Is the maths done with music or with a sine wave?
For instance using the maths the PSRR of an SRPP is -6dB, but in reality it isn't, is it?
Richwalters:
I think then that the inverted correction signal is not affected in the same way as the original - perhaps because it's the inverse!
So if NFB works perfectly and corrects all of the non-linearities in an amplifier, how is it that all amplifiers have their own sonic signature?
Or do they all really sound the same and we're just chasing our tails?
LOL..
Nice one..we know they all sound the same..its just that some sound more like the same than others...
Regards
M. Gregg
No feedback loop can do a "perfect rescue job". My statement that the amp should be as linear as possible before closing the loop is true. If you can halve the open loop distortion, then the extra distortion added by feedback will be only a quarter of what it would otherwise be. Of course it is possible to develop an amp which is so pathological (e.g. non-monotonic in Vout vs. Vin) that feedback can't help, which is why it is usually stipulated that a quasi-linear approximation is used - this excludes pathological horrors but includes most real amps.
The maths is not done with music or a sine wave but an arbitrary signal. Distortion is anything in the output which is not in the input.
I have never measured the PSRR of an SRPP, but in the usual balanced configuration with both cathode resistors unbypassed it will be around -6dB. If the low cathode is bypassed then it will be better, maybe -9dB. Are you saying that valves and resistors do not obey circuit theory? If your measurement differs markedly from these figures then there is something wrong with your measurement technique.
The maths is not done with music or a sine wave but an arbitrary signal. Distortion is anything in the output which is not in the input.
I have never measured the PSRR of an SRPP, but in the usual balanced configuration with both cathode resistors unbypassed it will be around -6dB. If the low cathode is bypassed then it will be better, maybe -9dB. Are you saying that valves and resistors do not obey circuit theory? If your measurement differs markedly from these figures then there is something wrong with your measurement technique.
I don't agree or disagree with this,
However here is another take on the same feedback argument:
Go down to Negative Feedback
Lenard Audio - Education - Valve Amps
And so all amps sound the same except for phase shift caused by component distortion?
Regards
M. Gregg
However here is another take on the same feedback argument:
Go down to Negative Feedback
Lenard Audio - Education - Valve Amps
And so all amps sound the same except for phase shift caused by component distortion?
Regards
M. Gregg
I hate to be blunt, but this part of this webpage is a bunch of stuff that fell outta the end of a horse. Most of the rest is not bad at all. Just my opinion, of course.
Thanks,
Chris
Has anyone done any papers on NFB through a non-linear amplifier? The phrase 'reasonably linear' worries me as it implies a 'not notice' factor whereas I am after the best solution, not the usual boringly mediocre ones
The earliest that I've heard of was by Baxandall and the latest is by Bruno Putzeys in Linear Audio Vol.1.
"Reasonably linear" means that we can use a simplified equation. The exact equation isn't simple, but it does exist - feedback is still completely predictable.
All good fortune,
Chris
The early stuff is not too bad (only a few errors) but his ideas about NFB are complete nonsense. His claim that 12dB is about the max that can be used would be surprising to the Mullard designers of the 5-20 which uses around 30dB of feedback.M Gregg said:
What is it about feedback that seems to create such ignorance and nonsense?
DF96, I suspect a lack of math background.
I'm slowly working backwards through various levels I no longer remember (I just ordered a college Algebra book) until I can understand it. Then I'll work forwards again until either I once again understand FB, or I expire.
Either way that page on the site was nonsense.
I'm slowly working backwards through various levels I no longer remember (I just ordered a college Algebra book) until I can understand it. Then I'll work forwards again until either I once again understand FB, or I expire.
Either way that page on the site was nonsense.
This illustrates perfectly how theory can be wrong. The theory and maths you are leaning on here simply doesn't cover the reality of what goes on inside an amplifier.
Think of an SRPP (or any stage with the output at B/2) driver - powered off 400V swinging 200Vpp, centred at 200V. So you say, PSRR is -6dB. Well at 200V you'd be right, but you'd be wrong at 100V where the PSSR would be -12dB, and wrong at 300V it would be -2.5dB.
So in reality your steady -6dB PSRR of theory is a variable AM ripple (and PSU hash) dependent on were the signal happens to be, with (in the example) a difference of 12-2.5 = 9.5dB between the extremes.
I know it's small (usually smaller) and many say it doesn't matter, but is this hi-fi right?
Surely we can only claim hi-fi if the signal is passed unmolested wherever the music makes it go?I don't recall saying that - could you refresh my memory?DF96 said:
Questioning convention is how new discoveries are made - it's just science guys. My concerns about phase shift is that the corrective signal is out of phase with the original: mainly it's only a few degrees - but the ear's phase response is pretty good, and it is still out.... and if it's out, it can't work perfectly as it relies on correcting the signal, not a phase shifted version of it.
Using Thorsten Loesch's ideas about the futility of GNFB over for instance the transformer (that many people disagree with) my SE amp now sounds fantastic, but also brutally powerful and tight, and clear even with complex music - without any GNFB at all, perhaps that's why I always question the line that GNFB is always needed and always the best solution
BTW: Judging by some comments on here we are almost reaching the religious fervour of the high priests of global warming, it's just a discussion to gain ideas guys
I know it's small (usually smaller) and many say it doesn't matter, but is this hi-fi right?
I don't recall saying that - could you refresh my memory?
Questioning convention is how new discoveries are made - it's just science guys. My concerns about phase shift is that the corrective signal is out of phase with the original: mainly it's only a few degrees - but the ear's phase response is pretty good, and it is still out.... and if it's out, it can't work perfectly as it relies on correcting the signal, not a phase shifted version of it.
Using Thorsten Loesch's ideas about the futility of GNFB over for instance the transformer (that many people disagree with) my SE amp now sounds fantastic, but also brutally powerful and tight, and clear even with complex music - without any GNFB at all, perhaps that's why I always question the line that GNFB is always needed and always the best solution
BTW: Judging by some comments on here we are almost reaching the religious fervour of the high priests of global warming, it's just a discussion to gain ideas guys
Two thumbs up!! Me thinks GNFB improves the measurements perhaps but not the listening enjoyment.
Last edited:
You are using a different, possibly personal, definition of PSRR. By PSRR I mean (and I believe others mean) the degree to which the output of a stage rejects signals on the supply rails. -6dB means the output noise/ripple is half the voltage of the noise/ripple on the supply rail. How this relates to likely signal levels is a separate issue. To calculate PSRR you just need to know how to calculate the circuit. Your figure, more related to signal-noise ratio, also requires knowledge of the PSU noise and the signal level. Your figure is useful, but it isn't PSRR.Globulator said:
This illustrates that there is more to measurement than measuring: you also have to correctly handle your raw data.
As I have said, NFB cannot achieve perfection. NFB is always roughly in phase with the input signal, even when the amp imposes nearly 90 degrees of phase shift. Otherwise it ceases to be NFB - this is what happens at frequency extremes and is how instability occurs. Either do the maths, or draw yourself a diagram of signal vectors. Failing to grasp basic theory is not how science proceeds.
It's only a separate issue if you listen to the amp while the input is grounded.
As soon as you start listening to music is becomes a PSRR issue, and as I tend to use my amps for music it is something I must consider! BTW it is not a level issue, it's a voltage divider issue.
The problem with PSRR theories and formulas is that they are incomplete simplifications of the real physics (or even the simulation) inside the amplifier. The same is true of GNFB.
Simplifications, approximations and rules of thumb can only take you so far IME.
I am not talking about listening to the amp, but calculating what you might hear when you do listen. PSRR is Power Supply Rejection Ratio - the ratio by which the power supply noise is rejected. You can refer the result to the input or the output, but output-referred PSRR avoids having to calculate gain too.
It is you that made it a level issue, but introducing signal level. I was talking about a ratio, calculated from a voltage divider.
All calculations are necessarily approximations, but an approximate value can be better than total ignorance. Anyway, perhaps you can tell me what is wrong with the usual calculation for SRPP PSRR? Is there some flaw in the derivation? What aspect of "real physics" has been overlooked?
It is you that made it a level issue, but introducing signal level. I was talking about a ratio, calculated from a voltage divider.
All calculations are necessarily approximations, but an approximate value can be better than total ignorance. Anyway, perhaps you can tell me what is wrong with the usual calculation for SRPP PSRR? Is there some flaw in the derivation? What aspect of "real physics" has been overlooked?
Ah - at last we are talking about the same thing!!
The simplistic PSRR calculation uses a constant ratio, which assumes that the voltage divider is fixed.
It isn't.
It is, for the usual definition of PSRR. If you want to see how PSRR varies with signal level then that can be done, but that is another issue. Don't change the subject.
You may, of course, have your own private definition of the meaning of PSRR but don't then complain that other people's calculations, based on the normal definition, are wrong.
You may, of course, have your own private definition of the meaning of PSRR but don't then complain that other people's calculations, based on the normal definition, are wrong.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.