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A two-stage AC power amplifier for home construction
Parmeko 25-watt amplifier kit assembled.
The name Parmeko has for long been associated with high-class public address equipment, and now, in order to meet the demand for a moderate-priced power amplifier, Partridge and Mee, Ltd., Percy Road, Aylestone Park, Leicester, have introduced a 25 Watt unit in kit form. It is AC operated and will deliver about six Watts undistorted power output, this being sufficient to operate several moving-coil loud speakers at full volume. Two stages are employed; the circuit consists of an input volume control, a Mazda AC/HL valve transformer coupled to a Marconi PX25 and a choke capacity output filter. Interposed between the LF transformer and the output valve is a small unit consisting of an HF choke, a resistance and a grid leak, its function being a parasitic oscillation suppressor.
In the Parmeko amplifier the input volume control has a resistance of 500,000 Ω, a somewhat higher value than usual, but it is fitted so that when following the detector valve of a wireless receiver it can be used as a variable grid leak in a resistance or choke-capacity inter-valve coupling.
The high tension supply is derived from a full-wave rectifying valve of the C class, giving some 450 Volts after smoothing on full load. Good quality components are employed throughout, the mains transformer, smoothing choke and output choke are the firm's own make, while the various resistances are wire-wound and all capacitors have an adequate factor of safety.
Using a high-grade gramophone pick-up, and one of the latest models in the Parmeko range of permanent-magnet moving-coil loud speakers particularly good results were obtained on test. The output is well balanced with an adequate treble response to give brilliance to the reproduction without accentuating record surface noises. Despite the good response at the lower end of the frequency scale, very little mains hum was noticed.
A more detailed investigation of the amplifier reveals that between 70 and 4,000 Hz the amplification is sensibly constant, but above this there is a slight falling off. At 6,000 Hz, however, an improvement occurs, and this is maintained up to 8,000 Hz, where the output, measured across a non-inductive resistance, was found to be about eleven decibels lower than at 1,000 Hz. Included with the kit are two blue prints, one showing the theoretical circuit and the other a full-size wiring diagram. The latter could be employed as a template for laying out the components if desired. The price of the complete kit, excluding valves, is £14 10s. For those who wish to en-close the amplifier totally, a 22-gauge steel case, cellulose enamel finished, is available, the price being 21s.
'Parmeko Senior' loud speaker.
The loud speaker used throughout our tests with the amplifier was the new Parmeko Senior - model. It is a permanent-magnet type fitted with a12 in. diameter cone and handles a large input. An input transformer is contained in the base, giving two alternative ratios, viz., 1:11 and 1:22, and the price, in chassis form, is £7.
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