GROUP STUDENT PROJECT: SINGLE-ENDED CLASS-A TUBE AMPLIFIER – AUG 10, 2007. 1. Abstract—The design and construction of a single-ended …

Abstract—The design and construction of a single-ended class-A amplifier is presented. By employing the use of tube technology and transformer coupling, a musical sounding amplifier is devised taking advantage of 2nd order harmonic distortion and a shortest possible signal path.

Index Terms—Class-A, Audio, Electron Tube

MODERN AUDIO AMPLIFIERS are efficient, noise-free, and can be as powerful as needed for any given application. Electron tube technology, despite being outdated, inefficient, and inherently noisy, still manages to provide a tonal quality that some ears find more pleasing, despite its pitfalls.

Setting out to design and eventually construct a tube amplifier provides several challenges and hazards. Scarce (and therefore expensive) component availability, dangerously high power-supply voltages, and large components with specific layout and construction constraints are among these. The main challenge, however, is a lack of available information on the use of this technology.

The design chosen for this project involves a single ended audio amplifier, which consists of a resistive input network, a driver stage, and an output stage to a typical 8 ohm loudspeaker load, all the while, using a minimum of supportive passive components for biasing and coupling duties. Power-supply voltage is provided by dual half-wave diode rectification of 120 VAC stepped up by a magnetic transformer. Although somewhat trivial, this design provides a quality audio amplifier that can be ealized at a reasonable cost.

Power supply section

The amplifier uses a simple linear power supply to develop 300VDC. Heater voltages are supplied directly from the 6.3VAC taps on the secondary of the power transformer. The other secondary windings, rated at 380VAC with a center tap, are used for the DC supply. Each half of the 380VAC winding is rectified separately during the positive1 half wave cycle only, using the center tap as the common. In this configuration, the output voltage of the power supply is halved, while the current capacity of the transformer is doubled, since each half-coil only has a 50% duty cycle. Following rectification, a bank of capacitors is used to filter AC ripple. A filter design utilizing a choke in series with the supply was briefly considered, but modern, low-cost high voltage electrolytic capacitors offer a simpler and more economical solution. To calculate the minimum amount of capacitance needed in the filter section, the following equation was used:

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