puresound L300 Amplificator

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L300
has 5 inputs and twin outputs (2 pairs single ended and 2 pairs
balanced via XLR). The
circuit uses output transformers and a
sophisticated full valve rectified, choke smoothed, regulated power
supply including
a 300B triode as the series element. A
Line Stage pre-amplifier needs to offer the facility to select between
different sources, to provide
some means of attenuating the chosen source
and then deliver it in an uncorrupted form to the power amplifier. Controlling the VolumeIn recent times, with the prevalence of digital
source components, many audio enthusiasts have adopted the use of
passive volume
controls incorporating resistive potential
dividers and even transformer based attenuators. These are volume
controls where
attenuation is achieved through the use of
tapped transformer windings. On the face of it either of these
might appear to offer the purest approach. However, its not so simple. Resistive attenuators are generally of quite a high
impedance so as not to load the source driving them but in turn confer
little drive
capability themselves. In some
circumstances they can sound quite transparent but its a commonly
experienced subjective impression, that such
attenuators leave the music with a lack of purpose and grip. Long connecting cables may also blunt the high
frequency response, a situation which may be worsened by using a
following amplifier
with low input impedance.
Transformer volume controls seem to address the issue of drive capability to some extent, but they have their
own problems. A wide bandwidth transformer can be made such that, when using both windings in full, i. e.
with no attenuation, it transfers a pretty good facsimile of the source signal. However, as attenuation
is applied and the transformer has to transform more and more, difficulties start to arise. By
the time
significant attenuation is applied, the
performance of the transformer no longer looks quite as impressive with
differing
patterns of resonance being visible at each
step. The reproduction of timbre suffers and the sound quality
realized alters slightly with each change in level. Auto-formers
which attenuate by tapping off a single
sided winding have actually proved to be the
most transparent passive controls we have tested and yet even they seem
to lack
a certain something.
The other issue with volume controls based on a
multi position switch is a sense that the right level for listening to a
piece
of music can never quite be reached. If the
steps of the attenuator are made close enough to suit a source with a
given output
or a following amplifier with a particular
sensitivity, it will be wrong with another source or another amplifier. A
conventional
potentiometer gives a much finer range of
adjustment.
In the L300, source selection is via a high
quality switch located near to the input sockets. The chosen source is
routed to
a high quality film potentiometer and from
there on to the audio circuit. This consists of 2 directly
coupled
triode stages, the second loaded by an output
transformer which steps down the amplified signal allowing it to be
coupled
to the power amplifier at a manageable level
and via a low impedance output. The output stage of the
L300
is impervious to long interconnecting cables
or lower impedance power amplifier input stages. The use of output
transformers
also allows the option of a true balanced
output configuration if desired. A low gain, low output impedance stage is in itself nothing new. Many transistor pre-amplifiers are
similarly specified. However, they usually achieve it by means of excessive negative feedback which is
highly detrimental to the subjective reproduction of music. Power Supplied properly The
L300 also incorporates an extremely sophisticated regulated power
supply to maintain constant operating
conditions for the audio circuitry. It
includes a valve rectifier, a choke filtered stage and then a series
regulator featuring
the 300B power triode as the passing tube. A
very elaborate error amplifier referenced to a gas voltage reference
valve allows
the circuit to precisely maintain the high
tension supply regardless of fluctuations in the mains supply and the
varying current
drawn by the audio circuit. Filament supplies
are also fully regulated to further reduce the potential for noise. The L300 preserves the verve and energy in recordings
that almost all pre-amplifiers and passive volume controls otherwise lose. It does this
without sacrificing transparency or introducing euphonic colouration to the sound.
Denumiri similare la Amplificator puresound L300: L 300
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