More Hybrid-Amplifier Design

The Journal of Tube Audio Circuit Design


Note from the Editor I thought that this issue would be up by Feb 1. Sorry for the delay. The sad fact is that this journal is always the last priority. I wish it were the my first priority, as I would like to cover many more topics and experimentations. But until it can economically justify the time and effort spent on it, the back burner is where it will stay. So please tell your friends about this journal, as the more readers it has, the better the chance it will have at attracting paying advertisers. The circulation count was removed not to hide the number, but because most readers have taken advantage of the PDF format. Speaking of which, I am buying the latest version of Adobe's Acrobat Distiller, so the files will become more user friendly. On a happier note, this issue is full of tube circuit information and analysis. The complaint that I have shunned the 300B is answered by using it in the design examples of the article on partial feedback. The tube is undoubtedly great, but even great things can bore one eventually. Seemingly, every article prompts an e-mail asking if the 300B could be used in the circuit; well, this time it can be. A second brief look at the problem of designing a portable headphone amplifier is made in this issue. Additionally, the e-mail section contains a lengthy reply concerning a hybrid Class-A SE amplifier. Remember, if you have a request or suggestion of your own for either an article topic or circuit explanation, please e-mail: Editor		Partial Feedback Amplifiers For the last two issues I have been promising to resurrect a worthy old tube circuit topology. Although this topology was used mostly in power amplifier output stages, its application could be (and should be) much wider. Going by the names of "the partial feedback amplifier" and "the inverse feedback pair amplifier," this circuit topology is defined by the use of a resistor spanning the output tube's output to its input, i.e. the wrapping of a feedback loop from the plate to the grid, and as the plate inverts the grid's signal, a negative feedback loop. Partial-Feedback Topology This feedback-loop delivers the goods: it lowers distortion, output impedance and noise at the plate, while extending the frequency bandwidth. Because the feedback loop is so short, this feedback system finds little of the instability problems usually associated with the use of feedback. Every additional stage (and its reactive components) burdens feedback stability. The analogy might be of the line of people passing a story from one listener to the next, with the usual outcome of the story straying from the first telling to something unrecognizable at the last. In an amplifier that uses feedback, if the signal fed back ever becomes phase inverted, the amplifier ceases being an amplifier and becomes an oscillator.
In This Issue
Partial Feedback Amplifiers Portable Tube Headphone Amplifier E-mail Previous Issue Index Glossary of Audio Terms Classic Magazine articles

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Partial Feedback Amplifiers