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At VTL we feel that tubes are the best technology
for audio, for the following primary reasons:
- Tubes use simple circuits
- Tubes overload gracefully
- Tubes are more linear, and therefore,
- Tubes require less overall negative feedback
Tubes are a more viable technology now than
in yesteryear, as better components are now available for
modern tube amplifiers.
 Simple
Circuitry 
At VTL we have found that in general, passing
the signal through a lesser number of components yields a
purer, more immediate sound. Simple circuits with fewer components
have been found to sound cleaner than complex circuits with
many components in the signal path. This is because
there are fewer components to change, or distort, the sound
(add to or degrade the signal).
Simple circuits also have inherently higher
reliability, since there are less parts to fail.
Tubes are more tolerant of circuit drift
and parts deviations, and thus can be used in the simplest
circuits.  top
Dynamic
Handling Capability
Music has great dynamic swings, and in tube
amplifiers the onset of clip/overload as maximum power is
reached is gradual, with a low even-order harmonic.
Even-order harmonic distortion is somewhat
benign, and less offensive to the ear than the harsh, odd-order
harmonic distortion characteristic displayed by solid-state
circuits, even though the distortion figures for tubes are
somewhat higher.
Transistor amplifiers generally reach their
power limit and clip in a mostly odd-order harmonic, which
is more fatiguing to listen to. In such a transistor amplifier
the distortion rises very quickly as the maximum power level
is reached, with an almost square wave characteristic, and
a high DC component, (which can destroy loudspeaker drivers).
As an example of the difference in the distortion
characteristics between the two technologies, tube guitar
amplifier manufacturers have traditionally designed their
equipment to drive the output stages into overload distortion,
using the resultant distortion to get the sound they like,
also known as "tone". In a tube amplifier this tone
contributes to the amplifier's sound, but in a solid-state
amplifier this distortion would be intolerable and would destroy
speakers. top
Linearity
and Feedback
Tubes are voltage amplifiers (as opposed
to solid-state current amplifiers), with the result that tubes
are a more linear amplification technology, requiring less
overall negative feedback to make the circuit linear.
Overall negative feedback is a sample of
the output of the amplifier re-injected into the input 180
degrees out of phase, and is used to correct non-linearities
and distortions.
Too much negative feedback in general tends
to slow the amplifier down and suck the emotion and life out
of the music. High feedback designs usually give a sterile
and boring, lifeless sound, while low feedback designs give
a more immediate sound to the music.
Negative feedback has other benefits, as
the use of feedback helps to lower output impedance of the
amplifier. Lower output impedances can control loudspeaker
loads better. Zero feedback designs tend to have very high
output impedances, and therefore tend to be very reactive
to the loudspeaker load, while high feedback designs can help
to provide the flattest response into a given loudspeaker
load.
Zero to 20dB of negative feedback is generally
considered acceptable, and is usually the maximum amount of
feedback needed to make a tube amplifier circuit linear and
to keep the output impedance down to an acceptable level.
Transistors (depending on technology and type of output device
used) generally need over 50dB of negative feedback overall
(either globally or within local loops).
The benefits of feedback are lower output
impedance, (and therefore higher damping factor), and generally
less reactance to the loudspeaker load and therefore better
control of the loudspeaker. Single ended amplifiers with zero
negative feedback, while being the most simple amplifiers
lowest in parts count, are very high output impedance. Because
of the high DC current in the output transformer, the output
transformer is easily saturated, with the result that such
amplifiers are generally extremely low powered and have very
narrow frequency response capabilities. top
Power
Supply
The higher working voltages present in tube
amplifiers generally allows better voltage swing capability
and better headroom. This higher working voltage yields higher
audible energy storage with a lower value capacitor (audible
energy storage is voltage squared divided by 2 multiplied
by capacitance) Compare 600 working volts of tube amplifiers
vs. 80 working volts of transistor amplifiers. This is most
likely why many listeners feel that tubes sound more powerful. top
Availability
of Modern Components
By using transistors in the area where they
are better: Solid state rectifier diodes offer faster rectification,
and have the ability to address a much higher capacitance
than the older tube rectifiers. (Tube rectifiers are designed
to handle only 50 microfarads (mf) at maximum, while solid
state rectifiers can handle capacitances greater than 4000
mf). Solid state rectifiers also offer far better reliability
and don't age as quickly as tube rectifiers. (Tube rectifiers
age and eventually short out the power supply cap, which is
an expensive repair).
Newer electrolytic capacitors available today
have much higher energy storage in a smaller package - 4000
mf and greater - than older paper in oil caps, which offer
just 50mf maximum. In any amplifier a better power supply
translates to wider high frequency response and better bass
control capability.
Better quality insulation materials available
today, as used in VTL Signature output transformers: The tighter
coupling between each layer, and the better interleaving of
the output transformers both help to keep capacitance (and
resultant high frequency roll off ) down, and offers more
efficient current transfer with lower insertion loss, to improve
current supply capability to the loudspeaker, thereby offering
much improved bass and top frequency performance capability.
PC boards available today offer a better
method to keep components apart, which is superior to point
to point assembly. PC boards offer easier manufacturability
and serviceability.
Teflon insulated wire is more suited to the
higher working voltages, which is a more stable insulation
than the old system of cloth wrapped wire. High voltages run
on separate wires avoids copper crystallization (whiskers,
due to high voltage) between layers of fiberglass when high
voltage is run on PC board traces. Teflon wire also has better
thermic stability and can withstand higher current for a short
time than PC board traces.
These multiple newer technologies allow us
to make tubes perform closer to the limit of their designed
capabilities, which couldn’t previously be done, and
allows tubes to offer better sonic performance than before. top
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