I have seen a few posts in various threads that criticize various amplifiers because of their high open loop gain. The reason for this criticism is the high open loop gain requires a large amount of negative feedback, and negative feedback is bad.
According to my understanding the closed loop gain is determined by the input and feedback impedance values. For sufficiently large values of open loop gain, the closed loop gain is independent of the open loop gain. So a further increase in open loop gain won't change the amount of negative feedback required.
Can I get some clarification please?
According to my understanding the closed loop gain is determined by the input and feedback impedance values. For sufficiently large values of open loop gain, the closed loop gain is independent of the open loop gain. So a further increase in open loop gain won't change the amount of negative feedback required.
Can I get some clarification please?
Negative feedback is not bad, it is a fundamental part of transistor amplifier design. Read this:
http://sound.westhost.com/articles/distortion+fb.htm
Feedback factor = Open loop gain - Closed loop gain
For example, an amplifier with an open loop gain of 60 dB and a closed loop gain of 20 dB will have a feedback factor of 40 dB. An increase in open loop gain by 10 dB will increase the feedback factor by 10 dB.
Rod Elliot gets some points right, but goes one claim too far:
http://www.diyaudio.com/forums/showthread.php?s=&postid=920583&highlight=#post920583
negative feedback does "create" new distortion products in the course of greatly reducing the total error
presumably the original question relates to Otala's "phase intermodulation distortion"- Otalta showed that the phase shift of integrating feedback can change the phase relations of the intermodulation distortion products from some static nonlinearities to resemble "phase modulation" spectrum vs"amplitude modulation" spectra that are seen with constant feedback over frequecy
the topic has been discussed several times - I like simulations to illustrate points:
http://www.diyaudio.com/forums/showthread.php?s=&postid=489927&highlight=#post489927
http://www.diyaudio.com/forums/showthread.php?s=&postid=920583&highlight=#post920583
negative feedback does "create" new distortion products in the course of greatly reducing the total error
presumably the original question relates to Otala's "phase intermodulation distortion"- Otalta showed that the phase shift of integrating feedback can change the phase relations of the intermodulation distortion products from some static nonlinearities to resemble "phase modulation" spectrum vs"amplitude modulation" spectra that are seen with constant feedback over frequecy
the topic has been discussed several times - I like simulations to illustrate points:
http://www.diyaudio.com/forums/showthread.php?s=&postid=489927&highlight=#post489927
')P.Lacombe said:
again I believe that opinion, while common, is easily demonstrated to be wrong
negative feedback does cause the increase of higher harmonics with low order distortion amplifying circuits, curious cancellations of particular distortion harmonics are possible with some device' distortion characteriscs and a specific (to that device, bias condition, load) small excess loop gain
some "low feedback" designers imply that they tune these cancellations to minimize the harmonic distortion component they claim is particularly dissonant such as the 7th
It is hard to believe in the practical possibility to tune the loop gain to the device' distortion characteristic to get even 40 dB reduction in one or two harmonics
It is not so hard today to get 60 dB excess loop gain over the audio frequency range, giving a greater reduction than the best cancellation for all distortion components
A high gain, low bandwidth negative feedback amplifier will have higher gains at lower frequencies - since music is commonly shown to have ~ 3KHz power bandwidth most musical signals in a "low bandwidth" amplifier will see > 20 dB more loop gain and corresponding reduction in distortion components at those frequencies compared to a low gain, high bandwidth amplifier with flat loop gain to 20KHz
If the low gain, high bandwidth amplifier has restricted excess loop gain to 20-30 dB then the high gain amplifier’s errors at all audio frequencies will be hundreds to thousands of times less – you wouldn’t think it would be hard for the low feedback crowd to show the PIM/PMD or other supposed high gain “dynamic” errors sound hundreds of times worse than the large distortion components in their amps
[implicit in the above is the assumption that the power output stage distortion is dominant and you compare high/low gain amps with the same output stage performance]
High feedback tends to create a false "hi-fi spectacular" sound quality. Some people describe it as "detail" but real guitars don't have that much finger sound on the strings and human voices don't have that sort of sheen to them. For me the deal killer is that it causes deterioration of the image. Real images don't sound that way.
I've built any number of circuits over the years and found that the less feedback I used the better they sounded.
People who claim otherwise rarely seem to get out and hear live, unamplified music so they lack a referent, but they sure do yap loudly about how perfect negative feedback makes things. I used to...until I started listening to classical and jazz. That changed my perspective, which previously had been based solely on rock. Once I started noticing that things sounded wrong at home on my stereo I started asking questions. The answers--and sometimes the lack of same--led me to experiment with differing levels of feedback.
Note that if negative feedback were truly the answer, we'd have reached audio nirvana in the late '70s when designers were routinely using grotesquely high levels of feedback. That the concept was an abject failure is demonstrated by the relative paucity of classic solid state designs from that era. And note that the few that you can name were not on the bleeding edge of the feedback wars. (There's always some joker who comes in and lists every bloody thing that was in production at the time...as though mere production constitutes a "classic." Jeez...)
You might also want to note that damping factor (which is always second or third on the list of so-called benefits of negative feedback) is vastly overrated. Why? Because tube amps, which are notoriously possessed of low damping factors, can sometimes outpace solid state amps in the bass.
Grey
I've built any number of circuits over the years and found that the less feedback I used the better they sounded.
People who claim otherwise rarely seem to get out and hear live, unamplified music so they lack a referent, but they sure do yap loudly about how perfect negative feedback makes things. I used to...until I started listening to classical and jazz. That changed my perspective, which previously had been based solely on rock. Once I started noticing that things sounded wrong at home on my stereo I started asking questions. The answers--and sometimes the lack of same--led me to experiment with differing levels of feedback.
Note that if negative feedback were truly the answer, we'd have reached audio nirvana in the late '70s when designers were routinely using grotesquely high levels of feedback. That the concept was an abject failure is demonstrated by the relative paucity of classic solid state designs from that era. And note that the few that you can name were not on the bleeding edge of the feedback wars. (There's always some joker who comes in and lists every bloody thing that was in production at the time...as though mere production constitutes a "classic." Jeez...)
You might also want to note that damping factor (which is always second or third on the list of so-called benefits of negative feedback) is vastly overrated. Why? Because tube amps, which are notoriously possessed of low damping factors, can sometimes outpace solid state amps in the bass.
Grey
GRollins, what can I say! I totally agree with all your observations and have come to similar conclusions.The majority of designs are all based on the same circuit topology, and while some have very impressive "on paper" specifications they are uninvolving to listen to.With my last design I opted for a different approach, single ended input stage(distortion mainly 2nd harmonic & rises at 6db/ octave compared to 18db/octave for long tailed pair), low open loop gain, mosfet output using lateral fets, and a low damping factor- 0.22ohm.A servo allows for d.c coupling.A simple preamp, OPA604, and discrete series/shunt FET input switching, which works better than any mechanical switch and which also allows for full remote control.This is an amp where 95% of recordings sound wonderful, not just a few "demo" discs.It has opened my ears to some amazing music, string quartets, solo violin, the list goes on.It can only be described as compulsive listening.
Regards Karl
Regards Karl
The 'quality' of the distortion, indeed...
If the feedback creates new distortion products, higher order distortion products, then your ear is far more sensitive to those. Don't be so certain that a low THD measurement is the whole game. It's a steady state measurement...
and music isn't steady state.
Get thee to a concert hall. Listen carefully. Then listen carefully to your stereo. Blind faith in distortion measurements isn't the answer.
But no one can make you listen, and if you think high feedback is the answer then go ahead and listen to it. There are certainly enough examples of it in the marketplace to keep you happy. Better still, just buy some late '70s gear off of ebay for pennies on the dollar and you'll think you died and went to heaven. Of course, the reason it's cheap is because no one wants it.
Grey
If the feedback creates new distortion products, higher order distortion products, then your ear is far more sensitive to those. Don't be so certain that a low THD measurement is the whole game. It's a steady state measurement...
and music isn't steady state.
Get thee to a concert hall. Listen carefully. Then listen carefully to your stereo. Blind faith in distortion measurements isn't the answer.
But no one can make you listen, and if you think high feedback is the answer then go ahead and listen to it. There are certainly enough examples of it in the marketplace to keep you happy. Better still, just buy some late '70s gear off of ebay for pennies on the dollar and you'll think you died and went to heaven. Of course, the reason it's cheap is because no one wants it.
Grey
why are you thrashing moldy strawmen? - I've tried to give a reasoned explaination of why people should believe well implemented high negative feedback can deliver accurate amplification
your all of your examples are logical non sequiturs
why do think any particular recording wasn't close miced, hit with eq, pan pot, digital reverb, ect. to give the mix that met the engineer's idea of "stereo image" and "tonal balance"? - just because you can distort or unintentionally eq some recording to your definition of "better" on your system leaves many logical gaps in you indictment of high negative feedback
why would you look to feedback in amplification to fix source, production, loudspeakers, room interaction? shouldn't amps just amplify - as perfectly as possible?
if a particular loudspeaker benifits from some particular drive impedance I would argue that the Loudspeaker designer is (ir)responsable, adding impedance to a high damping, high feedback amp seems to convince some professional "golden ears":
lots was learned by the audio design community in the 70's and 80's - why should bad examples of high negative feedback amplifiers that violate today's "best practice" guidelines for sucessful application of feedback be the basis for comparison?
Halcro's products are no different from QSC or Behringer PA amps?
your all of your examples are logical non sequiturs
why do think any particular recording wasn't close miced, hit with eq, pan pot, digital reverb, ect. to give the mix that met the engineer's idea of "stereo image" and "tonal balance"? - just because you can distort or unintentionally eq some recording to your definition of "better" on your system leaves many logical gaps in you indictment of high negative feedback
why would you look to feedback in amplification to fix source, production, loudspeakers, room interaction? shouldn't amps just amplify - as perfectly as possible?
if a particular loudspeaker benifits from some particular drive impedance I would argue that the Loudspeaker designer is (ir)responsable, adding impedance to a high damping, high feedback amp seems to convince some professional "golden ears":
jcx said:
lots was learned by the audio design community in the 70's and 80's - why should bad examples of high negative feedback amplifiers that violate today's "best practice" guidelines for sucessful application of feedback be the basis for comparison?
Halcro's products are no different from QSC or Behringer PA amps?
GRollins said:
I don't understand how an amplifier with >0.2% THD can have fewer 9th and 11th order harmonics than one with <0.02% THD. My intention is not to be condescending but I would like to know. Don't be affraid to get mathematical or technical with me.
Is it cheap because of the high feedback that hardly anybody knows about or the cheesy wood cabinets that everybody can see?
GRollins said:
That goes without saying.
fizzard said:
fizzard,
I already answered this one. Steady state vs. dynamic.
jcz,
You mention non sequiturs, then proceed to turn your post into one, huge non sequitur. I never said anything about microphones, equalization, or reverb. I trust that anyone with the IQ of a 2x4" can grasp that the quality of reproduction is dependent on the quality of the recording.
But then again, perhaps there are people who look up to 2x4"s as mental giants.
Obviously there are bad recordings. Lots of them. It in no way follows that there aren't good recordings. Some really neat music was poorly recorded. Some really bad music was recorded well. Sometimes you get lucky and get a recording that has good music that was played well and also recorded well. Gosh, I know that's a really difficult concept to grasp, but I'll try it out on a 2x4" when I get home tonight.
I never said, and don't know anyone who believes, that high NFB is supposed to cure a speaker's ills or room interaction. I'd like some of whatever you folks are smoking. It sounds as though it must be some really good stuff!
Where on Earth do people get these ideas? Anyone this lost isn't going to be able to handle reality. Kinda like that line from the movie: The truth? You can't handle the truth!
And Bonsai lauds jcx's post as a good rebuttal? A rebuttal to what or to whom? Not to anything I wrote or even implied.
This is just sad.
The next thing you know, someone with sawdust for brains will try to advance the inane argument that because there are opamps in studios it doesn't matter. As though it was an all-or-nothing process; one opamp and the signal is rendered useless--unworthy of playback on anything better than a boombox. Just to save anyone the trouble of making said post...short of rail-to-rail clipping (which pretty much completely trashes any signal) damage is done incrementally, not all at once. Your job in building circuits is to handle it with as much care as possible so as not to damage it further.
But that simple concept will only generate more off the wall comments relating to comets and cabbages and cows and who knows what else...followed by more empty-headed posts cheering the 'logic' that produced such scintillating wit.
Better yet, go listen to live, unamplified music, then to your stereo. Then you'll know why I say these things. But that would take effort, and it's much easier to be an armchair critic and claim that everything necessary is already known.
Grey
Hi Grey,
Hi fizzard,
I am listening to the new Cyrus Mono X right now (a no feedback design - really) against my Marantz 300DC (high feedback design) and also use an Eico HF-87 tube amp. They all sound great, but the Marantz I souped up sounds slightly better than the Cyrus. The Cyrus sounds amazing though and I am very impressed with them. I could live with them.
So, what does this say? They are all great in the mids, very smooth. The highs as well and the bass contest excludes the poor Eico. This shows that high feedback does not have to sound bad and many times will sound great. I've also noticed that some amp designs have a happy spot for the amount of feedback used. This depends on the amp design. Let's just say that the design must be done properly before you start using feedback. In the early days, feedback was used as a band aid rather than a valuable tool. High feedback wrapped around a poor design will generate many high order harmonics but may measure low. Also beware the THD meter with poor bandwidth that will favour substandard high feedback designs. A low or no feedback design will tend to create more of the low order harmonics unless carefully designed. Ironically you could then apply some feedback to improve the performance still further. What a twist!
-Chris
I can't agree with you on that one. There are differences with dynamic signals yes, but steady state signals also show different harmonic structures for different amps. So "Steady state vs. dynamic" isn't really saying anything useful.
Hi fizzard,
No deep math required here. A low feedback design will generate higher harmonics just as a high feedback design will. The trick is in using a very linear design before you apply the feedback. Less nasties are generated that way. The SymAsym is pretty good in that way compared to some other simple amplifiers. A no feedback design (okay, no overall feedback) will show less of a tendency to create higher order harmonics. This will depend on the amp and layout.
I am listening to the new Cyrus Mono X right now (a no feedback design - really) against my Marantz 300DC (high feedback design) and also use an Eico HF-87 tube amp. They all sound great, but the Marantz I souped up sounds slightly better than the Cyrus. The Cyrus sounds amazing though and I am very impressed with them. I could live with them.
So, what does this say? They are all great in the mids, very smooth. The highs as well and the bass contest excludes the poor Eico. This shows that high feedback does not have to sound bad and many times will sound great. I've also noticed that some amp designs have a happy spot for the amount of feedback used. This depends on the amp design. Let's just say that the design must be done properly before you start using feedback. In the early days, feedback was used as a band aid rather than a valuable tool. High feedback wrapped around a poor design will generate many high order harmonics but may measure low. Also beware the THD meter with poor bandwidth that will favour substandard high feedback designs. A low or no feedback design will tend to create more of the low order harmonics unless carefully designed. Ironically you could then apply some feedback to improve the performance still further. What a twist!
-Chris
jcx, though I agree with 99% of your post, why do you feel that zero source impedance to drive a speaker is somehow privileged? What is irresponsible about designing a speaker fro some other source Z as long as the buyer understands that it needs X ohms to eq the bass properly and X ohms isn't unreasonable to provide?
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