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scutterflux 11th January 2013 02:15 AM

help with local nfb
 
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Hello

I was wondering if anyone could provide suggestions for an implementation of a local negative feedback from the anode/plate of the 805 tube back to the 300b driver stage?

History: I removed the gnfb loop, and I ain't going back. I have way more resolution without it, but I would like to see if a local feedback from the power tube provides the decrease in impedance to tighten up bass and possibly extend freq. response without the same loss of resolution with a gnfb loop. (as per another discussion on this forum)

Schematics for that section of the amp below.

Also I can try provide any additional info if need be.

DF96 11th January 2013 10:58 AM

Two problems:
1. The driver is a cathode follower so already contains 100% local feedback within itself. You can't apply feedback directly from the output cathode to the driver grid, as that would be bootstrapping - a type of positive feedback.
2. Feedback taken from the output cathode will sense the output current, not voltage, so will raise, not lower, the output impedance.

If the amp was designed with GNFB then removing it will raise distortion and output impedance and reduce bandwidth. The extra distortion may be what you hear as 'resolution', as our ears are easily fooled. An amp without GNFB needs to be designed for that purpose. Feedback cannot simply be added or removed at will.

scutterflux 11th January 2013 09:15 PM

Quote:

Originally Posted by DF96 (Post 3320916)
Two problems:
1. The driver is a cathode follower so already contains 100% local feedback within itself. You can't apply feedback directly from the output cathode to the driver grid, as that would be bootstrapping - a type of positive feedback.

100% local feedback would mean no sound right???

I meant off the anode or plates, my mistake... that's where the phase is 180 degrees out.

Quote:

Originally Posted by DF96 (Post 3320916)
2. Feedback taken from the output cathode will sense the output current, not voltage, so will raise, not lower, the output impedance.

I've read plenty of articles and discussions where a local negative feedback from the output tube lowers impedance, and reduces the phase issues with a GNFB loop. This problem #2 you mention is still the same problem as you mentioned in #1 which is related to my fault of saying cathode when I meant anode... My appologies for the confusion.

Quote:

Originally Posted by DF96 (Post 3320916)
If the amp was designed with GNFB then removing it will raise distortion and output impedance and reduce bandwidth. The extra distortion may be what you hear as 'resolution', as our ears are easily fooled. An amp without GNFB needs to be designed for that purpose. Feedback cannot simply be added or removed at will.

OK, some of that that may be true, but with GNFB there was some image smearing now there is not, it is now defined and precice, it is more detailed, more open, more space, more organic natural, images have finer defined lines, greater soundstaging, etc... but yes the bass is more relaxed and looser and there doesn't sound like a loss in HF extention but treble is more fleshed out, tangible, palpatable... of course there is a loss, I understand that but what good is having the greater frequency response if it sounds smeared and less defined? Also I'm trying to redesign parts of the amp so GNFB is not needed, and I'm having varying success, this is how I learn. Also yes of course you can add and remove feedback at will who say's you can't??? I say I can, I give myself permission too, and I'll learn a whole heck of allot in the process.

I had to remove the first gain stage to remove what was clearly distortion. GNFB decreases distortion, like 2nd order harmonics and increases odd order relatively. 2nd order harmonics is distortion, right? Well I've probably traded some ratio of higher odd in exchange for greater distortion of the 2nd order, it's a nice trade off I think, very musicaly pleasing even if THD is greater.

So I could just try things until I get it just right, that's normally what I do. I just thought I'd ask for help for a good starting point.

Thank You

DF96 11th January 2013 10:18 PM

100% NFB means gain=1, which is how a cathode follower works.

You could try adding feedback from the output anode to the CF grid. This would reduce output impedance, as you want, but it will also reduce input impedance which will load the previous stage and increase distortion there. Electronic design is always a matter of compromise.

Quote:

Also yes of course you can add and remove feedback at will who say's you can't??? I say I can, I give myself permission too, and I'll learn a whole heck of allot in the process.
OK. Most people who 'learn' about feedback by experiment usually end up misunderstanding it, but not realising this. You may be an exception.


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