Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

THD spectrum and negative feedback.
THD spectrum and negative feedback.
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
Thread Tools Search this Thread
Old 19th December 2010, 01:48 AM   #1
gaetan8888 is offline gaetan8888  Canada
diyAudio Member
Join Date: Apr 2007
Default THD spectrum and negative feedback.


Nelson Pass wrote an interesting article about thd spectrum and negative feedback.


Any opinions about that subject ?

Myself I've found, for all my amps, that keeping the open loop gain under
arround 65 db and using a global negative feedback lower than 35 db,
do sound better.


Attached Images
File Type: gif Nelson Pass and thd spectrum vs nfb.gif (37.1 KB, 204 views)

Last edited by gaetan8888; 19th December 2010 at 01:56 AM.
  Reply With Quote
Old 19th December 2010, 02:06 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
nigelwright7557's Avatar
Join Date: Apr 2008
Location: Carlisle, England
I designed a class d amp that has no feedback at all and sounds great.
Very clear and a good bandwidth.

Too much OLG can cause oscilation which needs slugging out.
PCBCAD51 and PCBCAD360 out now >>> https://www.murtonpikesystems.co.uk
  Reply With Quote
Old 19th December 2010, 05:22 AM   #3
jcx is offline jcx  United States
diyAudio Member
Join Date: Feb 2003
Location: ..
we've beat the subject to death in the Cordell negative feedback thread

Baxandall (and others before and after) have done the negative feedback "harmonic growth" math - among the the conclusions:

that it is worst at ~ 10-15 dB feedback

it doesn't depend on wether the feedback is "global" or local" - it can be seen with degeneration too

just use lots of negative feedback once youv'e started using any at all and above ~40 dB of feedback all harmonic distortion orders are reduced, and they assume the decreasing level as order increases pattern so many claim is important

higher order compensation, fast output devices allow up to 60 dB loop gain in audio power amps at 20 KHz so all conventional "audio" signals can benefit from high feedback factor distortion reduction

not directly related but Otala's TIM concerns can be addressed in input, VAS, compensation design and Is Not made worse by high negative feedback factor - or even a sloping feedback factor in the audio band - the "flat open loop bandwidth over the audio range" perscription is simply wrong on TIM grounds
(although Bode's maximal feedback design does use flat feedback factor over the working band to boost the bandwidth benefiting from high feedback factor by an octave)

I think Cherry did the best feedback "distortion growth" math job in:

Cherry, “Estimates of Nonlinear Distortion in Feedback Amplifiers” JAES V48#4 2000

a bad use of high feedback is to try to "fix" a bad amplifier design with static or dynamic underbiased output devices which show deadbands with expected loads and V,I slewing requirements

build linear stages with generous bias that can handle the required real world loads, signals with only smooth nonlinearities and there is no indication that high negative feedback does anything except improve the amp's linearity at the usual and quite controllable cost in frequency compensation for stability

Last edited by jcx; 19th December 2010 at 05:43 AM.
  Reply With Quote
Old 19th December 2010, 02:09 PM   #4
Bigun is offline Bigun  Canada
diyAudio Member
Bigun's Avatar
Join Date: Jan 2009
Location: Waterloo, ON or Herefordshire UK
THD spectrum and negative feedback.
Sometimes the simple maths is too simple. For example, degenerative feedback can be quite different in it's impact compared with global - it generally has much less phase shift and rarely requires stability compensation. We know from our own experience that compensation can have a profound effect on sonics. Degenerative feedback is very accurate - it uses the device itself as the error amplifier. To me, it's intuitively kinder on the music than any kind of loop feedback.

I'm not so sure about CFP yet, it has a reputation for good sound and the feedback loop is very tight between the two devices - not tight enough to avoid the danger of instability though.

Feedback has many other consequences not just the mathematical impact on harmonics. It affects clipping behaviour, PSRR, and provides a path into the amplifier for any rf pickup or speaker driver back emf on the output. It changes the effective output impedance.

Good or bad I reckon it's interesting to have the option of using it or not using it. Not using gnf with a Class AB amplifier is not easy. It's difficult to control distortion at the output and its difficult to design a VAS with a gain of order 10dB (it's easier to make a gain of 0.98 or > 10dB). You have to take care with PSRR and frequency response because there's no gnf to fix all the warts.
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 19th December 2010, 08:18 PM   #5
gaetan8888 is offline gaetan8888  Canada
diyAudio Member
Join Date: Apr 2007

To have more gain you need multiple stages in the amp and as wrote in the Nelson Pass's document, the more stage you have the more IM thd you will have with a complex signal like the musics.

To have gain and drive I use this buffered Vas and I ad a local feedback, I keep the number of stages to minimum in my amps.

A sine wave in a simulations or on a resistive load are easy for an amp but all things change on a real speakers load with a complex signal like musics.


Attached Images
File Type: gif vas 4.gif (2.1 KB, 137 views)

Last edited by gaetan8888; 19th December 2010 at 08:41 PM.
  Reply With Quote


THD spectrum and negative feedback.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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
National Instruments DAQ for THD / Spectrum measurements? hartze11 Equipment & Tools 3 2nd June 2010 10:25 PM
Question - THD Measrement/Spectrum Analyzis LinuksGuru Tubes / Valves 3 30th March 2009 03:10 AM
Question - THD Measrement/Spectrum Analyzis LinuksGuru Solid State 1 30th March 2009 01:57 AM
THD has a price, and a negative one! Bricolo Class D 9 11th June 2007 06:47 PM
dead-time, feedback, thd aborigini Class D 1 19th September 2006 10:53 PM

New To Site? Need Help?

All times are GMT. The time now is 08:46 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio