Octave HP 500 SE, Manual

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8
1. OCTAVE TECHNOLOGY
1.2. Description HP 500 SE
Negative
feedback
The HP 500 SE Line is a two-stage tube amplifier with overall negative
feedback. Amplifiers with feedback make ideal preamplifiers. Feedback
reduces the sensitivity to manufacturing tolerances to a negligible level and
also produces a low impedance output. Unless the output impedance of the
line stage is kept to below 200 ohms, dynamics are compromised and fine
detail is lost.
Tubes In the HP 500 SE, an ECC 82 is responsible for the input section of both
channels, with 2 x EF 184 (D 3A) wide-bandwidth pentodes in the output. The
tubes operate in class A. Their extremely low impedance allows them to
deliver a high output current.
Bandwidth The SE version differs from the standard HP 500 in the way its output
configuration has been dramatically overdimensioned. There are also major
differences in the power supply. The output stage has been designed for
maximum bandwidth with a view to extending the low frequency response
downwards rather than extending the upper frequency limit.
As a result, this amplifier reproduces bass instruments with great definition
and authority - without a trace of bloom. The ability to reproduce
fundamentals cleanly is a prerequisite for a natural midrange, and a high
upper frequency bandwidth is important for consistently low distortion. The
designer's intention is that the amplifier should not add any colouration of its
own to the sound. The bandwidth of the line stage extends from 0.2 Hz - 2.3
MHz, +0 - 6 dB at low gain.
Stabilized
power supply The power supply has now been fitted with electronic stabilization throughout.
A low noise floor is only possible when the supply current is absolutely clean.
We therefore use MOSFET transistors to regulate the high-voltage supply.
The heating and auxiliary circuits are also stabilized. All voltages have a
noise and hum component less than 300 µV.
The benefit of all these measures can be seen in the signal-to-noise ratio of
the line stage, which demonstrates an unusually high value of -103 dB
Power
Management Preamplifier stabilization is integrated with the electronic soft start surge
control (Power Management). Soft start, which carefully ramps up the heating
and supply voltages, is extremely important in extending the life of the tubes
and maintaining a consistent sound from the tubes. It also relieves the power
supply components of the strain of handling huge surge currents at turn-on
and assists the tubes in achieving a service life as long as 10 years.
Gain switch You can adjust the gain of the line stage in the HP 500 SE. This means you
can tailor the effective adjustment range of the volume control to suit your
particular power amp-loudspeaker system.
True
symmetry Also new is the transformer-coupled XLR output. The only way to achieve
true, fully floating balanced signals is with a transformer. Balanced inputs
depends on the + and - signals being exactly 180 degrees apart. This is
where audio transformers come into their own because transformer coupling
can provide two perfectly out-of-phase signals from the very lowest
frequencies up – without introducing any group delay distortion.
Another advantage of transformers is that they provide galvanic isolation,
which makes it impossible for earth loops to develop between the
preamplifier and power amplifiers or active loudspeakers.
Switchable
phono Phono input
The HP 500 SE can be supplied with an optional phono preamp, providing
separate inputs for MM and MC. The input impedance for MC is adjustable
between 37 and 500 ohms.