What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Radford STA.100 (1967)

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 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

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