Negative Feedback

2 Articles

Tube Design Tips To Save A Writer’s Project

December 30, 2023 by 24 Comments

Most of the stories we cover here are fresh from the firehose, the newest and coolest stuff to interest you during your idle moments. Sometimes though, we come across a page that’s not new, but is interesting in its own right enough to bring to your attention. So it is with our subject here, because when faced with a tube circuit design problem, we found salvation in a page from [The Valve Wizard].

Do you need to apply negative feedback to a triode amplifier? The circuit is simplicity itself, but sadly when we were at university they had long ago stopped teaching the mathematics behind the component values. Step forward everything you need to know about triode amplifier negative feedback.

Negative feedback is a pretty simple idea: subtract a little of the amplifier’s output from the input. It reduces the amplifier’s gain with a flat response, so it’s useful for removing humps in the frequency response and reducing the tendency for distortion. In a single-ended triode amp it’s done with a resistor and capacitor from anode to grid, but the question is, just what resistor or capacitor?. Here the page has all the answers, taking the reader through calculating the desired gain, and picking the value of the capacitor to avoid affecting the frequency response. We wish that someone had taught us this three decades ago!

The website is full of really useful info about valve or tube amps, and it’s worth mentioning that he’s made it available in book format too. There’s no reason not to have a go at vacuum electronics. Meanwhile in case you are wondering what project prompted this, it was a quest to improve upon this cheap Chinese kit amplifier.

Stop Touching My Face

July 13, 2021 by 12 Comments

We all have a habit or two that we’re not terribly proud of and have probably thought of any number of ways to help rid ourselves of them. Well, [Friedlc] wondered if he could create a mechanism that would get him to stop touching his face using a bit of negative conditioning. He rigged up a head brace that slaps his forehead whenever reaching for his face.

The first thing he needed to do was to detect a hand approaching his face. He decided to use a few cheap IR motion sensors he had laying around but noted they had a few limitations. He had a tough time tuning the sensitivity of the motion sensors to prevent false positives and they were completely useless in direct sunlight as the sun’s radiation saturated the photodetector. Despite these problems, [Friedlc] figured he would mostly need his device indoors so he stuck with the IR detectors.

For the “hitter” as he called it, he thought of a few different ideas. Maybe a rotating drum with a flap that would hit his hand or maybe a hitting arm on a bar linkage. He admitted that his rudimentary mechanical design knowledge made thinking of the perfect “hitter” a bit challenging, but like any good hacker, [Friedlc] just kept working at it. He decided on using a cam mechanism which allowed him to separate the motor from the hitting action. This choice actually put a lot less load on the motor which kept the motor from stalling and giving him other kinds of trouble.

[Friedlc] was pretty proud of his invention and noted that it really helped him stop touching his face as the successive strikes to the head were definitely quite a deterrent. This certainly isn’t the first time we’ve seen a Pavlovian Conditioning project on Hackaday. We could probably all use a bit of help curing a few bad habits. But maybe you prefer positive reinforcement instead.

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