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With an active buffer stage, which has a gain factor
of 1,
the best of two worlds can be combined resulting in no
losses of dynamics together with the high transparency
achieved when using a minimum of components as in a
passive unit
A buffer stage is characterized by high input and low,
constant output impedances. The circuit is very flexible
in that it can drive any power amplifier without problems
of impedance mismatch. The construction also has a
simplified circuit with a minimum of components resulting
in a short and uncomplicated signal path. Due to its impedance
characteristics and to its function as a buffer between
the signal source and power amp, a well-constructed buffer
stage results in audible improvements even when compared
to a short piece of high quality signal wire.
Construction
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Nirvana can not be achieved by simply building a buffer
stage, any kind of way. Practically everything in an audio
design can affect the sensitive audio signals. Rigorous
attention must be taken to all construction details and
quality of topology and parts. The buffer stage is constructed
along the following criteria:
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hard wire technique without the use of any printed circuit boards
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component layout which eliminates connecting wires
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air as dielectric-no plastic isolation
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no magnetic materials
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as short signal-,ground- and voltage supply connections as possible
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consistent use of high-grade components
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passive regulation of the power supply
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minimization of distortion already at it's potential source
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separate and low impedance ground planes
Passive components
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Conductors made up by the copper laminate of printed circuit
boards have too high impedance and result in noticeable
degradation of the audio information. Instead,
the components are "hard wired". In comparison with thick
copper-laminated boards the conductor's cross-sectional areas
are 25-50 times larger. Where interconnect cables are used,
the hard wire technique also has the advantage of freedom of
choice for the most sound-neutral cable type and material.
Considering cable-isolation, teflon is amongst the best
materials. It has a relatively low dielectric constant but
compared to that of air, it is several times higher.
Listening tests reveal however, that even teflon isolation
of the same cable has an audible signature. We have radically
solved this problem by using air-suspended non-insulated
cables; thus utilizing air as dielectric which is second
best only to vacuum.
Even air-suspended cables of the best material can affect
the sound so all cables are kept to a minimum or totally
eliminated in most places. Through careful component
positioning there are no interconnects whatsoever in the
signal path, from the volume control to the outputs.
All passive and active components including the output
contacts are directly attached to each other.
Audiolive's active and passive preamps are outfitted with
a new type of stepped
volume control
where only one resistor is in the signalpath. For every volume
setting, there is an individual voltage-divider
(1 signal & 1 ground resistor) which are tailored for
constant input impedances of all 24 dividing networks.
Power supply
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Power supplies can have a large audible impact upon the music
information, even in buffer stages with a gain of only 1.
The supplies must have low noise and freedom of disturbing
distortion products. Various parameters such as musical
dynamics are not easily measurable or can be related to
technical data. It is during design and development of
audio equipment very important to evaluate the results
also by listening tests. So far, I have not experienced
any power supply with active regulation and feedback which
does not result in more or less compression of dynamics.
The buffer unit's one active component is fed off
a power supply which uses oversized RC filter banks
and no active regulation. Special concern has been
taken to an effective filtering of RFI, which is also
important for a neutral reproduction of the audio signals.
There are several principles to bias the active device
and all have been tested. Fixed-bias circuits have a
tendency of dynamic compression whereas self-bias is the
preferred system. It has the advantage of having a stabilizing
effect towards component tolerances and aging. Moreover,
no audible side-effects could be detected when using this principle.
Tube or transistor?
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To make this large subject short, tubes and especially triodes
are far more linear compared to transistors. In principle,
it is a better
electronic design
to use a circuit which produces less distortion to begin with
rather than fixing up the damage later with the help of excessive
feedback.
Compared to transistor circuits, tube amplifiers also have
the advantage of 10-30 times higher supply voltages while the
audio signal-levels are the same in both cases. These
relationships imply that tubed constructions are less sensitive
to any variations of the voltage supply.
An additional bonus with tube amplifiers are their capacities
to handle large signal voltages. The buffer stage has a
voltage swing in excess of 200 volts which gives a generous
margin to the signal levels which normally reach a maximum
of 2 volts. In other words, there is no risk of any distortion
caused by excessive signal levels-unless you plug the inputs
directly to the wall socket!
Tube circuit
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There are many various circuit designs for a tube in
a buffer or cathode follower function. During the development
of the buffer stage, many designs were optimized and tested and
improvements were accomplished as the circuit became more
sophisticated. The ultimate solution is a specially developed
cascode-coupling of two triodes, both utilizing self bias.
It measures very well with exceptionally
low distortion
and low output impedance. Finally, I believe to have accomplished
the goal of making a buffer stage which has no audible
effects!
Biamplification
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The active preamp is outfitted with two pairs of signal outputs,
for use in biamplified systems. These outputs come as standard
with 80 ohm resistors for impedance matching of normal signal cables
but can easily be custom-made to suit cables of other impedances.
Traditional loudspeaker systems are designed with passive dividing
filters consisting of large inductors and capacitors in the
signal path. These have detrimental side-effects. For good sound quality,
other systems of frequency dividing networks must be used. It is highly
recommended to drive the speakers directly with individual power
amplifiers! Also electronic crossovers may beneficially be eliminated and
instead, apply bandwidth limitation of the amps (internal modifications)
without the use of any extra filters. The amplifiers in this case are
directly connected, in parallel, to the outputs of the preamp. However,
filter technology and loudspeaker design are other topics so I will stop
here in mentioning that our preamp is designed for use in any kind of
audiophile system, standard or biamped.
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