The STA.100 is a
dual channel amplifier of large power output intended for sound reproduction of
the highest quality. Performance characteristics are similar to those of the
Series 3 Amplifiers but various circuit differences exist due to the higher
voltage level of operation, although the basic principles are the same.
This amplifier has
been designed to satisfy the need for a larger power output than normally
required in the home, such as in recording, monitoring, and high level sound
distribution where a dual channel amplifier is required, or can be used to
advantage.
Two impedances are
provided, 8 - 16 Ohms (nominal) and 100V line.
The normal input impedance is high, but to satisfy professional
requirements the amplifier is available with a balanced 600 ohms input,
additionally. Thestandard chassis is pierced for the inclusion of transformers
and 3 pin input sockets, to permit for conversion for balanced input,
additionally, at any time.
The STA.100 is
designed on a very generous basis for continuous duty operation with minimum
maintenance. Once installed and adjusted after a running-in period it should be
necessary to replace valves at regular intervals only depending on the
operating duty cycle. The amplifier is
substantially constructed, is totally enclosed and able to withstand the rigors
of frequent handling and transportation in commercial vehicles.
The basic amplifier
circuit is a 3 stage design having overall feedback from the secondary of the
output transformer to the cathode of the input stage. The conventional pentode
input, triode pentode phase splitter arrangement as used in the Series 3. Amplifiers
is unsuitable in this design due to the larger voltage swing required at the
output valve grids. The STA.100 circuit is a new design using the latest valve
techniques resulting in extremely low phase shift with consequent high feedback
capability and absolute stability.
Input Stage
The input stage uses
a low impedance twin triode valve (ECC88) in a cascode circuit. Secondary
feedback is taken into an undecoupled
resistor in the cathode circuit. A high frequency phase correcting network is
connected across the anode resistor and the output is direct coupled to the
phase splitter through a low frequency phase correcting network.
Phase Splitting
In order to obtain
yhe high voltage swing necessary to drive the output stage to a power in excess
of 100 watts, a substantial phase splitter is required. Two EF184 video power
pentodes are used in a balanced circuit. Pentodes have a low anode-to-grid Miller reflection and a very low phase shift
design is possible. Drive to the output stages is balanced by the means of a 2
kilohm potentiometer in the power supply anode circuit of the EF184's. The potentiometer is adjusted for equal
clipping in the output stages to obtain a maximum available power output
condition at low distortion.
Output Stage
The output stage
uses KT88 beam tetrodes connected in an ultra-linear mode with the screen taps
at 40% of the winding. The valves are operated in class AB condition with fixed
bias drawing a quiescent current of 60mA per valve. The output transformer is a multi-section
type having series and parallel windings for maximum performance and
efficiency. In the 8-16 ohms output condition the three secondary winding are
connected in parallel, and for the 100V
line output they are connected in series.
As the feedback requirements are different for each output condition,
separate networks are incorporated. To change from one output condition to the
other it is necessary to adjust the winding connections on the output
transformer bobbin face, and change a link for the feedback circuit on the
printed circuit board.
Power Supplies
To eliminate
feedback through the power supplies from large current swings of the output
stages a separate power supply is provided for the voltage ampllifying and
phase splitting stages. Two power transformers are used; one providing 600V for
the output stage from a silicon diode bridge and the other supplies 350V for
the drive stages. The high voltage circuit has very low internal resistance in
order to meet the demands of the output stage for high transient power. The
350V supply transformer energises a silicon diode biphase rectifier with
choke-capacitor smoothing DC bias for the output stages, plus the heater
supplies are also provided for each output valve, fitted to a sub-panel at the
rear of the chassis, and readily accessible for adjustment.
Construction and
Finish
The amplifier is
constructed on a 14 s.w.g. mild steel chassis Sub-assembly modules are used
however, to assist in manufacture and service. The valveholders are mounted on
a separate subchassis inside the main chassis to which the main circuit board
is fitted. Valves and transformers are enclosed in a perforated metal cover,
and vertical carrying handles are fitted to each side of the chassis.
Ventilation is provided by perforations in the amplifier base and chassis. The
amplifier chassis and cover are finished with two coats of light grey stove
enamel after an etching primer, each coat stoved. Handles are chromiumplated
and internal metalwork is finished bright zinc. An anodised aluminium screen
printed panel is fixed to the front of the amplifer illustrating the input and
output facilities, and a similar panel is fitted to the rear showing power
stage biasing arrangements.
Transformers
Transformers are
manufactured to the highest possible atandard. Bobbines are made from synthetic
resin bonede paper and coils are wound with a vinyl-acetal covered wire. No
oleo-resin enamelled wires are used. Polyester film is used as
insulationbetween high voltage windings and between sections in output
transformers. Wound coils are impregnated with a thermo-bond varnish and cured
by baking at 120°C for 10 hours. Copper screens are used between the primary
and secondary windings in mains transformers. Assembled transformers are sealed
with a tropical enveloping varnish making it vitrtually failure proof under
normal working conditions.
Performance
Characteristics
Power Output
Rating: 100 watts nominal, continuous
RMS sine wave per channel
Voltage Frequency
Response (at 1 watt output): 20 Hz to
20,000 Hz, -1 dB +0 dB
Power Frequency
Response (at rated power): 20 Hz to
20,000 Hz, -1 dB +0 dB
Power Frequency
Bandwidth (at rated power): 20 Hz to
20,000 Hz
Output Load
Characteristics
Damping Factor
(Referred to 60 watts):
8 Ohms load at 1 kHz
: 22
8 Ohms load at 50 Hz
: 15
16 Ohms load at 1
kHz : 60
16 Ohms load at 50
Hz : 30
Matching: 8 -16 Ohms nominal and 100V lone
Stability:
Unconditionally stable in accordance with BS3860 specification
Input Sensitivity:
350 mV for 60 watts
output into 8 Ohms load.
450 mV for 60 watts
output into 16 Ohms load.
Input
Impedance: 100 kohms
Signal-to-Noise
Ratio: 95 dB below 60 watts output into
16 Ohms
Crosstalk: 5 kHz and below, better than 85 dB
At 10 kHz : 80 dB
At 20 kHz: 65 dB
Transient
Characteridtics: Rise time less than 5
µsecs. Overshoot and ringing less than 3%
Powe Supply: AC 100 -140V / 200 - 250V, 50/60 Hz