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Universal 15VA HI-FI amplifier.
Radioamator, Rok X, kwiecień 1960, Nr 4 (Radioamateur, Year X, April 1960, No. 4)
Editorial office: The following description concerns a system, the model of which was built at our request and practically tested by the designer.
The amplifier described here is perfect as a final power amplifier for playing music from records or tapes in the apartment. It is also suitable for broadcasting dance music in medium-sized halls and clubs.
Construction is easy; it can be performed by any radioamateur with basic theoretical and practical knowledge.
Due to the need to obtain a power equal to at least 10VA with very low nonlinear distortions, I decided to use the output stage amplifier in a push-pull configuration with negative feedback. This is called "ultralinear" system. The feedback voltage in this system is obtained from taps, or from separate windings on the output transformer. By changing the ratio of the alternating voltage of the screen grid to the anode alternating voltage, we change the working conditions of the end tubes. With this ratio equal to one, the tubes work as triodes, because the shielding grids are connected with the anodes, while with the ratio equal to zero, they act as pentodes (Fig. 1).
Fig. 1. "Ultralinear" system.
For the ratios with intermediate values, the circuit has a number of advantages over push-pull circuits, both with triodes and pentodes, and above all, less nonlinear distortions at large and small signals, at the cost of low power loss. Moreover, such feedback significantly reduces the internal resistance of the system.
For EL34 tubes, the 20% tap is the most advantageous (Usz / Ua = 0.2, of course, from the power supply side).
The "ultralinear" circuit with EL84 tubes gives about twice less nonlinear distortions compared to the conventional push-pull circuit with 10% lower power. The "ultralinear" circuit requires a special design of the output transformer.
Both halves of the anode windings of the transformer should be placed symmetrically by dividing the window into two sections, each for one half of the anode winding. The unbalance has a significant impact on the phase shifts and amplitude differences between the anode current and the screen voltage, so that even harmonics of higher frequencies may not reduce each other.
Fig. 2 shows a diagram of the windings of such a transformer, and Fig. 3 shows the arrangement of the windings.
Fig. 2. Diagram of the windings of the output transformer.
Primary windings: 0.18mm copper wire in enamel.
Secondary windings: 0.75mm copper wire in enamel.