A Note on Feedback

This is a copy of a comment I made during a forum discussion on “improving” the response of an amplifier.

So lets talk feedback for a minute. There are lots of ways to apply feedback and all feedback performs the same function. Any differences in feedback response are usually due to the different impedance functions applied to the feedback loop. Contrary to popular opinion, there is no magic in any feedback function. The effects of feedback are to reduce gain, increase bandwidth, reduce circuit induced distortion and noise, and decrease gain function sensitivity to circuit parameter variation. And it works. We’ve been using feedback amplifiers widely since Harold Black introduced the concept back in 1934.
Continue reading

I’m Making a Request

When writing for a webpage or a blog it is sometimes very difficult to get a feel for your audience. You never really know if you’re talking to a few friends over coffee, standing in front of a classroom of twenty, or lecturing to an auditorium of two hundred. I know there are a few people with whom I regularly converse, but my web traffic suggests that there may be more. So in order to help me get a feel for all of you, I am proposing a CascadeTubes version of Groundhog Day. Allow me to explain. Continue reading

A Lesson in Power Stage Distortion

Lately I’ve been looking at a project that’s been sitting on the back burner for quite a while. It’s a stereo power amp for which the 6EM7 Vertical amp chassis was a proof of concept. This amp uses an 807 tetrode in a triode strapped configuration. Now the one thing that I didn’t care for in the load line design was the level of distortion. From the load line paper design, distortion peaked at around 8% at 4.31W output, yielding a Ds of 1.8%/W. This is larger than I generally like in my amps. Then I had an idea. Continue reading

Low Frequency Output Transformer Performance

There are a lot of opinions out there considering what’s acceptable for low frequency amplifier performance. It seems like everyone has their own very strong opinion about how low the frequency response must go for an amp to be “acceptable”. And it seems that usually these opinions range from somewhere between 40Hz and 20Hz as the minimum acceptable “roll off” frequency with definitions of “roll off” ranging from “no loss”, to “-1dB”, to the Engineering standard of -3dB. And strangely enough, virtually no one ever addresses the technical performance of amplifiers at their low end rolloff frequency. Amplifier distortion is usually only measured (when mentioned at all) at 1kHz and not across the frequency band. Continue reading

Why Cascaded Drivers?

Lately I’ve been looking into drive topologies for Single Ended Triode (SET) power stages. The major benefit of power triodes is of course how great they sound. The major down side, from the perspective of the amplifier designer, is the fact that their power sensitivity is very low. What this means is that it requires a large voltage swing to drive these tubes. Some examples from my recent designs: a 6AS7 SET design that requires ±75v, a 6336 SET that requires ±81v, and a triode strapped 807 “SET” that requires ±72v. All from a line level inputs of between 1v and 2v peak. Continue reading

SET Driver Revisited

So I was looking over the prototype work I’d done for a 6336 Dual Triode SET amp driver stage. This is the unit that I discussed on the “Brassboard Prototypes” page. However, when examining the pictures and performance, I noticed that something wasn’t quite right. So I took apart the brassboard and took a look. Sure enough, I had built the unit slightly differently than what was in the written schematic on that page. Continue reading

A Tale of Two Chokes

As part of my fascination with vacuum tubes I am almost constantly laying out different amplifier chassis designs. I usually have between two and four different designs in process at any given time. This generally follows a very predictable pattern: design the amplifier schematic, choose a size and layout which is pleasing to the eye, then try to cram everything into the resulting (usually too small) chassis. It seems I am nothing if not predictable.

Because of this personal failing I am usually looking for component choices which are as small as possible. Which brings me to the point of this post; power filter chokes. I have gotten to the point where I simply will not build a tube amp power supply without one. There is too much benefit to performance to not have a good choke in the main filter. There is only one problem, chokes tend to be big. Continue reading

Triode Sound. As Good As It Gets!

Sometimes when you’ve been working on technical stuff and just listening to music in the background you can kind of forget why we do what we do. Then there are times like right now. I just reached deep down in my music collection and put on the original 1984 mix of Stevie Ray Vaughan’s “Couldn’t Stand the Weather“. Now I’m listening to 8 minutes of Stevie doing the Hendrix classic “Voodoo Chile (Slight Return)” through an all triode signal chain and I’ve got goose bumps it sounds so good.

If ever asked the question about why you do tubes, please let me recommend this demonstration. It’s truly awesome.