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#1
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Feedback in audio esp wrt op-amps.
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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? Graham |
#2
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? That negative feedback linearizes the transfer function at the expensive of adding higher-order harmonics has been long-known. What you say is perfectly logical. However, the presence of higher-order harmonics is not the only factor, but their amplitude. Below a certain percentage (I'm sure Arny will be able to tell us what that is), they're inaudible. A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. |
#3
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Feedback in audio esp wrt op-amps.
On Aug 19, 8:39 am, 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration Degeneration is NFB. It is just applied locally. What you really want is to go with a topology that is naturally more linear, of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? One huge problem with including a lot of local NFB is that it makes the overall system harder to close. Local feedback often creates 2 pole systems with modest Q values within the system. When you go to close the loop, you have to keep a good phase margin so you are forced to use a lower overall loop gain. Try spice modeling a thing like this: Vcc ---------------+------------ ! \ / Vbias \ ! ! / +-------------- \ ! ! ! !/ e ! ---!!--+------! PNP --- !\ --- +--Out ! \ ! / ! \ ! ! ! +-------------- ! V D1 --- ! GND Change D1 to be a resistor and back and you will see quite a difference in the amount of degeneration needed to get the same distortion values for a modest signal of lets say 10mV in. |
#4
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
MooseFET 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration Degeneration is NFB. It is just applied locally. What you really want is to go with a topology that is naturally more linear, Sorry I didn't make that clearer. Yes, I'm referring to the reduction of overall loop feedback. Graham |
#5
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
MooseFET wrote: One huge problem with including a lot of local NFB is that it makes the overall system harder to close. That's not my experience. Quite the reverse actually. But then I do tend to incorporate internal lead-lag compensation. This results in a far BETTER phase margin. Graham |
#6
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Sun, 19 Aug 2007 16:39:55 +0100, 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? Graham Just speaky from some audio hobby work.... *Like with most things in electronics, there are frequency limits. I think feedback decreases with frequency. The harmonic distortion becomes an ultrasonic problem. *Feedback is a correction signal.. If nothing messes up this process then all's well. *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. D from BC |
#7
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
D from BC wrote: Just speaky from some audio hobby work.... *Like with most things in electronics, there are frequency limits. I think feedback decreases with frequency. The harmonic distortion becomes an ultrasonic problem. *Feedback is a correction signal.. If nothing messes up this process then all's well. *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. You need to learn more. I appreciate your interest but your grasp of the issues is beginner level. Graham |
#8
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: 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? That negative feedback linearizes the transfer function at the expensive of adding higher-order harmonics has been long-known. What you say is perfectly logical. However, the presence of higher-order harmonics is not the only factor, but their amplitude. Below a certain percentage (I'm sure Arny will be able to tell us what that is), they're inaudible. There's more than a little discussion about what level that is, and indeed it's known that audibility varies according to harmonic number. A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. As a buffer it has 100% NFB and I hope that's the case.. As a gain stage with say 40dB of voltage gain that isn't the case however. Really, part of what I'm saying is that the classic op-amp isn't really the ideal gain stage for audio circuits if you want to produce totally 'technically blameless' performance. Graham |
#9
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 19, 9:57 am, Eeyore
wrote: MooseFET wrote: One huge problem with including a lot of local NFB is that it makes the overall system harder to close. That's not my experience. Quite the reverse actually. But then I do tend to incorporate internal lead-lag compensation. This results in a far BETTER phase margin. This means that you have lowered the outter loop gain in the process. If the internal part looks kind of like this: --!!-/\/\-- ! ! ---/\/\-+--!-\ ! ! ------+--- !+/ The amplification stage you are placing the NFB around must have a great enough bandwidth to make the feedback determine the responce. The local feedback has all of the problems a global feedback has with creating upper harmonics. Your global feedback is now at a lower gain and thus can't remove them. This is just a case of the lack of a free lunch. The pole and zero inside the loop is a good thing to do to improve the phase margin when you have other poles in the system. It allows you to determine where the gain crossover happens and the phase at the crossover. It is a method of lowering the overall loop gain. It doesn't however get rid of the harmonics issue. It also is something that you can only do a few times inside the loop. When the system starts to look like 3 of those in series, you are back in trouble. In audio stuff, you generally want to put the pole-zero thing near the output, ideally enclosing the output. This makes the system apply a low pass filter to any distortion products that the feedback can't get rid of. |
#10
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 19, 9:43 am, "William Sommerwerck"
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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? That negative feedback linearizes the transfer function at the expensive of adding higher-order harmonics has been long-known. What you say is perfectly logical. However, the presence of higher-order harmonics is not the only factor, but their amplitude. Below a certain percentage (I'm sure Arny will be able to tell us what that is), they're inaudible. A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. Even at reasonable gains, there are many that will perform well enough that nobody will hear the difference. Power amplifiers are the place where it gets very hard to keep distortion low at reasonable efficiencies. |
#11
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 19, 10:03 am, D from BC wrote:
On Sun, 19 Aug 2007 16:39:55 +0100, 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? Graham Just speaky from some audio hobby work.... *Like with most things in electronics, there are frequency limits. I think feedback decreases with frequency. Yes it typically does generally decrease. It also has a phase shift. If you add feedforward, you can have a band in which the feedback increases with frequency. The harmonic distortion becomes an ultrasonic problem. One problem is that ultasonic things can interact on any nonlinear part of the system. This can lead to frequencies that are things like 7*F1 - 9*F2 in the circuit. It is like someone injected a signal at that frequency into that point in the circuit. How the system responds to it determines whether it will be heard or not. *Feedback is a correction signal.. If nothing messes up this process then all's well. *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. D from BC |
#12
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
On Aug 19, 10:03 am, D from BC wrote:
[...] *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. For some reason my cursor went away. This makes it harder to edit what I'm typing. Making the "best linear open loop" is for all practical purposes never enough. You need a very linear open loop design with a low enough phase shift to make the NFB work and ideally to have a lowpass effect applied to any distortion that is created. You also have to trade off performance against water cooling. A simple class A power MOSFET common source stage can be used as an example. If you use about 10 power MOSFETs in parallel, have each one passing about 0.5 Amps, and run with a 50V supply, you will have a circuit that is darn linear for a 1mV input signal. |
#13
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Feedback in audio esp wrt op-amps.
On Aug 19, 8:39 am, Eeyore
wrote: ...about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. All amplifiers have characteristic curves; the gain isn't completely linear. Feedback components can be (very linear) resistors. So you get some combination of amplification and negative feedback in most useful low-distortion amplifiers. A single transistor can have power gain of 10,000; a single vacuum tube or MOSFET can have more. Giving up gain for linearity is a good trade. It's never perfect (even resistors are distortion sources, if you have signals at 2 Hz and the self-heating of the resistors isn't insignificant), but it's good enough. Listen. Enjoy. |
#14
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Feedback in audio esp wrt op-amps.
A good op amp can be used as a buffer and be sonically transparent,
its output indistinguishable from its input. As a buffer it has 100% NFB and I hope that's the case. As a gain stage with say 40dB of voltage gain that isn't the case however. Really, part of what I'm saying is that the classic op-amp isn't really the ideal gain stage for audio circuits if you want to produce totally "technically blameless" performance. That's certainly true. But does it matter what type of circuit or components you use if the performance is audibly blameless? |
#15
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Feedback in audio esp wrt op-amps.
On Aug 19, 2:42 pm, "William Sommerwerck"
wrote: A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. As a buffer it has 100% NFB and I hope that's the case. As a gain stage with say 40dB of voltage gain that isn't the case however. Really, part of what I'm saying is that the classic op-amp isn't really the ideal gain stage for audio circuits if you want to produce totally "technically blameless" performance. That's certainly true. But does it matter what type of circuit or components you use if the performance is audibly blameless? It also has to work for a reasonable time, be easy to manufacture and look good. |
#16
Posted to sci.electronics.design,rec.audio.pro,rec.audio.tech
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Feedback in audio esp wrt op-amps.
MooseFET wrote: Eeyore wrote: MooseFET wrote: One huge problem with including a lot of local NFB is that it makes the overall system harder to close. That's not my experience. Quite the reverse actually. But then I do tend to incorporate internal lead-lag compensation. This results in a far BETTER phase margin. This means that you have lowered the outter loop gain in the process. Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop gain makes no sense for audio. Besides, gain is cheap these days. I have no objection to the introduction of another gain stage for example. I'd rather have a sensible amount of very linear and well defined gain than oodles of 'poor quality' gain. If the internal part looks kind of like this: --!!-/\/\-- ! ! ---/\/\-+--!-\ ! ! ------+--- !+/ The amplification stage you are placing the NFB around must have a great enough bandwidth to make the feedback determine the responce. Yes and yes. Many IC op-amps used for audio have GBPs in the 10MHz region so this isn't too difficult even if using one of those inside the loop (which I did in a recent design ). Discrete stages suitably degenerated can have higher GBPs than that. The local feedback has all of the problems a global feedback has with creating upper harmonics. Your global feedback is now at a lower gain and thus can't remove them. This is just a case of the lack of a free lunch. I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one to imagine that pursuit of linearity in individual stages was a pointless pursuit and you might as well have tons of non-linear gain and I know that's not the case, not least because the very hugh gain system has to be stable and that tends to lead to rolling off the gain (and the advantage of NFB) from very low frequencies. The pole and zero inside the loop is a good thing to do to improve the phase margin when you have other poles in the system. It allows you to determine where the gain crossover happens and the phase at the crossover. It is a method of lowering the overall loop gain. It doesn't however get rid of the harmonics issue. It also is something that you can only do a few times inside the loop. When the system starts to look like 3 of those in series, you are back in trouble. 3 of them would be a bit much. I've not used more than 2 inside the loop in fact. In audio stuff, you generally want to put the pole-zero thing near the output, ideally enclosing the output. This makes the system apply a low pass filter to any distortion products that the feedback can't get rid of. Interesting idea. Graham |
#17
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Feedback in audio esp wrt op-amps.
whit3rd wrote: Eeyore wrote: ...about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. All amplifiers have characteristic curves; the gain isn't completely linear. I know. That's why I said the individual stages should be degenerated to linearise them. This results in a lower gain but this may not be a problem in practice as long as GBP is maintained. Feedback components can be (very linear) resistors. So you get some combination of amplification and negative feedback in most useful low-distortion amplifiers. A single transistor can have power gain of 10,000; In every audio amplifier stage I know, POWER gain is of little importance. Voltage gain is what's required. Cuurent gain can be readily added where needed by using emiiter followers. a single vacuum tube or MOSFET can have more. Giving up gain for linearity is a good trade. That was indeed my point wrt giving up some of that *open-loop* gain in a gain block. Graham |
#18
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Feedback in audio esp wrt op-amps.
William Sommerwerck wrote: A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. As a buffer it has 100% NFB and I hope that's the case. As a gain stage with say 40dB of voltage gain that isn't the case however. Really, part of what I'm saying is that the classic op-amp isn't really the ideal gain stage for audio circuits if you want to produce totally "technically blameless" performance. That's certainly true. But does it matter what type of circuit or components you use if the performance is audibly blameless? You can (and people do) argue forever about what is or isn't subjectively audible. The '990C' discrete op-amp was mentioned in another thread for example. With THD of 0.06% (-64dB) under some conditions it strikes me that those distortion products could easily be audible yet ppl leapt to its defence. If it can be shown that the defects must be inaudible from first principles (such as distortion below 100dB for example) you're on firmer ground IMHO. Graham |
#19
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Feedback in audio esp wrt op-amps.
On Aug 19, 4:34 pm, Eeyore
wrote: MooseFET wrote: Eeyore wrote: MooseFET wrote: One huge problem with including a lot of local NFB is that it makes the overall system harder to close. That's not my experience. Quite the reverse actually. But then I do tend to incorporate internal lead-lag compensation. This results in a far BETTER phase margin. This means that you have lowered the outter loop gain in the process. Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop gain makes no sense for audio. You do need merely "high gain" however. This high gain needs to be true at the frequencies of interest so the GBP does have to be at least some reasonable amount. Very high values of loop gain makes for very large amounts of reductions in the harmonics within the band. This can argue for much more gain than it would normally appear you need if you only needed enough gain to be sure that the feedback resistors were what was setting the gain. Besides, gain is cheap these days. I have no objection to the introduction of another gain stage for example. I'd rather have a sensible amount of very linear and well defined gain than oodles of 'poor quality' gain. Adding stages adds to the phase shifts. This is another "no free lunch situation". When you increase the number of stages you also want to increase the bandwidths of most of the stages to keep the phase shift near the gain cross over within reason. Keeping this for reference later: --!!-/\/\-- ! ! ---/\/\-+--!-\ ! ! ------+--- !+/ [.. stuff we have covered and agree on ... The local feedback has all of the problems a global feedback has with creating upper harmonics. Your global feedback is now at a lower gain and thus can't remove them. This is just a case of the lack of a free lunch. I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one to imagine that pursuit of linearity in individual stages was a pointless pursuit and you might as well have tons of non-linear gain and I know that's not the case, not least because the very hugh gain system has to be stable and that tends to lead to rolling off the gain (and the advantage of NFB) from very low frequencies. No what I am pointing out is that local feedback is not a good substitute for a naturally more linear stage. Consider this sort of a situation: ---------- ------ ---------- ------ Signal---! Subtract !---! Gain !---! Subtract !---! Bad !--+---- ---------- ------ ---------- ! gain ! ! ^ ^ ------ ! ! ! ! ------------------------+--------------------- You can trade back and forth how much subtracting you do in the two subtraction circuits but you can't really fix the "bad gain" section. I am for not leaving out the idea that the good gain has a limited bandwidth and assuming all of the bandwidth limiting happens in the "bad gain". I think this makes the idea obvious in it simple form. To take a bit of a real example, consider a dreadful output stage that works like this: ----+--------+--- ! ! \ \ R1 / / R2 \ \ ! ! ! !/e +-------! PNP ! !\ !/ ! ! NPN ! !\e ! R3 ----------+----/\/\--- --- mirror for other half R3 is providing a measure of local feedback. R2 also is doing so. This stage will still be a horror story. Adding a diode in series with R1 to match to the E-B drop of the PNP makes it much less so. The diode makes the PNP act much more like a linear current mirror and thus reduces the natural distortion. Since the transistors used in power stages are usually slower than the others in the design. The output is almost always where the pole you didn't design in lives. The pole and zero inside the loop is a good thing to do to improve the phase margin when you have other poles in the system. It allows you to determine where the gain crossover happens and the phase at the crossover. It is a method of lowering the overall loop gain. It doesn't however get rid of the harmonics issue. It also is something that you can only do a few times inside the loop. When the system starts to look like 3 of those in series, you are back in trouble. 3 of them would be a bit much. I've not used more than 2 inside the loop in fact. Trust me on this: Don't put three inside the loop. Reconsider the design if you find yourself going there. Two is ok. One plus a feedforwards is ok but three always seems to mean trouble. In audio stuff, you generally want to put the pole-zero thing near the output, ideally enclosing the output. This makes the system apply a low pass filter to any distortion products that the feedback can't get rid of. Interesting idea. It only works up to a point. It also requires largish (mechanically) parts be involved. You have a capacitor and a resistor with fairly large swings on them and are working at lowish impedances. |
#20
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Feedback in audio esp wrt op-amps.
MooseFET wrote: Eeyore wrote: MooseFET wrote: Eeyore wrote: MooseFET wrote: One huge problem with including a lot of local NFB is that it makes the overall system harder to close. That's not my experience. Quite the reverse actually. But then I do tend to incorporate internal lead-lag compensation. This results in a far BETTER phase margin. This means that you have lowered the outter loop gain in the process. Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop gain makes no sense for audio. You do need merely "high gain" however. This high gain needs to be true at the frequencies of interest so the GBP does have to be at least some reasonable amount. 10MHz seems to work reasonably well but 120dB gain at LF is not a requirement. Very high values of loop gain makes for very large amounts of reductions in the harmonics within the band. This can argue for much more gain than it would normally appear you need if you only needed enough gain to be sure that the feedback resistors were what was setting the gain. That's sort of what I'm after. Besides, gain is cheap these days. I have no objection to the introduction of another gain stage for example. I'd rather have a sensible amount of very linear and well defined gain than oodles of 'poor quality' gain. Adding stages adds to the phase shifts. Needn't be a very significant phase shift. Plus, if the 'natural' phase shift of the existing stages is reduced through degeneration, that's all fine. This is another "no free lunch situation". When you increase the number of stages you also want to increase the bandwidths of most of the stages to keep the phase shift near the gain cross over within reason. Oh yes and degeneration will do that of course. Graham |
#21
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Feedback in audio esp wrt op-amps.
MooseFET wrote: addressing individual points here Since the transistors used in power stages are usually slower than the others in the design. The output is almost always where the pole you didn't design in lives. In a power amplifier design this is pretty much invariably true (although power devices from the likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly need not have such a limitation. I'd expect to be using parts with a 100MHz fT. As an example of designing around the problem where you do need some watts of dissipation, where I once needed to provide a highish current drive stage to drive some Mosfet gates I used several reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device. Graham |
#22
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Feedback in audio esp wrt op-amps.
" Graham Stevenson Total ****** " You can (and people do) argue forever about what is or isn't subjectively audible. The '990C' discrete op-amp was mentioned in another thread for example. With THD of 0.06% (-64dB) under some conditions it strikes me that those distortion products could easily be audible yet ppl leapt to its defence. ** Shame how the incorrigible, self aggrandising Graham Stevenson charlatan deliberately did not provide a link to this obscure product from the audio ******'s brigade. Here it is: http://www.johnhardyco.com/pdf/990.pdf The 0.06% figure is there in the specs. If refers to operation at 20 kHz, with 40 dB of gain and 19 volts peak into a 75 ohm load - a power level of 2.5 watts !!! Naturally, the THD figures improve dramatically at lower frequencies, power levels and with common load impedances. WHAT a CROCK OF **** !!! This link has some actual test results with range of popular audio op-amps. http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm ....... Phil |
#23
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Feedback in audio esp wrt op-amps.
On Aug 19, 5:49 pm, Eeyore
wrote: [....] You do need merely "high gain" however. This high gain needs to be true at the frequencies of interest so the GBP does have to be at least some reasonable amount. 10MHz seems to work reasonably well but 120dB gain at LF is not a requirement. I quite agree. The point that matters more is the gain at the top of the band. At the low end you almost always have more than enough gain. Besides, gain is cheap these days. I have no objection to the introduction of another gain stage for example. I'd rather have a sensible amount of very linear and well defined gain than oodles of 'poor quality' gain. Adding stages adds to the phase shifts. Needn't be a very significant phase shift. Plus, if the 'natural' phase shift of the existing stages is reduced through degeneration, that's all fine. The local feedback is lowering the gain and thus shifts the gain cross over downwards. When you add the stage, you never get quite the full gain increase. At least, this is generally true for gains greater than about e. |
#24
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Feedback in audio esp wrt op-amps.
On Aug 19, 5:57 pm, Eeyore
wrote: MooseFET wrote: addressing individual points here Since the transistors used in power stages are usually slower than the others in the design. The output is almost always where the pole you didn't design in lives. In a power amplifier design this is pretty much invariably true (although power devices from the likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly need not have such a limitation. I'd expect to be using parts with a 100MHz fT. As an example of designing around the problem where you do need some watts of dissipation, where I once needed to provide a highish current drive stage to drive some Mosfet gates I used several reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device. No cursor again damit! Zetex makes the 2N2222 is a SOT223 package. Each one is good for about 0.5A of gate drive and are quite fast. I've used them as RF devices at 90MHZ. Graham |
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Feedback in audio esp wrt op-amps.
MooseFET wrote: Eeyore wrote: MooseFET wrote: addressing individual points here Since the transistors used in power stages are usually slower than the others in the design. The output is almost always where the pole you didn't design in lives. In a power amplifier design this is pretty much invariably true (although power devices from the likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly need not have such a limitation. I'd expect to be using parts with a 100MHz fT. As an example of designing around the problem where you do need some watts of dissipation, where I once needed to provide a highish current drive stage to drive some Mosfet gates I used several reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device. No cursor again damit! Zetex makes the 2N2222 is a SOT223 package. Each one is good for about 0.5A of gate drive and are quite fast. I've used them as RF devices at 90MHZ. In my case quiescent dissipation was a factor so SOT223 wouldn't have been helpful. Also I was using +/- 105V supplies. MPSA42 and 92 did the job. Graham |
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Feedback in audio esp wrt op-amps.
On Sun, 19 Aug 2007 11:38:53 -0700, MooseFET
wrote: On Aug 19, 10:03 am, D from BC wrote: On Sun, 19 Aug 2007 16:39:55 +0100, 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? Graham Just speaky from some audio hobby work.... *Like with most things in electronics, there are frequency limits. I think feedback decreases with frequency. Yes it typically does generally decrease. It also has a phase shift. If you add feedforward, you can have a band in which the feedback increases with frequency. The harmonic distortion becomes an ultrasonic problem. One problem is that ultasonic things can interact on any nonlinear part of the system. This can lead to frequencies that are things like 7*F1 - 9*F2 in the circuit. It is like someone injected a signal at that frequency into that point in the circuit. How the system responds to it determines whether it will be heard or not. *Feedback is a correction signal.. If nothing messes up this process then all's well. *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. D from BC I have to wonder how often BW limiting (say cutoff at 20khz) is practiced in audio electronics design to filter out ultrasonic harmonics produced by op amp stages. For example: Active crossovers, sound cards, mixing boards... D from BC |
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Feedback in audio esp wrt op-amps.
On Sun, 19 Aug 2007 11:49:37 -0700, MooseFET
wrote: On Aug 19, 10:03 am, D from BC wrote: [...] *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. For some reason my cursor went away. This makes it harder to edit what I'm typing. Making the "best linear open loop" is for all practical purposes never enough. You need a very linear open loop design with a low enough phase shift to make the NFB work and ideally to have a lowpass effect applied to any distortion that is created. You also have to trade off performance against water cooling. A simple class A power MOSFET common source stage can be used as an example. If you use about 10 power MOSFETs in parallel, have each one passing about 0.5 Amps, and run with a 50V supply, you will have a circuit that is darn linear for a 1mV input signal. Lowpass effect?? First time I read it that way... but I think I know what you mean. It's the increasing distortion with increasing frequency. All happening due to decreasing open loop gain with increasing frequency. (With a open loop phase(f) such that the amp is stable.) Speaking of phase... Here's something I find fuzzy.. For an amplifier.. the input signal is summed with the output signal. The result of the summation is the input signal + call it an anti-distortion signal. The more fed back the more the gain goes down but the more linear the amp acts.. Great if it all happens instantly.. But I can't imagine it does. Electronics have time delays. Feedback kinda looks like a late arrival. It's just amazing the amplifier can keep up and fix its own nonlinearity with chaotic audio jumping around at all differant rates. D from BC |
#28
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Feedback in audio esp wrt op-amps.
D from BC wrote: I have to wonder how often BW limiting (say cutoff at 20khz) is practiced in audio electronics design to filter out ultrasonic harmonics produced by op amp stages. For example: Active crossovers, sound cards, mixing boards... Never IME. Flat to 100kHz is the order of the day. Graham |
#29
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Feedback in audio esp wrt op-amps.
D from BC wrote: It's the increasing distortion with increasing frequency. Open loop gain reduces with frequency (Miller Effect). Reduced open loop gain at higher frequencies means less negative feedback available. Less 'correcting' NFB increased THD. Graham |
#30
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Feedback in audio esp wrt op-amps.
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 |
#31
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Feedback in audio esp wrt op-amps.
On Sun, 19 Aug 2007 18:41:42 +0100, Eeyore
wrote: D from BC wrote: Just speaky from some audio hobby work.... *Like with most things in electronics, there are frequency limits. I think feedback decreases with frequency. The harmonic distortion becomes an ultrasonic problem. *Feedback is a correction signal.. If nothing messes up this process then all's well. *For large signals, doesn't every semiconductor naturally distort? Developing the best linear open loop design may not be enough. You need to learn more. I appreciate your interest but your grasp of the issues is beginner level. Graham Learn more..No wayy... :P I gave up audio electronics in 98. Is there still money to be made in that area? D from BC |
#32
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Feedback in audio esp wrt op-amps.
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. 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 |
#33
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Feedback in audio esp wrt op-amps.
In article ,
William Sommerwerck wrote: A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. As a buffer it has 100% NFB and I hope that's the case. As a gain stage with say 40dB of voltage gain that isn't the case however. Really, part of what I'm saying is that the classic op-amp isn't really the ideal gain stage for audio circuits if you want to produce totally "technically blameless" performance. That's certainly true. But does it matter what type of circuit or components you use if the performance is audibly blameless? 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. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#34
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Feedback in audio esp wrt op-amps.
On Sun, 19 Aug 2007 11:29:26 -0700, MooseFET
wrote: On Aug 19, 9:43 am, "William Sommerwerck" 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 made me think about the application of op-amps in audio generally. Negative feedback is used primarily to linearise the transfer function and is used in huge quantites as much as 80dB @ 1 kHz for example. Since this amount of NFB is not required to provide an accurate gain setting, it struck me that it's somewhat counter productive. If instead the open-loop transfer characteritic was made more linear by degeneration of the open-loop gain for example, when NFB is applied, the overall result should be largely similar (i.e. no worse) but would presumably also suffer less from the creation of these new distortion products . Comments ? That negative feedback linearizes the transfer function at the expensive of adding higher-order harmonics has been long-known. What you say is perfectly logical. However, the presence of higher-order harmonics is not the only factor, but their amplitude. Below a certain percentage (I'm sure Arny will be able to tell us what that is), they're inaudible. A good op amp can be used as a buffer and be sonically transparent, its output indistinguishable from its input. Even at reasonable gains, there are many that will perform well enough that nobody will hear the difference. Power amplifiers are the place where it gets very hard to keep distortion low at reasonable efficiencies. Consider how the sound got onto a CD or a slab of vinyl: microphones, preamps, mixers, equalizers, time synchronizers, echo adders, synthesizers, fake drums, distortion adders, digitizers. All this supervised by some egomaniac producer who has his own opinion about what sounds good and what the public wants to hear on whatever equipment they are likely use, like a Panasonic receiver or a boom box. 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. Ludicrous. John |
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Feedback in audio esp wrt op-amps.
On Aug 19, 11:39 am, 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. Mark |
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Feedback in audio esp wrt op-amps.
On Mon, 20 Aug 2007 09:32:21 -0700, Mark wrote:
On Aug 19, 11:39 am, 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. Mark The way poorly implemented overall feedback can increase the level of higher order harmonics is by permitting a marginal stability margin at the top end. This is usually a result of a misguided attempt to extract maximum possible bandwidth by not using a dominant pole at a low enough frequency. Instead of a smooth top-end roll-off you get a dip, then a rise. It is in the frequency range of this rise that the feedback is tending towards positive rather than negative, and can result in increased harmonic levels. Hopefully this (if it happens at all) is well beyond the audible range. d -- Pearce Consulting http://www.pearce.uk.com |
#37
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Feedback in audio esp wrt op-amps.
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. When I reviewed, I made final judgements with my own live, undoctored recordings. |
#38
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Feedback in audio esp wrt op-amps.
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. |
#39
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Feedback in audio esp wrt op-amps.
William Sommerwerck 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'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. 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. -- Kevin Aylward |
#40
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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. 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. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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