I think one listen speaks a 1000 words. Try it with your amp.
It's one thing talking about distortion and all, it's another thing to listen to it.
Is low output impedance that important? Speaker is reactive, you really get flat response even you have absolute 0ohm output impedance? There is a school that design amps that have relative higher output impedance. Tube amps being one.
Is zero distortion really important? I listened to Krell and Mac, I really not impressed. I favorite is YBA, looking at schematic, it's a distortion master.
Speaker is the one single piece in the system that define the sound. Then your room. Given these, you match your amp to the speakers and the home you have. Try and listen to it to decide. It's not the spec, it's the sound.
It's one thing talking about distortion and all, it's another thing to listen to it.
Is low output impedance that important? Speaker is reactive, you really get flat response even you have absolute 0ohm output impedance? There is a school that design amps that have relative higher output impedance. Tube amps being one.
Is zero distortion really important? I listened to Krell and Mac, I really not impressed. I favorite is YBA, looking at schematic, it's a distortion master.
Speaker is the one single piece in the system that define the sound. Then your room. Given these, you match your amp to the speakers and the home you have. Try and listen to it to decide. It's not the spec, it's the sound.
One issue that would worry me is the speaker EMF wreaking havoc when there's no NFB to keep it in check.
http://linearaudionet.solide-ict.nl/sites/linearaudio.net/files/volume1bp.pdf
Jan
http://linearaudionet.solide-ict.nl/sites/linearaudio.net/files/volume1bp.pdf
Jan
Lots of input from people who view 'GNFB' as yet another black boax as if the effects had nothing to do with the topology (and hence, in some sense technology, as it's not equally easy and indeed sometimes not even possible to make completely equivalent circuits using semiconductors and tubes, also not even specific types of semis and tubes in some cases). Lots of oversimplifications lead to incorrect modeling and results.
An example - amp dominated by input stage gm. Really? As if there is only one possible type of output stage, capable of only one interdependence with the actual load. Not to mention mixing up things like single ended and class A, follower type and gain type output stages (or for that matter transformer coupled output stages). How GNFB will behave and what it will do varies GREATLY depending on where and how it's applied (with caveats that there are quite a lot of cases in which certain combinations of where and how can be shown to give bad results just by using common sense).
GNFB - the usual basic stuff it does always applies - lowers distortion and improves linearity and sensitivity to parts tolerances, at the expense of gain. BUT that is not the only expense. In particular, our ear-brain combo does not perform linearity analysis according to classical math, in stead it has evolved to tolerate some non-linearities more than others, also be fairly insensitive to some and very sensitive to others. A mathematically 'more linear' transfer function does not by any stretch necessarily translate to being perceived as more 'natural sounding.
Here are some 'rules of thumb' that I have collected during the years from experience of designing and restoring amplifiers:
1) Generally, you want your amplifier to be as linear as possible before feedback is applied.
2) The less linear the amp is without feedback, the more excess gain and bandwidth is required for it to sound good once GNFB is applied. Note I said gain and bandwidth, not gain-bandwidth product.
3) The more stages the GNFB loop contains, the more problems you can expect.
4) Do not even try to fix time delay/hysteresis problems with GNFB, in other words, avoid putting gnfb loops around, for instance, transformers or any circuit that has non-amplification related time constants, such as blocking distortion, sticky clipping and other dead zones in the transfer function like excessive crossover distortion, ESPECIALLY if the actual manifestation of dead zones depends heavily on the loading of the stage (mot notably output stages suffer from this). In some cases like output protection schemes you can't avoid this but then have to thoroughly test for stability when these cases happen, the good thing being one does not expect the amp to continue providing signal fidelity, just be able to get back to it when the error condition is removed.
5) There can be a definite NEGATIVE sum effect when applying small amounts of feedback to fairly nonlinear circuits. For instance, if a stage produces fairly large but nicely distributed levels of low order harmonics, and a few of them, the ear will tolerate or even suppress them, whereas applying GNFB in small amounts around such a stage will produce harmonics of harmonics, which can indeed sum to a higher distortion leve even though the original harmonics can be suppressed, and even worse, upset their distribution in such a way that the ear will not be able to mask them. Consider a stage that makes only 2nd and 3rd harmonics. Small levels of GNFB will produce 2nd, 3rd, 4th, 6th, 9th etc, where the 2nd and 3rd will be suppressed by GNFB in some amount, not at alll necesairly both by the same amount, which can sometimes result in changing which of them is dominant. Conspicuously, some harmonics will be missing almost entirely, where the ear expects a falling series of harmonics, either all of them or even and odd falling separately but by a similar basic monotonous rule. This sort of situation is not what you find in nature and hence not masked. This is a very simplistic example, much worse things happen with intermodulation, which is FAR more significant where perception of 'naturally sounding' is concerned.
6) Be very careful incorporating 'filtering elements' - high or low pass - in feedback loops especially if the fiter corner frequency depends on input or output impedances connected to the amp that cannot be controlled for by the designer (this problem most often happens in tube amps). There may be unwanted behavior inside the loop that you will not see after an output filter (i.e. coupling cap). This can seriously worsen cases where the action of the 'filter' is still felt at the edges of the audio band - especially on the high side (but also can lead to some strange effect in bass reproduction on the low side). While the ear's sensitivity to sound falls drastically in the top octave of the audio band, experiments with location and reverb show that our ear]b]S[/b]-brain system (not ear as in single!) can discriminate delays between the left and right ear with a precision greater than the reciprocal of the higher frequency we can hear suggests, so this is something that should be carefully taken care of when you need two identical amps such as for stereo reproduction.
7) If you can avoid giving NFB extra correction jobs to do, avoid it - for instance, rather than rely on GNFB to sort out your PSRR problems, employ separate measures for this in the circuit topology or by adding filtered or stabilized power rails where appropriate.
An example - amp dominated by input stage gm. Really? As if there is only one possible type of output stage, capable of only one interdependence with the actual load. Not to mention mixing up things like single ended and class A, follower type and gain type output stages (or for that matter transformer coupled output stages). How GNFB will behave and what it will do varies GREATLY depending on where and how it's applied (with caveats that there are quite a lot of cases in which certain combinations of where and how can be shown to give bad results just by using common sense).
GNFB - the usual basic stuff it does always applies - lowers distortion and improves linearity and sensitivity to parts tolerances, at the expense of gain. BUT that is not the only expense. In particular, our ear-brain combo does not perform linearity analysis according to classical math, in stead it has evolved to tolerate some non-linearities more than others, also be fairly insensitive to some and very sensitive to others. A mathematically 'more linear' transfer function does not by any stretch necessarily translate to being perceived as more 'natural sounding.
Here are some 'rules of thumb' that I have collected during the years from experience of designing and restoring amplifiers:
1) Generally, you want your amplifier to be as linear as possible before feedback is applied.
2) The less linear the amp is without feedback, the more excess gain and bandwidth is required for it to sound good once GNFB is applied. Note I said gain and bandwidth, not gain-bandwidth product.
3) The more stages the GNFB loop contains, the more problems you can expect.
4) Do not even try to fix time delay/hysteresis problems with GNFB, in other words, avoid putting gnfb loops around, for instance, transformers or any circuit that has non-amplification related time constants, such as blocking distortion, sticky clipping and other dead zones in the transfer function like excessive crossover distortion, ESPECIALLY if the actual manifestation of dead zones depends heavily on the loading of the stage (mot notably output stages suffer from this). In some cases like output protection schemes you can't avoid this but then have to thoroughly test for stability when these cases happen, the good thing being one does not expect the amp to continue providing signal fidelity, just be able to get back to it when the error condition is removed.
5) There can be a definite NEGATIVE sum effect when applying small amounts of feedback to fairly nonlinear circuits. For instance, if a stage produces fairly large but nicely distributed levels of low order harmonics, and a few of them, the ear will tolerate or even suppress them, whereas applying GNFB in small amounts around such a stage will produce harmonics of harmonics, which can indeed sum to a higher distortion leve even though the original harmonics can be suppressed, and even worse, upset their distribution in such a way that the ear will not be able to mask them. Consider a stage that makes only 2nd and 3rd harmonics. Small levels of GNFB will produce 2nd, 3rd, 4th, 6th, 9th etc, where the 2nd and 3rd will be suppressed by GNFB in some amount, not at alll necesairly both by the same amount, which can sometimes result in changing which of them is dominant. Conspicuously, some harmonics will be missing almost entirely, where the ear expects a falling series of harmonics, either all of them or even and odd falling separately but by a similar basic monotonous rule. This sort of situation is not what you find in nature and hence not masked. This is a very simplistic example, much worse things happen with intermodulation, which is FAR more significant where perception of 'naturally sounding' is concerned.
6) Be very careful incorporating 'filtering elements' - high or low pass - in feedback loops especially if the fiter corner frequency depends on input or output impedances connected to the amp that cannot be controlled for by the designer (this problem most often happens in tube amps). There may be unwanted behavior inside the loop that you will not see after an output filter (i.e. coupling cap). This can seriously worsen cases where the action of the 'filter' is still felt at the edges of the audio band - especially on the high side (but also can lead to some strange effect in bass reproduction on the low side). While the ear's sensitivity to sound falls drastically in the top octave of the audio band, experiments with location and reverb show that our ear]b]S[/b]-brain system (not ear as in single!) can discriminate delays between the left and right ear with a precision greater than the reciprocal of the higher frequency we can hear suggests, so this is something that should be carefully taken care of when you need two identical amps such as for stereo reproduction.
7) If you can avoid giving NFB extra correction jobs to do, avoid it - for instance, rather than rely on GNFB to sort out your PSRR problems, employ separate measures for this in the circuit topology or by adding filtered or stabilized power rails where appropriate.
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Short version: you're looking for an effects box. Which is fine as long as you understand that this is the goal. That's a different design path than something whose output is indistinguishable from its input.
That would be difficult to arrange. As you vary the feedback you have to adjust the frequency compensation too, so that stability is maintained. As I said, feedback cannot be simply added or removed; it has to be designed in or designed out.Hiten said:
I would certainly hope so! Just the basic effects of feedback (less distortion, flatter frequency response) would "change the sound" from more distortion and earlier rolloff.AKSA said:
Perhaps for some, maybe including you. Others want accurate amplification, and are prepared to put up with the sound; this is exactly what happens at a live acoustic event. When I go to a concert I don't want to have tone controls (variable 'knob' based or inflexible 'tube/chip rolling' based) or 'niceness' controls to fiddle with - I just accept what the musicians produce.ivanlukic said:
Things are much simpler than that. Some people want to reproduce music. Others want to produce a sound they enjoy. Nothing wrong with that, except when the latter people start claiming that their preferred distortion is somehow more accurate.
Oh dear. It's absolute drivel.
Luckily I can just reach for my Self or Cordell books, or Putzeys 'The 'F' Word' for a dose of reality.
That's the whole point. Ultimately, these amps are for listening, not for measuring. I am new, I have no idea what is important. So choose to have low distortion wide band for my first amp. So 5 pairs output, 3EF, higher bias, complementary IPS, darlington push pull VAS...........All about low crossover distortion and all. I plan to try a Hiraga which has higher output impedance and CFA next. Then I definitely going to design a tube hifi amp. Hopefully by that time, I will have a much better understanding.
I know this is an amp forum here, BUT really, the sound is totally governed by the speaker, which is the single most non linear element of the the system. People really need to start by first identify a pair of speaker to work with, then choose the amp. I also question people here that talk about amps what kind of speakers they have. You can design an amp better than Krell, Threshold, but if you only have a pair of Kef, Klipch etal, it's unlikely it's going to make that big a difference. Even if you have the top line Sonus fabus, Wilson or JMLab type of high end stuffs, it still depends on the characteristics. If you have a panel speaker, it's going to be a different ball game.
I heard arguments between feedback or no feedback, tubes or SS. I heard it all. Ultimately, it's your own preference.
I don't know what people that design amps to sell use as standard, but I sure would strongly advice hobbyist that want to improve the sound of their own system like me to first make sure your speaker is up to par. Go listen in the store first before diving into this designing amps. Spend the money on the speaker first, then worry about the amp. I am designing the amp because I had the chance to compare my own Acurus with a YBA amp when I was buying the speaker, that I know how much better my speaker will sound with the right amp.
If you insist on building an amp, bring your amp to the store to compare, or better yet, some stores let you check out an amp for a day to take home and try. Find one you like, then come here to look for schematic to build.
I heard arguments between feedback or no feedback, tubes or SS. I heard it all. Ultimately, it's your own preference.
I don't know what people that design amps to sell use as standard, but I sure would strongly advice hobbyist that want to improve the sound of their own system like me to first make sure your speaker is up to par. Go listen in the store first before diving into this designing amps. Spend the money on the speaker first, then worry about the amp. I am designing the amp because I had the chance to compare my own Acurus with a YBA amp when I was buying the speaker, that I know how much better my speaker will sound with the right amp.
If you insist on building an amp, bring your amp to the store to compare, or better yet, some stores let you check out an amp for a day to take home and try. Find one you like, then come here to look for schematic to build.
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Current drive for speakers will not work unless you fundamentally change the way speaker drivers work, or the physics in how mass spring systems behave under resonance.
Current conveyer blocks for amplification works quite well. Dadod has a gainblock with passiv IV and good specs
Current conveyer blocks for amplification works quite well. Dadod has a gainblock with passiv IV and good specs
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Why try and make this a black and white discussion and put everyone in one box or the other? No product is designed without real-world testing to ensure it delivers what it is designed to do. An amplifier is intended to be part of a reproduction system for the enjoyment of music. If an amplifier design sounds better when it has less feedback yet its standard measurements are worse, why would anyone want to buy or listen to the version with better measurements but worse sound? Surely the version that sounds best is better suited to its purpose?
Obviously the problem is working out objectively what sounds better. Usually the folks who insist that an amplifier must be measurably accurate to be any good aren't interested in listening for differences, they are convinced that the test gear is a better judge than their own ears at what sounds good. I both envy and pity them.
I've read a lot of amplifier reviews lately in the top UK and US Hifi mags, and if the descriptions are accurate, the majority of top rated amps do not use global feedback. Maybe this in itself is an answer to Hugh's original question?
...first specifying what the target is. It has nothing to do with boxes or whatever, the question is do you want the amplifier to be audibly transparent or not? If the former, then there's specific sets of design rules. If the latter, you have to then figure out what alterations you're looking for and design for them. Otherwise, you're just doing stuff at random and hoping that the monkeys really will type out Shakespeare.
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