Negative feedback? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th February 2003, 05:32 PM   #11
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: GŲteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by Sch3mat1c
You know, despite the >40dB (probably 60) NFB in my SS stereo (Onkyo TX-2500), it is still incredibly noisy (part of why I don't use it). Makes me wonder........
Feedback isn't the medicine. Feedback can't make the noise go away but you can increase noise by poor design. You can also get more noise with bad parts. I got evetially very much unregular noise in my old LF357 based preamp. I had bad sockets but the chips had turned bad. Maybe the heat had something to do with this?

If your amp allways have been noisy it's one thing but has the amp become noisier by the time it's probely someting which has been worn out. Bad electrolythic caps, bad chips? Old chips can absorb pollutions and this can create bad behavior, irritating noise. Plastic package isn't totally tight against enviromental things.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 28th February 2003, 06:54 PM   #12
diyAudio Member
 
Join Date: Feb 2003
I was wondering how you measure feedback in dB. Is this the reduction in gain caused by the NFB?
  Reply With Quote
Old 28th February 2003, 07:37 PM   #13
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: GŲteborg, Sweden
Blog Entries: 4
I have got a very good file about feedback, 160 pages but you don't have to read it all.

SLOD006A.pdf at www.ti.com

Unfortunately I can't find it. Anyone else who can?

The file is 800k and is a very good peice and I think selected chapters can be very educational.

If noone can help you, drop me a note if you want the file emialed.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 28th February 2003, 08:27 PM   #14
diyAudio Member
 
Steve Eddy's Avatar
 
Join Date: Sep 2002
Location: Sacramento, CA
Quote:
Originally posted by Solid Snake
I was wondering how you measure feedback in dB. Is this the reduction in gain caused by the NFB?
Basically, yup. If your open loop gain is 100dB, and your closed loop gain is 20dB, you've got 80dB of feedback.

se
  Reply With Quote
Old 28th February 2003, 09:37 PM   #15
diyAudio Member
 
Pjotr's Avatar
 
Join Date: Nov 2002
Location: Netherlands
Hi,

The dominant source of noise in an amplifier is usually the input stage as already mentioned. Overall NFB reduces the overall gain (closed loop gain) of an amplifier. Thus you can say because of the reduced overall gain of the amplifier due to the NFB the noise generated by the input stage is less amplified. The noise itself canít be reduced. Thatís all.

  Reply With Quote
Old 1st March 2003, 12:07 AM   #16
diyAudio Member
 
Join Date: Feb 2003
This is an explaination I found on a site:

Quote:
For one thing, the inverted feedback is, while reducing the original signal by a factor determined by the network, also reducing itself to a very great degree. To put it mathematically, if the distortion is 10% of the output signal and the output signal is 10 volts, then the distortion is 1 volt. If the input signal is 1 volt, then the gain is abviously 10 (actually it is a bit more. I think the formula stipulates a +1, for an actual gain of about 11. this is why I say the inverted signal is about 0.9 volts). The feedback network is reducing the ouput by about a factor of ten. So, the inverted feedback is getting a signal of close to probably about 0.9 volt including the 1/10th of the distortion that is only 10% of the output, or 0.09 volts. This will then reduce greatly the distortion at the output.
I think what he's trying to say is that when you feedback part of the signal, the input signal fights the inverted signal, but there is nothing to counteract the noise in the NFB, so the inverted noise is much greater than the normal signal, greatly reducing it???
  Reply With Quote
Old 1st March 2003, 04:03 PM   #17
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by Pjotr
Hi,

The dominant source of noise in an amplifier is usually the input stage as already mentioned. Overall NFB reduces the overall gain (closed loop gain) of an amplifier. Thus you can say because of the reduced overall gain of the amplifier due to the NFB the noise generated by the input stage is less amplified. The noise itself canít be reduced. Thatís all.

I don't know where you guys all get the idea that noise can't be reduced, but that's wrong. Noise isn't very special, just something that is generated in amp stages just like harmonics due to THD. Same with DC offset, temperature drift, etc. NFB reduces it, IF it is generated inside the FB loop. NFB cannot reduce anything outside the loop, where it has no control, be it noise, THD, etc.

Jan Didden
  Reply With Quote
Old 1st March 2003, 04:05 PM   #18
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Default nfb

To Pjotr:

I just re-read your post, maybe I misunderstood. You are right, the noise itself will not be reduced, but the effect on the output signal is. I think we are on the same line Pjotr?

Jan Didden
  Reply With Quote
Old 1st March 2003, 04:38 PM   #19
diyAudio Member
 
Pjotr's Avatar
 
Join Date: Nov 2002
Location: Netherlands
Hi Jan,

Maybe I was not that clear in my post. I intended to say the noise of the input stage itself canít be reduced.

In general when you have a chain of amplifiers the noise of the amplifier stage that is most amplified in the chain dominates. In most amplifier chains this is usually the noise of the input stage. On the OUTPUT the noise is reduced by the amount of overall feedback when applied to such a chain. And an audio amp is usually a chain of amplifier stages. The noise of the source itself is not reduced by NFB, only the effect of it at the output. Same holds for distortion in the chain.

It is just basic textbook electronics. Indeed, I think we are on the same line.

Cheers
  Reply With Quote
Old 1st March 2003, 06:16 PM   #20
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Default nfb

Right.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Negative Feedback? Binaural Tubes / Valves 15 12th March 2009 07:05 PM
Negative Feedback AmpBuilder225 Tubes / Valves 7 20th February 2009 08:05 PM
Less Negative Feedback Is Better? CarlosT Chip Amps 25 14th December 2006 10:18 PM
Negative Feedback Jack Thomas Tubes / Valves 6 5th May 2003 12:38 AM


New To Site? Need Help?

All times are GMT. The time now is 07:22 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2