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Vintage threads from the first ten years

Re: Advanced feedback loop questions..

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11/9/1999 7:08 PM

Re: Advanced feedback loop questions..

You're getting all this negative feedback about positive feedback for a reason.

It's not _always_ bad. It's just very hard to control because it tends to exaggerate any instability or tendency to oscillation. An oscilloscope is a necessary tool here because of the near-certainty of ultrasonic oscillation that you will need to obviate.

Do experiment, just protect the bejeezus out of the expensive parts and your work area (and yourself).

Consider yourself both warned _and_ encouraged. We all wish you luck and success with this project - you're gonna need it.

11/8/1999 7:37 PM

Hi,

The difference in sound is quite large. As you increase the impedance, the bass gets, well I can only describe it as more loose and flubby. The mids get more prominent, and if the speaker was bright already they get very icepick. Overall it seems to me the sound loses clarity and definition. I think too much NFB makes the sound a bit flat and boring though. As a compromise I like to use a low-gain 12AU7 tube in the phase splitter, and only apply about 6dB NFB. You need to use a different phase splitter circuit for the low-mu tube.

The Art of Electronics is a great book about electronics in general. There isn't anything in it about tubes in particular.

Swapping your negative feedback for positive will cause the amp to be unstable and probably damage it. It certainly won't be very musical. I can't emphasise enough, DON'T BOTHER

Steve.

11/9/1999 6:05 PM
Gus	mixed feedback One could have nfb at low fregs and less at highs I believe I read about that in an AES mag years ago this was for home tube amps.

11/9/1999 11:05 PM
Ken Gilbert	When you take your (traditionally hooked up) presence control and turn up the highs, that's what you get. Happens all the time! KG

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