Tube-based Buffer amplifier


Posted on Feb 5, 2014

The solid-state, unity-gain power buffer is easy to imagine, as many already exist. But what about corresponding tube equivalents Have they been made Could one be made Actually, super cathode followers (complex-and-augmented cathode followers with a gain of 0. 99) have a long history, with many interesting designs appearing in the 1940s and 1950


Tube-based Buffer amplifier
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s. But a tube power buffer robust enough to drive directly loudspeakers is rare, and with good reason: the impedance mismatch can only be easily addressed with an output transformer. Because of the transformer`s phase shifts, however, the amount of feedback that can be wrapped around a tube amplifier and its output is limited. And all buffers use some form of feedback to deliver low distortion and output impedance. Couldn`t an OTL tube-based power buffer be created that would allow much greater amounts of feedback to be used Sure; but it, like all tube OTL amplifiers, will be a beast. In the schematic above, we see a power pentode configured in a simple single-ended topology, with the only interesting twist being the pentode`s cathode direct connection to the transformer`s secondary. This connection eliminates the pentode functioning as an amplifier, as the cathode follows the grid, thus providing 100% degenerative feedback. This short feedback loop gives the pentode`s output stage a low impedance output that it would otherwise be altogether lacking. It also lowers the distortion, improves the PSRR, and extends the bandwidth. Actually, the PSRR is fairly good in a pentode-based single ended amplifier, even without the feedback loop, as the pentode`s high rp, unlike the triode`s low rp, makes a poor voltage divider against the output transformer`s primary impedance. In the schematic below, we can see the noise relationships...




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