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#41
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Feedback in audio esp wrt op-amps.
"William Sommerwerck" wrote in message . .. And somehow, magically, the golden-ear boys (is's almost always boys) think that it matters that what they do to the signal that comes *off* the CD makes so much difference that they can hear the difference in the oxygen content of the interconnect wiring, or 0.06 percent distortion when the producer added 30% of his own, because he liked the effect. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. 30 years of experience with bias-controlled listening tests says that *seems* and *is* can be two different things. Intellectual logic has this interesting tendency to rule, once the comparison is based on just listening. When I reviewed, I made final judgements with my own live, undoctored recordings. The fun begins when you level-match, time-synch and eliminate other non-audible cues. |
#42
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"William Sommerwerck" wrote in message . .. There was part of a thread a while back about how adding negative feedback can create higher order harmonic distortion products than exist open-loop in an amplifier stage. This premise is NOT correct. Do not believe everything you read on the Internet. Feedback done correctly ADDS nothing. Perhaps what you are thinking about is that feedback is generally more effective at reducing low-order distortion compared to reducing high order distortion. Feedback (implemented correctly) does not INCREASE either form of distortion. It reduces them both. I'm sorry, Mark, but this has been known for decades, and was not established by audiophile reviewers -- the reduction of the overall distortion level is accompanied by an increase in higher-order harmonics. I apologize for not having a reference. Something to do with reality intruding on fantasy. Normally, a reduction of the overall distortion level is accompanied by a similar but possibly smaller decrease in higher-order harmonics. A decrease in all forms of distortion is the primary effect. The shift towards larger percentages (but not larger amounts) of higher order distortion is a secondary effect. One possible exception was described by Don - relating to marginal stability. Another common situation is where the open-loop gain of the amplifier inside the loop simply falls with increasing frequency. Very common, particularly with op amps. The higher harmonics are still reduced, but they may be reduced by a smaller amount than the lower harmonics. This leads to the higher harmonics being a bigger slice of a significantly smaller pie. The smaller pie is the stronger effect, so the size of all harmonics is still reduced. When the pie is as sour-tasting as nonlinear distortion is in reproduction equipment, I'm always in favor of significantly smaller pies! The source of this myth is the mistaken idea that negative feedback regenerates the audio signal, and the nonlinearity of the amplifier leads to higher order products of the regenerated harmonics and the nonlinearity of the amp. This ignores the fact that the regenerated signal is brought back in out-of-phase, and has the primary result of reducing the high order harmonics. |
#43
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Kevin Aylward" wrote in message ... Well, it is trivially obvious that a pure square law device, with a *small* amount of feedback will generate 3rd harmonic distortion, that was never orginally there, from the mixing of the second and the fundamental. It is also true that for such low levels of feedback, although the total thd is less, the new 3rd component may sound more objectionable to those goldern ears. However, assuming *sufficient* feedback is applied, the final distortion will be audiable less noticable. Let's put this into a real-world perspective. The LM 4562 has a typical GBW of 55 MHz. If it is a typical compensated op amp, that means that its gain at 1 KHz is 55,000. The feedback factor at 60 dB gain (x1,000) and 1 KHz is thus 55. Based on its specs, its open-loop nonlinear distortion at 1 KHz is 55,000 times its unity gain distortion @ 1KHz is less than 2%. At 60 dB gain, negative feedback drops this to about 0.035 %. Let's assume the worst case - the nonlinear distortion is all second harmonic. Then, when fed back, 0.035% or less of the 0.035% second order distortion undergoes conversion to third harmonic. IOW with feedback, there is now 0.00001225% or less third harmonic in addition to the 0.035% second. I wouldn't expect anybody to hear the 0.035% second order nonlinear distortion, and definitely not the 0.00001225% third. |
#44
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Mark wrote: Eeyore wrote: There was part of a thread a while back about how adding negative feedback can create higher order harmonic distortion products than exist open-loop in an amplifier stage. This premise is NOT correct. Do not believe everything you read on the Internet. Feedback done correctly ADDS nothing. Perhaps what you are thinking about is that feedback is generally more effective at reducing low order distortion compared to reducing high order distortion. Feedback (implemented correctly) does not INCREASE either form of distortion. It reduces them both. I know it decreases overall THD numbers. I'm not one of those nuts who's anti-NFB per se. What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. Graham |
#45
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
William Sommerwerck wrote: And somehow, magically, the golden-ear boys (is's almost always boys) think that it matters that what they do to the signal that comes *off* the CD makes so much difference that they can hear the difference in the oxygen content of the interconnect wiring, or 0.06 percent distortion when the producer added 30% of his own, because he liked the effect. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. Absolutely true. The idea that you can 'get away' with sloppy circuitry for replay because the source was in some way 'impaired' is totally false. Graham |
#46
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Scott Dorsey wrote: William Sommerwerck wrote: And somehow, magically, the golden-ear boys (is's almost always boys) think that it matters that what they do to the signal that comes *off* the CD makes so much difference that they can hear the difference in the oxygen content of the interconnect wiring, or 0.06 percent distortion when the producer added 30% of his own, because he liked the effect. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. Oh, absolutely, but sometimes that's because of what the distortions do to the artifacts in the original recording. I like to use a particular track from Hair for listening to speaker systems... something in the vocal chain on that track (2-4-0-0) is right on the edge of clipping and the problem is much more audible on good speakers than bad ones. The irony being that it sounds 'worse' on 'good' speakers. This is why domestic hi-fi tends to have little in common with studio monitors. Horses for courses and all that. Graham |
#47
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Arny Krueger wrote: "William Sommerwerck" wrote in message . .. And somehow, magically, the golden-ear boys (is's almost always boys) think that it matters that what they do to the signal that comes *off* the CD makes so much difference that they can hear the difference in the oxygen content of the interconnect wiring, or 0.06 percent distortion when the producer added 30% of his own, because he liked the effect. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. 30 years of experience with bias-controlled listening tests says that *seems* and *is* can be two different things. Intellectual logic has this interesting tendency to rule, once the comparison is based on just listening. When I reviewed, I made final judgements with my own live, undoctored recordings. The fun begins when you level-match, time-synch and eliminate other non-audible cues. I'd love to see you level match the sound field from different loudspeakers ! Graham |
#48
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Eeyore" wrote in message ... Mark wrote: Eeyore wrote: There was part of a thread a while back about how adding negative feedback can create higher order harmonic distortion products than exist open-loop in an amplifier stage. This premise is NOT correct. Do not believe everything you read on the Internet. Feedback done correctly ADDS nothing. Perhaps what you are thinking about is that feedback is generally more effective at reducing low order distortion compared to reducing high order distortion. Feedback (implemented correctly) does not INCREASE either form of distortion. It reduces them both. I know it decreases overall THD numbers. I'm not one of those nuts who's anti-NFB per se. What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. The actual fraction of new higher harmonic is very low in practice. Take a really crappy power amp that has a 1% nonlinearity. If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC that is 46 dB down, and 2 KHz that is 46 dB down. Run the output back through again, and you get a nearly FS 1 KHz tone, DC that is still about 46 dB down, a 2 KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92 dB down. 1. Higher harmonics *are* more audible, but the additional 46 dB down is far, far less than enough to make up the difference. 2. If the amp has 0.1 % nonlinearity (still fairly crappy by modern standards), the numbers are 66 dB down for the DC and second harmonic, and 132 dB down for the third harmonic. 3. If the amp has 0.01 % nonlinearity (very good modern standards), the numbers are 86 dB down for the DC and second harmonic, and 172 dB down for the third harmonic. Most modern power amps will be someplace between (2) and (3). Most modern op amp applications will be closer to (3), on the good side. |
#49
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 20, 2:30 pm, Eeyore
wrote: Mark wrote: Eeyore wrote: There was part of a thread a while back about how adding negative feedback can create higher order harmonic distortion products than exist open-loop in an amplifier stage. This premise is NOT correct. Do not believe everything you read on the Internet. Feedback done correctly ADDS nothing. Perhaps what you are thinking about is that feedback is generally more effective at reducing low order distortion compared to reducing high order distortion. Feedback (implemented correctly) does not INCREASE either form of distortion. It reduces them both. I know it decreases overall THD numbers. I'm not one of those nuts who's anti-NFB per se. What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. Graham And I am saying NFB CANNOT create new higher harmonics. OK maybe what you are thinking aobut is this. If you take an amplifier without feedback and overdrive it to clipping, it will compress softly (2nd and 3rd order). If you take the same amplifier and add NFB to it and now overdrive it to clippiong, it will remain linear until it clips hard and this creates more high order harmoincs comapred to the first case. If you are talking about gutiar amp output stages where you are overdriving into clipping intentionally, yeah feedback will make the amp linear until it hard clips and lots of high order products are created. An amp without feddback will overdrive with softer compression and less higher harmonics. But this is comparing apples and oranges. If you operate both below clipping, the amplifer with neg feedback will have less overall distortion and less high order distortion. So are you talking about amps that are intentionally overdriven or amps that are operated in their linear range? Mark |
#50
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
The source of this myth is the mistaken idea that negative feedback
regenerates the audio signal, and the nonlinearity of the amplifier leads to higher order products of the regenerated harmonics and the nonlinearity of the amp. This ignores the fact that the regenerated signal is brought back in out-of-phase, and has the primary result of reducing the high order harmonics. I'm not sure that's right. My memory (which could be faulty) is that this can be shown mathematically. I'll ask around (I know a few people in high places) and see if I can get a reference. |
#51
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the recording. 30 years of experience with bias-controlled listening tests says that *seems* and *is* can be two different things. Intellectual logic has this interesting tendency to rule, once the comparison is based on just listening. When I reviewed, I made final judgements with my own live, undoctored recordings. The fun begins when you level-match, time-synch and eliminate other non-audible cues. Arny, I was talking in general terms. The distortion in recordings does not automatically mask distortions further down the playback chain. |
#52
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Mark" wrote in message oups.com... What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. Graham And I am saying NFB CANNOT create new higher harmonics. I'd like to see what Graham is using re the "maths of how feedback works." The complete result for a negative-feedback amplifier, using the configuration typically shown for an "inverting amp" application of an op-amp (but which may be generalized to any amplifier) winds up looking like this (assuming high enough input impedance at the amplifier such that current into that path is negligible): Vout/Vin = Rf/[Rin(A-1)] where A is the open-loop gain of the amp and Rf and Rin are the feedback and input resistors, respectively. Let A get big enough, and this simplifies to the more common Vout/Vin = -Rf/Rin ....but I sure don't see anything in the above which would *necessarily* result in additional harmonics. To be sure, in the practical case, the open-loop gain of the amplifier is non-linear, but even then you can clearly create an amplifier employing negative feedback which does NOT "create new harmonics" to an appreciably greater degree than its open-loop cousin. As a side note, it's really, REALLY hard to find any practical amplifier design which doesn't involve SOME negative feedback, somewhere - although it may not be as easy to spot as in this case. Bob M. |
#53
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Eeyore" wrote in message ... Arny Krueger wrote: "William Sommerwerck" wrote in message . .. And somehow, magically, the golden-ear boys (is's almost always boys) think that it matters that what they do to the signal that comes *off* the CD makes so much difference that they can hear the difference in the oxygen content of the interconnect wiring, or 0.06 percent distortion when the producer added 30% of his own, because he liked the effect. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. 30 years of experience with bias-controlled listening tests says that *seems* and *is* can be two different things. Intellectual logic has this interesting tendency to rule, once the comparison is based on just listening. When I reviewed, I made final judgements with my own live, undoctored recordings. The fun begins when you level-match, time-synch and eliminate other non-audible cues. I'd love to see you level match the sound field from different loudspeakers! Of course we can't do that with anything like the precision that is readily obtainable with amps, preamps, converters, digital players and recorders, etc. OTOH, it can be done well enough to be revealatory to many. |
#54
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"William Sommerwerck" wrote in message . .. What you say is intellectually logical, but it seems that post-recording distortions can be plainly audible, regardless of the quality of the recording. 30 years of experience with bias-controlled listening tests says that *seems* and *is* can be two different things. Intellectual logic has this interesting tendency to rule, once the comparison is based on just listening. When I reviewed, I made final judgements with my own live, undoctored recordings. The fun begins when you level-match, time-synch and eliminate other non-audible cues. Arny, I was talking in general terms. The distortion in recordings does not automatically mask distortions further down the playback chain. Since almost nothing in audio is automatic, that sounds a lot like an excluded middle argument. ;-) It is quite clear that the background noise in recordings is typically so much greater than that in good equipment (other than microphones and rooms), that it *does* mask the noise in much good equipment. True for good digital recorders and players. True for many mic preamps, at least when used with typical condenser mics. The most common cause of audible distortion in audio gear is clipping due to importune gain setting. |
#55
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"William Sommerwerck" wrote in message . .. The source of this myth is the mistaken idea that negative feedback regenerates the audio signal, and the nonlinearity of the amplifier leads to higher order products of the regenerated harmonics and the nonlinearity of the amp. This ignores the fact that the regenerated signal is brought back in out-of-phase, and has the primary result of reducing the high order harmonics. I'm not sure that's right. My memory (which could be faulty) is that this can be shown mathematically. I have just laid out the math results in two other posts. I've confirmed it with simulations. |
#56
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Mark" wrote in message oups.com... And I am saying NFB CANNOT create new higher harmonics. But it can. However, if the NFB is working reasonably, the higher harmonics are at vanishing low levels. OK maybe what you are thinking aobut is this. If you take an amplifier without feedback and overdrive it to clipping, it will compress softly (2nd and 3rd order). That avoiding NFB necessarily leads to soft clipping is a myth. NFB does do a nice job of making such clipping as might happen have sharper corners. Tubes saturating can be a little soft. Transistors saturating or cutting off give pretty sharp corners. If you take the same amplifier and add NFB to it and now overdrive it to clippiong, it will remain linear until it clips hard and this creates more high order harmoincs compared to the first case. If the equipment is clipping, there are going to be plenty of higher harmonics like 5,7 (presuming no P-P), even without NFB. But this is comparing apples and oranges. If you operate both below clipping, the amplifer with neg feedback will have less overall distortion and less high order distortion. Agreed. |
#57
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
The most common cause of audible distortion in audio gear
is clipping due to importune gain setting. Actually, clipping is not "in" the gear. I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. |
#58
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
The complete result for a negative-feedback amplifier,
using the configuration typically shown for an "inverting amp" application of an op-amp (but which may be generalized to any amplifier) winds up looking like this (assuming high enough input impedance at the amplifier such that current into that path is negligible): Vout/Vin = Rf/[Rin(A-1)] where A is the open-loop gain of the amp and Rf and Rin are the feedback and input resistors, respectively. Let A get big enough, and this simplifies to the more common Vout/Vin = -Rf/Rin ...but I sure don't see anything in the above which would *necessarily* result in additional harmonics. Because the mathematical model you're using doesn't include distortion! To be sure, in the practical case, the open-loop gain of the amplifier is non-linear, but even then you can clearly create an amplifier employing negative feedback which does NOT "create new harmonics" to an appreciably greater degree than its open-loop cousin. How do you know that? As a side note, it's really, REALLY hard to find any practical amplifier design which doesn't involve SOME negative feedback, somewhere - although it may not be as easy to spot as in this case. Many amplifiers include negative feedback, if only to stabilized local gain. |
#59
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"William Sommerwerck" wrote in message . .. The most common cause of audible distortion in audio gear is clipping due to importune gain setting. Actually, clipping is not "in" the gear. Please explain. I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. Your'e externalizing again William. It is you who need to run your K1 nd a QSC USA 400 or Dyna ST-120 through some level-matched bias-controlled listening tests. |
#60
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Mark wrote:
And I am saying NFB CANNOT create new higher harmonics. Sure it can. Just put something that creates harmonics in the feedback path. Negative feedback relies on the feedback path being linear and having low group delay. If these aren't the case, bad things can happen. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#61
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). Graham |
#62
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
William Sommerwerck wrote: The most common cause of audible distortion in audio gear is clipping due to importune gain setting. Actually, clipping is not "in" the gear. I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. What's the K1 like then ? Graham |
#63
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Eeyore" wrote in message ... William Sommerwerck wrote: The most common cause of audible distortion in audio gear is clipping due to importune gain setting. Actually, clipping is not "in" the gear. I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. What's the K1 like then ? Something like The Second Coming? |
#64
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 20, 6:06 pm, Eeyore
wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). Graham take something like crossover distortiuon for example... in an open loop amp, crossover dist. creates lots of harmonics. add neg feedback and they are all reduced. The high order ones are not reduced AS MUCH as the low order ones, but they are certainly not increased (assumming a proper design not on the verge of instability and assuming the feedback componets themselves are linear, resistors are usually linear for our purposes). so after you add neg feedback the proportion of high order to low order will change and realative to the low order there will be more high order, but in absolute terms that are all reduced. Someone else already said this so I am repeating... I don't know how else to say it... Mark |
#65
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
"Scott Dorsey Notorious Charlatan" It does, because a stage which is audibly blameless by itself may turn into a sonic disaster when it appears a few hundred times in the signal path. ** Huh ?? A few HUNDRED times ??????? The colossal fool must be on LSD. "C'est un Nagra. C'est suisse, et tres, tres precis." ** C'est Dorsey - so you know it is total crapology. ......... Phil |
#66
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Mark wrote: Eeyore wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). take something like crossover distortiuon for example... in an open loop amp, crossover dist. creates lots of harmonics. add neg feedback and they are all reduced. The high order ones are not reduced AS MUCH as the low order ones, That's because of the falling loop gain with frequency of the amplifier. Not what I was referring to. but they are certainly not increased (assumming a proper design not on the verge of instability and assuming the feedback componets themselves are linear, resistors are usually linear for our purposes). so after you add neg feedback the proportion of high order to low order will change and realative to the low order there will be more high order, but in absolute terms that are all reduced. Someone else already said this so I am repeating... I don't know how else to say it... You've missed the point I was making entirely. Other posters have explained it better than myself however. Graham |
#67
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Phil Allison wrote: "Scott Dorsey Notorious Charlatan" It does, because a stage which is audibly blameless by itself may turn into a sonic disaster when it appears a few hundred times in the signal path. ** Huh ?? A few HUNDRED times ??????? The colossal fool must be on LSD. The EQ section alone on a Neve V series (and derivatives) has 18 op-amp stages. Graham |
#68
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
" Graham Stevenson Mentally Deranged Pile of Autistic ****." Phil Allison wrote: "Scott Dorsey Notorious Charlatan" It does, because a stage which is audibly blameless by itself may turn into a sonic disaster when it appears a few hundred times in the signal path. ** Huh ?? A few HUNDRED times ??????? The colossal fool must be on LSD. The EQ section alone on a Neve V series (and derivatives) has 18 op-amp stages. ** **** OFF YOU ASININE TROLLING MORON !! ....... Phil |
#69
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
Arny Krueger wrote:
"Eeyore" wrote in message I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. What's the K1 like then ? Something like The Second Coming? No, more like something out of Revelations. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#70
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Feedback in audio esp wrt op-amps.
On Aug 20, 7:20 pm, Eeyore
wrote: Phil Allison wrote: "Scott Dorsey Notorious Charlatan" It does, because a stage which is audibly blameless by itself may turn into a sonic disaster when it appears a few hundred times in the signal path. ** Huh ?? A few HUNDRED times ??????? The colossal fool must be on LSD. The EQ section alone on a Neve V series (and derivatives) has 18 op-amp stages. It isn't hard to end up with that many. 1 per band per channel plus a few will get you to 20 without working at it. To get above 100, you are talking about a serious amount of more signal processing. |
#71
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Feedback in audio esp wrt op-amps.
MooseFET wrote: Eeyore wrote: Phil Allison wrote: "Scott Dorsey Notorious Charlatan" It does, because a stage which is audibly blameless by itself may turn into a sonic disaster when it appears a few hundred times in the signal path. ** Huh ?? A few HUNDRED times ??????? The colossal fool must be on LSD. The EQ section alone on a Neve V series (and derivatives) has 18 op-amp stages. It isn't hard to end up with that many. 1 per band per channel plus a few will get you to 20 without working at it. To get above 100, you are talking about a serious amount of more signal processing. 100 sounds pretty extreme certainly. Graham |
#72
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 20, 6:13 pm, Mark wrote:
On Aug 20, 6:06 pm, Eeyore wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). Graham take something like crossover distortiuon for example... No, I don't want crossover distortion. How about thinking about a distortion that only adds, lets say the 2nd harmonic to a sine wave. Think about what happens when that is enclosed in a feedback loop. You take some of that second harmonic from the output and feed it back into the input. The nonlinear circuit takes the 2nd harmonic of the 2nd harmonic giving the forth and sends that out the output. That forth comes back around and around and around. A nonlinear cicrcuit that only made 2nd a harmonic is now resulting in an infinite chain of frequencies. in an open loop amp, crossover dist. creates lots of harmonics. add neg feedback and they are all reduced. The high order ones are not reduced AS MUCH as the low order ones, but they are certainly not increased (assumming a proper design not on the verge of instability and assuming the feedback componets themselves are linear, resistors are usually linear for our purposes). This is not correct. You have to have an extraordinarily large phase margin to not have a boost in the harmonic near the gain crossover. If G is the forward gain from the point where the distortion is made to the output and H is the rest feedback the math looks like: G /(1 + GH) Here's the very ugly bit: The distortion is often created in the output section making the G part unity or nearly so. A stable servo loop can have a phase margin of 30 degrees. 1/(1 + 1 * 1@(180-30)) = 1/(1 - 0.866 + j0.5) = 1/(0.134 + j0.5) Take ABS() ABS(1/(0.134 + j0.5)) = 1/sqrt(0.134^2 + 0.5^2) = 1.93 Even though this amplifier is very stable, the feedback loop doubles the amplitude of the harmonic near the gain crossover. |
#73
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Feedback in audio esp wrt op-amps.
On Aug 20, 7:16 pm, Eeyore
wrote: Mark wrote: Eeyore wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). take something like crossover distortiuon for example... in an open loop amp, crossover dist. creates lots of harmonics. add neg feedback and they are all reduced. The high order ones are not reduced AS MUCH as the low order ones, That's because of the falling loop gain with frequency of the amplifier. Not what I was referring to. ..... plus the increasing phase lag. Must not forget that nasty detail : but they are certainly not increased (assumming a proper design not on the verge of instability and assuming the feedback componets themselves are linear, resistors are usually linear for our purposes). so after you add neg feedback the proportion of high order to low order will change and realative to the low order there will be more high order, but in absolute terms that are all reduced. Someone else already said this so I am repeating... I don't know how else to say it... You've missed the point I was making entirely. Other posters have explained it better than myself however. I tried my harmonic of the harmonic argument again. Sometimes it works sometimes not. |
#74
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Feedback in audio esp wrt op-amps.
On Aug 19, 10:35 pm, D from BC wrote:
On Mon, 20 Aug 2007 05:19:10 +0100, Eeyore wrote: D from BC wrote: Electronics have time delays. Switching circuits have time delays ( Ton - Toff - Tstg etc ) . Amplifier circuits are not normally hard switching. It's more useful to look at phase shift with them. Graham I guess I think phase for repeating waveforms. Audio is like noise. I haven't heard someone say "That noise is lagging by 40 degrees." 2 sine waves out of sync can be expressed by degrees or time delay. Any complex signal can be broken down into a combination of sine waves. So you can still use the notion of phase shift. But yeah... when it comes to feedback, time delay within a 1/2 cycle is of concern..so I guess that's why phase is the better term. I mentioned time delay to express the time it takes for a signal to pass through x amount of transistors in an op amp. After that, feeding back the signal kinda doesn't look like instantaneous correction. In some ways feedback is seems like continuously breaking wine glasses on the floor.. If the clean up is done fast enough...it doesn't look like any glasses are being broken. Well...that's probably a crappy analogy but best I can think of... D from BC |
#75
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Feedback in audio esp wrt op-amps.
On Aug 20, 3:12 pm, (Scott Dorsey) wrote:
Mark wrote: And I am saying NFB CANNOT create new higher harmonics. Sure it can. Just put something that creates harmonics in the feedback path. Negative feedback relies on the feedback path being linear and having low group delay. If these aren't the case, bad things can happen. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." I'm not sure I'd bring group delay into the discussion. What you need is phase margin. |
#76
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Feedback in audio esp wrt op-amps.
MooseFET wrote:
On Aug 20, 6:13 pm, Mark wrote: On Aug 20, 6:06 pm, Eeyore wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). Graham take something like crossover distortiuon for example... No, I don't want crossover distortion. How about thinking about a distortion that only adds, lets say the 2nd harmonic to a sine wave. Think about what happens when that is enclosed in a feedback loop. You take some of that second harmonic from the output and feed it back into the input. The nonlinear circuit takes the 2nd harmonic of the 2nd harmonic giving the forth and sends that out the output. That forth comes back around and around and around. A nonlinear cicrcuit that only made 2nd a harmonic is now resulting in an infinite chain of frequencies. in an open loop amp, crossover dist. creates lots of harmonics. add neg feedback and they are all reduced. The high order ones are not reduced AS MUCH as the low order ones, but they are certainly not increased (assumming a proper design not on the verge of instability and assuming the feedback componets themselves are linear, resistors are usually linear for our purposes). This is not correct. You have to have an extraordinarily large phase margin to not have a boost in the harmonic near the gain crossover. If G is the forward gain from the point where the distortion is made to the output and H is the rest feedback the math looks like: G /(1 + GH) Here's the very ugly bit: The distortion is often created in the output section making the G part unity or nearly so. A stable servo loop can have a phase margin of 30 degrees. 1/(1 + 1 * 1@(180-30)) = 1/(1 - 0.866 + j0.5) = 1/(0.134 + j0.5) Take ABS() ABS(1/(0.134 + j0.5)) = 1/sqrt(0.134^2 + 0.5^2) = 1.93 Even though this amplifier is very stable, the feedback loop doubles the amplitude of the harmonic near the gain crossover. So for audio, put the gain crossover way out of band. Right? -- Les Cargill |
#77
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Feedback in audio esp wrt op-amps.
On Mon, 20 Aug 2007 21:31:17 -0700, MooseFET
wrote: On Aug 20, 6:13 pm, Mark wrote: On Aug 20, 6:06 pm, Eeyore wrote: Mark wrote: Eeyore wrote: What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't exist with the open-loop situation. It's down to the maths of how feedback works. And I am saying NFB CANNOT create new higher harmonics. And it seems you are incorrect (at least when the amplifier having the feedback applied has some non-linearity). Graham take something like crossover distortiuon for example... No, I don't want crossover distortion. How about thinking about a distortion that only adds, lets say the 2nd harmonic to a sine wave. Think about what happens when that is enclosed in a feedback loop. You take some of that second harmonic from the output and feed it back into the input. The nonlinear circuit takes the 2nd harmonic of the 2nd harmonic giving the forth and sends that out the output. That forth comes back around and around and around. A nonlinear cicrcuit that only made 2nd a harmonic is now resulting in an infinite chain of frequencies. [snip] Cool... Maybe call it a distortion loop. :P +---------------------------------------+ | | sine--summation-------nonlinear transfer (inverting)-+ | Not completely containing a signal to cancel out the nonlinear transfer. So some 2nd harmonic gets to pass through the nonlinear transfer again to make...the 4th....and so and so on.. (IIRC that would be the harmonic generation sequence for a 2nd order nonlinear transfer.) Take 2 tone and then there's the intermodulation products. What a painful thing to think about... Significant magnitudes??? Cheerleader in electronics... "2,4,6,8 what distortion do I hate." D from BC |
#78
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Feedback in audio esp wrt op-amps.
"Eeyore" wrote in message ... I do wish you'd glom onto a Crown K1. You really need to hear this amplifier -- and run it through some blind tests. What's the K1 like then ? Like most other good amps, so good you can forget about it being the problem, unless it's broken of course :-) MrT. |
#79
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Feedback in audio esp wrt op-amps.
"MooseFET" wrote in message ups.com... It isn't hard to end up with that many. 1 per band per channel plus a few will get you to 20 without working at it. To get above 100, you are talking about a serious amount of more signal processing. 100 op amps on parallel channels is a far different situation than 100 *ALL in series* with the signal. Of course in the real world the situation is somewhere in between those extremes. MrT. |
#80
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Feedback in audio esp wrt op-amps.
["Followup-To:" header set to sci.electronics.design.]
Eeyore wrote: The idea that you can 'get away' with sloppy circuitry for replay because the source was in some way 'impaired' is totally false. I don't think anybody proposed "sloppy" circuitry for replay. The point is that studio audio gear is just solid, reliable, conventional good audio stuff (none of that high-end low-oxygen power cord crap). Plenty of opamps, plenty of NFB, plenty of digital processing, plenty of all the things that high-enders loathe. Since the recording studio already did 90% of the work of completely destroying the audio signal beyond repair, it doesn't matter how much your home audio gear adds to that. Sometimes when I hear the golden earers talk I'm surprised that I can make out any music at all when listening with my Cantons fed from an old Sony amp through particularly oxygen-rich cables. robert |
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