput impedance6 ohms50 ohmsFrequency response10 Hz to 50 kHz10 Hz to 50 kHz(+0 / -0.5 dB)Signal to noise ratio> 110 dB > 105 dB(ref. 1 V)(ASA-A weighting)(ASA-A weighting)Dynamic range>125 dB> 112 dB(gain at +12 dB)Minimum input levelto achieve 0 dB (meter)0.28 V RMS0.25 V RMS(gain at +12 dB)Maximum input levelto achieve 0 dB (meter)25 V RMS25 V RMS(gain at 0 dB)Total harmonicdistortion (THD)< 0.01 %< 0.02 %(1 kHz, 1 V RMS,no load)Crosstalk>85 dB> 78 dBPower supplyMains 115 or 230 VACExternal Nagra ACPS IIExternal Nagra MPS (option)AccessoriesVFS L long vibration free supportVFS vibration free supportConsumptionless than 10 mW in standbyless than 10 mW in standbyDimensions (DxWxH)379 x 277 x 76 mm 254 x 277 x 76 mm(overall)Weight4.9 kg 3.3 kgSpecifications may change without notice ................
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