Feedback

In my last post I threw in a comment at the end pointing out that lowering plate load resistance will play a part in how the cathode bypass capacitor behaves. Someone asked, so I thought an example of how this affects things might be in order.

There seems to be a little confusion concerning the true effects of cathode bypass on low frequency amplifier performance. The truth is, gain benefits from cathode bypass are limited. And not by capacitor size, but by the basic design of the amplifier stage.

This topic comes up a lot! Someone will email me with, “I want to build {insert project here} from your web site. But…”. Sometimes it’s just that part availability varies. Sometimes people want to use parts on hand which are slightly different. Sometimes people want to substitute tubes, transformers, or whole assemblies. And sometimes people want to actually change the topology! So at what point is the amp being built no longer the design on my website?

For anyone who is interested in a little more of the history of feedback development, I have posted an article from the Proceedings of the IEEE from 1999. It’s a relatively quick but informative read about the topic.

This may seem a non-sequitur to some. After all, I don’t post designs on this website that make use of feedback. This is simply because I don’t like the way feedback affects the sound of an audio amplifier. But that doesn’t mean that I don’t understand feedback and make use of it in other applications.

When looking at schematics and builds of vacuum tube amplifiers employing output transformers some people notice a curious inconsistency. On some amplifiers, the output (i.e. secondary) side of the transformer is grounded, on others, it is not. This can lead to a dilemma for amplifier builders: To ground, or not to ground? Luckily, there are some simple guidelines which will almost always yield the correct answer.