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What can I expect by removing the NFB?

206.101.197.213
Posted on April 25, 2000 at 10:24:16
Francis S


 
I finally got the Japanese manual of the TU-872 2A3 amp translated! Now, it has a section explaining how to remove the NFB from the circuit but does not say what this will do to the sound.

Questions:
1. What can I expect sonically by removing the NFB circuit (this requires taking out 2 caps and a resistor/channel)?

2. Why is the NFB installed in the first place? hmmmm...

Thanks,

Francis

 

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Re: What can I expect by removing the NFB?, posted on April 25, 2000 at 10:39:06
Thorsten


 
Hi there,

>1. What can I expect sonically by removing the NFB circuit
>(this requires taking out 2 caps and a resistor/channel)?

More open Midrange, possibly less but more defined bass, more "thereness".

>2. Why is the NFB installed in the first place?

NFB has a "classic" valve sound and helps out substandard Output transformers for better measurements. In Japan the tendency is not as much towards Zero Negative Feedback and often Specifications count.

I would remove the negative feedback. On all Amplifers where I tried it I preferred the sound without negative feedback. I have found that to substantially improve either frequency response or distortion (substantially meaning 10db wrt distortion and 3db wrt frequency response) you need a lot of Negative feedback, at least 10 - 12db.

This means as much extra gain as that muct be present, meaning that you normally need two stages where usually one suffices. Ideally the amount of feedback applied would significantly exceed 20db, but such is difficult without risking all the (very audible) drawbacks of negative feedback audible in less than ideally implemented Solid State Amplifiers.

So basically, I prefer to do without negative Feedback as I can never enough spare gain to make it's use wortwhile. And certainly using only 3 - 6db Negative Feedback ALLWAYS made the circuit sound DRAMATICALLY worse, much more so than introducing a Cathode Follower or a SRRP into the Circuit.

My take, anyway. YMMV.

Later Thorsten

 

Re: What can I expect by removing the NFB?, posted on April 25, 2000 at 11:42:51
GEO


 
Could you go over the gain issue one more time. Will you have more or less gain with the negative feedback eliminated and if there is more gain how do you attenuate or reduce the gain or is it even desirable to reduce the gain in your experience.

 

Re: What can I expect by removing the NFB?, posted on April 25, 2000 at 13:36:48
Thorsten


 
Hi there,

It's simple, feedback and "closed loop" Gain (after feedback is applied) are inversely proprtional.

That means that as the amount of negative feedback goes up, the gain goes down and reverse, reducing negative feedback increases gain.

Essntially, negative ffedback improves the aparent linearity of a circuit while reducing it's gain. Hence you need excess gain to start with in order to be able to use negative feedback.

Later Thorsten

 

Thanks!, posted on April 25, 2000 at 18:38:31
Francis S


 
I'll have the NFB circuit removed this PM and will report findings. Hopefully, this will tame the slightly fat bass I hear with the NFB.

Thanks again.

Francis

 

Re: What can I expect by removing the NFB?, posted on April 25, 2000 at 19:07:13
GEO


 
Thanks

 

Re: What can I expect by removing the NFB?, posted on April 28, 2000 at 05:42:32
Steven R. Rochlin


 
Francis,

Agreed with Thorsten. While playing around with the Cary 805's and their adjustable feedback loop, i felt it sounded best with the feedback off (read: no NFB). Thorsten hit the proverbial nail on the head.

Enjoy the music,

Steven R. Rochlin

 

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