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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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