Valve audio amplifier technical specification

  
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This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. Valves (also known as vacuum tubes) are very high input impedance (near infinite in most circuits) and high-output impedance devices. They are also hig
Valve audio amplifier technical specification - schematic

h-voltage / low-current devices. The characteristics of valves as gain devices have direct implications for their use as audio amplifiers, notably that power amplifiers need output transformers (OPTs) to translate a high-output-impedance high-voltage low-current signal into a lower-voltage high-current signal needed to drive modern low-impedance loudspeakers (cf. transistors and FETs which are relatively low voltage devices but able to carry large currents directly). Another consequence is that since the output of one stage is often at ~100 V offset from the input of the next stage, direct coupling is normally not possible and stages need to be coupled using a capacitor or transformer. Capacitors have little effect on the performance of amplifiers. Interstage transformer coupling is a source of distortion and phase shift, and was avoided from the 1940s for high-quality applications; transformers also add cost, bulk, and weight. The following circuits are simplified conceptual circuits only, real world circuits also require a smoothed or regulated power supply, heater for the filaments (the details depending on if the selected valve types are directly or indirectly heated), and the cathode resistors are often bypassed, etc. The basic gain stage for a valve amplifier is the auto-biased common cathode stage, in which an anode resistor, the valve, and a cathode resistor form a potential divider across the supply rails. The...



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