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ST-7

Class A Single Ended pre-amp

ST-7 can match to ST-1 Master built the all single end class A combo
http://www.audio-gd.com/Pro/amp/ST/ST1MasterEN.htm


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   Applies all audiophile grade components, including DALE resistors, RA resistors, EVOX and SOLEN caps, custom order NOVER (UK brand) caps, all audiophile grade input & output sockets.

   ST-7 Single end pre-amp overview:
   
The ST-7 pre-amp has been designed as a complimentary pair of Class A single ended preamps.

     The single ended Class A amplifier is a highly respected design with a tradition going back to Linsley Hood. More recently it has been made famous by Nelson Pass and other respected audio engineers. Class A, SE amps have the characteristic of operating over the whole of the input cycle in such a way that the output signal is an exact scaled-up replica of the input with no clipping. Power draw on the mains power supply is high and close to constant regardless of signal strength and thus Class A amps are much less efficient than, for example, class AB amps. In return, Class A SE amps produce even-order harmonics which makes them highly regarded by audiophiles for their warmth, sound stage and resolution.

     While the original Linsley Hood circuit was elegant and simple, it had a low power output. The increases in power gained by later designs like those of PASS brought with them certain problems. A differential input circuit has a tendency to reduce the SE character of the design and though it can be improved by re-tuning circuit components, the result is not as natural. Another approach used in class A SE amps has been to use bridging techniques but this is arguably an even less satisfactory solution than the differential input method.

     The design of the ST-7 pre-amp and the ST-1 power amp is as much philosophical as it is technical and brings to fruition many years of experience. Combined in these amps is the driving power of an SS amp along with the resolution, tonal colour and flavour of a SET valve amp. On top of this comes easy maintenance and low cost.

  Circuit design analysis
     The design of an outstanding electronic circuit must be based not just on technical specifications but should also take into account an understanding of the acoustic fidelity of the individual components used. Beyond this, a designer has also to anticipate how these components will combine to produce a neutral and natural sound. It is much more satisfactory to make good initial choices than to have to apply add-on corrections later.
 
     Inherent in this classic SE amplification circuit is the problem that variations of the operating current and output voltage can occur with temperature changes. Control of these fluctuations is essential to prevent catastrophic and expensive damage to speakers. Traditional solutions to this problem have been:

       1. the addition of several thousand uF of high grade capacitance just prior to the output terminal to block DC.

     
       2. to employ a differential circuit to limit DC drift [as exemplified by PASS circuitry]

       In pursuit of perfection, a different approach has been taken with the design of the ST-7 using dc servo to lock the operating points. The DC servo circuit circuitry used here is based on a fresh examination of theory and thus breaks away from more conventional thinking. The first stage of the servo circuit uses an integrated amplifier to extract DC from the signal while reducing AC. In addition a -6db buffer is present to reduce any noise introduced by the integrated amplifier.

       The output of the DC servo again passes through the RC filter, then to the feedback coupling capacitor to get rid of the alternating component, yielding the pure dc component. When the DC level in the output is changing, the DC servo output point TP1 changes accordingly. The V/I variations pass the resistor and alter the operating point of the input stage to restore OV output. The output dc is limited to 2MV during the actual circuit test.

        Looking at the main amplifier, the circuit is quite straightforward, though it deserves explanation. If the first stage of the circuit was to use a constant current to obtain a high S/N ratio this would necessarily also reduce the Class-A single-ended character of the amplifier. Therefore this stage does not use constant current source but instead allows other parts of the circuit to enhance the S/N ratio.
 
       The second part is the critical main gain stage where most of the gain occurs. The operational characteristic here significantly influences the whole sound. If this stage only uses common-emitter circuit, the sound is dense and warm, but blurred and lacking delicacy. With a desire to reproduce the mid/high frequencies to convey the feeling of running water, floating cloud and high transparency a different approach is needed. So, a cascode circuit which has the desired timbre, becomes the logical choice here. The next challenge is to keep the cherished timbre but at the same time to reduce the wideband characteristics.
 
        The loading of this circuit is Constant current source that can enhance load capacity, maximize output effectively and concurrently lower the distortion. When the high current Darlington transistor's [MJ11032/MJ11033] operating current is 80MA, the sound quality is well balanced and delightful. Hence, this circuit is set at 80MA approximately. The output current is enough to drive the output transistors without using any pre-driver transistors.

         To bring out an open and singing nature requires low open-loop gain. This circuit's open-loop gain is 30DB only. The -3DB frequency response is at 70Khz in open-loop station. Closed loop gain is 13DB. The local feedback loop is connected from the output of the voltage amplification parts but does not rejoin at the front-end round to give global feedback. Audiophiles might appreciate this aversion to global feedback.

 
        The design of this system also tested out a theory that a tube-like soft and deep low frequency presentation would be favoured by the use of a low damping factor. This was achieved by not using a pre-drive transistor. Auditioning the completed system proved the theory to be correct though interestingly, this form of design is counter to that conventionally used in hi-fi equipment with huge dynamics.
 
       
Along with the high operating current comes an increase in supply ripple, and potentially hum.  An alternative to enhance SNR is to provide a voltage regulator to the output stage. The stabilized voltage supply on the left side of the power supply diagram is for output and the supply on right side is for voltage gain circuits.
These voltage regulators perform well and are of a well respected and recognized design. The circuits have been re-designed to enhance temperature stability and to suppress ripple capability.
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      The circuit structure is the same as the voltage gain stage of the power amp, only the gain is altered to 13DB.

   The volume control is of the parallel divergence type. At maximum volume there is -3db attenuation. Also at maximum volume the overall gain of the ST-7 is 10db.                                                                      

 The design of this preamp emphasises the characteristics of the SE preamp with the slight differences to the circuit that add richness to the sound.

 By replacing the C-3 with the ST-7 pre-amp, the sound immediately changes to another flavor, clarity and sound stage displayed as before, bass is resilient and dynamic, but the high-mid becomes softly round and rich, female voice appears slender, tender and full of sentiment, the harp imitates like the falling bead to the jade plate, the violin sounds touching, Ancient Zheng sounds less sharp, seems surreal. But the sound is joyfully easy, like dreaming. A more expensive hi-end tube product could not achieve this balance.

  Specs

  Weight: 15KG

  Dimension:  435]W^*435]L^*85(H) MM  


     How to choose the function in ST-7 :
      With the ST-7, the user has a choice of 70 steps and 99 steps of volume control, and you can also choose whether to have volume memory.
1. Choose 70/99 steps volume control:
Step A: Push in the "Debug" switch.
Step B: The display shows as pictures below.
Step C: Turn the volume knob; choose between 70 or 99 on right.
Step D: Push out the "Debug" switch.

           70 step volume control                      99 step volume control
             
2. Choose volume memory:
Step A: Push in the "Debug" switch.
Step B: Turn the selector knob; the display shows as pictures below.
Step C: Turn the volume knob; choose between "ON" or "OF" on left.
Step D: Push out the "Debug" switch.

         Volume memory active.                          Volume memory off.
              

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