The basic design philosophy
The Fairman KT88 Tube Stereo Amplifier operates in a dual tube push-pull
configuration. The preamplifier and phase reverse circuitry is entirely DC couplet to
exclude capacitors in the driver stage signal path. The four necessary capacitors for
the KT88 input grids are “Supreme Silver/Gold” (dry/non oil) manufactured by
Mundorf in Germany.
The KT88 is a beam power output tube with 4 internal elements, a so called Tetrode.
This tube type characteristic is combining the qualities of the Triode with 3 internal
elements and the Pentode with 5 internal elements. The typical KT88 maximum
plate power dissipation is 40 W. The basic major aim for this amplifier is to operate
all stages in about the first half of their maximum ratings.
As tubes are high impedance devices and dynamic speakers are low impedance
there is a need for transforming the output signal.
The output transformers are from the legendary Swedish Audio transformer
manufacturer Lars Lundahl. They are C-core types. The core consists of one left C
and one right C assembled together. A double C-core acts similar as a toroidal core
with almost no loss and no stray fields to its surroundings and are immune against
external influences. The C-core construction allows the transformer coil to be made
of two separate identical and symmetric coil systems. Each coil system, one for each
KT88 is wounded in five sections, giving a total of ten. Most common audio
transformers using EI cores are wounded in five or six layers.
The essence of sectionizing is to split up the primary and secondary coils into several
sections and shift them into each other. This results in a significant reduction of the
many capacities between the many copper layers, essential for the frequency
linearity and very low distortion.
The Power supply uses also a Lars Lundahl C-core transformer with a 250 W power
rating. The main focus point for the B+ high voltage supply is to serve the KT88’s
with enormous energy resources at all levels. A rock steady supply voltage is
essential for the power linearity especially at the lowest frequencies. For that reason
the bridge rectifier, one for each channel consists of four single high power high
voltages silicon diodes for being able to charge and supply the two enormous
2200 uF/450V capacitors. Good normal values for two channels in that position
would be about 500 uF
The amplifier is designed to work in class AB. There is a separate small transformer
for the stabilized Bias voltage supply. The Bias adjustments and test points for the
four KT88 are placed at the amplifier top. The recommended 500 mV test points are
idle the KT88’s at 20 W.
The Amplifier Back Panel
The XLR inputs are transformer balance by Lars Lundahl 1:1 line transformers
The gold plated XLR and RCA A and B inputs are selected by the front panel Input
Switch. The “OFF” position mutes the amplifier.
The “TO SUB WOOFER AMP” output RCA is the signal feed from the Left and Right.
speaker output terminals. The signal level meets normal AUX and active Sub Woofer
sensitivity. It is mono and linear and follows the amplifier volume control.
The Speaker LEFT and RIGTH output terminal pairs i.e. four terminals per channel
are connected vertical/parallel prepared for speaker Bi-Wiring.
The gold plated terminals accept 4mm Banana plug and/or up to Ø 5mm wires.
The IEC Mains input for 230/115 Volt 50/60Hz internal selected by soldering..
Mains fuse 3A @ 230 Volt // 5A @ 115 Volt
Power Output @ 8 ohm: 2x 40 W
Inputs: RCA A and B, XLR transformer balanced
Input sensitivity at max volume: 1V for 40 W
Input impedance: 50 Kohm
Sub Woofer output gain @ max volume: +10dB
12 Hz-22 KHz +0/- 0.15 dB // 8 Hz-56 KHz +0/ -1 dB
10 W 0.04% 20 W 0.10% 30 W 0.15% 40 W 0.175%
AC mains 230V / 115V
Power consumption 165 W
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