I am the original owner of this unit which saw only light use in my bedroom system. Tubes are 6DJ8 Dutch Nationals. They have 50 hours, at most, on them. This is truly an amazing premp for the money. If you are trying to assemble a highly musical system on a budget, this would be the perfect preamp to use! The Musical Design line of preamps was developed by John Hillig whose Musical Concepts was well regarded as a modifier of Hafler, Adcom, Magnavox, as well as other amps and CD players. The SP-2 Vacuum Tube Line Preamplifier was designed for maximum musicality and neutrality and Hillig really achieved this goal. Here is his Design Statement: The Musical Design SP-2 is a direct result of years of investigation which revealed how topology and component parts affect sound quality. At Musical Design, we believe that there is great virtue in simplicity. Therefore our preamplifier has the minimum of stages and complexity. The Musical Design team has a great depth of experience in selecting the essential passive parts that make up a great audio design. It should be clear, to even casual observers, that traditional measurement techniques fail to predict the sound quality of a preamplifier. Given this argument you might think we are suggesting that audio design still smacks a bit of alchemy. You would be right! However, there are technical details to explain, so read on. CIRCUIT DESCRIPTION
The input signal is DC coupled. This vacuum tube line stage preamplifier uses only two tubes, one for each channel. EAch twin triode (6DJ8) operates as a single push-pull voltage gain stage. Therefore, the active circuitry is essentially composed of one stage per channel, not two. The circuitry operates in Shunt Regulated Push Pull (SRPP) mode. This has been called “The most linear circuit design possible for vacuum tubes.” SRPP operation provides extremely wide bandwidth and low distortion without feedback. POWER SUPPLY
Separate B+ voltage pass transistors and capacitor reservoirs are used in each channel. Very large capacitors are used for the ‘raw B+’ and outputs of each regulator. These capacitors are larger than those used in many tube POWER amplifiers. Filament voltage is heavily filtered and then regulated. A huge capacitor is used for the ‘raw’ heater supply and heater voltage regulation ensures no variation in cathode emission during operation. The power transformer is custom manufactured. Its windings are would side by side, not one on top of the other like conventional designs. This is a so-called ‘Split Bobbin’ design which minimizes the intrusion of sonically harmful RFI energy. The overall power supply design emphasizes RFI attenuation and minimum wideband noise. FEATURES
Many features of the SP-2 are unique and expensive. When it comes to value received, we believe the SP-2 is in a class by itself. • The separate tube circuit board is acoustically isolated (decoupled) from the main PCB and the chassis.
• “Direct” input switch is hard-wired with Musical Design AURA interconnect. This input bypasses all other switching and the balance control.
• Stepped balance ‘switch’ does not degrade audio signal like a typical pot.
• Dual ‘Main Out’ jacks simplify the connection of bi-amplified systems.
• ‘Mute’ switch provides full muting of the output at your command. The output relay mutes the output signal for the first 30 seconds at turn-on and instantly at turn-off.
• All RCA input/output jacks are heavily gold plated and have Teflon insulation.
• Cabinet feet are specially designed to minimize acoustic feedback transfer to the preamp chassis.
• Cabinet is fashioned from heavy-gauge damped steel which has good vibration absorption characteristics...it is solid and quite dead.
• Faceplate is machined from .25 inch aluminum which is brushed and anodized.
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