Hi,
I have a couple of questions regarding DC/AC coupled amplifiers:
1) Is it true that removing the NFB capacitor results in a less dynamic sound, or is it a mith? Will an AC coupled amp have more energy due to the caps and therefore more dynamics?
2) Is it possible to make the NFB capacitor 'invisible' by choosing the right time constants for input and NFB RC filters? I tried this with input RC = 90 ms and NFB RC = 150 ms but I think I still can hear the cap! It's not like distortion but lesser resolution with the cap in place. DC coupled I believe I can hear more details and a more precise sound. Am I hearing imaginary things?
Thank you.
I have a couple of questions regarding DC/AC coupled amplifiers:
1) Is it true that removing the NFB capacitor results in a less dynamic sound, or is it a mith? Will an AC coupled amp have more energy due to the caps and therefore more dynamics?
2) Is it possible to make the NFB capacitor 'invisible' by choosing the right time constants for input and NFB RC filters? I tried this with input RC = 90 ms and NFB RC = 150 ms but I think I still can hear the cap! It's not like distortion but lesser resolution with the cap in place. DC coupled I believe I can hear more details and a more precise sound. Am I hearing imaginary things?
Thank you.
Hi,
No you are not hearing things in your imagination. Less resolution is what everyone hears about caps in the signal path. But you might not be able to just remove it because it is eliminating a dc offset on your output. You would have to add a dc servo to keep the output having no dc.
-RNM
No you are not hearing things in your imagination. Less resolution is what everyone hears about caps in the signal path. But you might not be able to just remove it because it is eliminating a dc offset on your output. You would have to add a dc servo to keep the output having no dc.
-RNM
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Joined 2009
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A servo is another place for caps to hide, so is the power supply.
It's like dreaming of a world with no bacteria, all clean. It turns out that it isn't possible nor desirable (we wouldn't survive without them).
I've generally found no issue having caps in my amplifiers regarding the sound. Only once did I 'hear' a cap sound 'bad' and that was after putting a cheapo electro on the output of a dual-rail amplifier just to find out.
It's like dreaming of a world with no bacteria, all clean. It turns out that it isn't possible nor desirable (we wouldn't survive without them).
I've generally found no issue having caps in my amplifiers regarding the sound. Only once did I 'hear' a cap sound 'bad' and that was after putting a cheapo electro on the output of a dual-rail amplifier just to find out.
I realise a servo has its issues but I suppose a properly design one has less influence in the sound than a capacitor. My preamplifier has servos and sounds very good.
I also realise we always have a great deal of caps in a power amp and they are good thing and unavoidable. I love PSU with lots of them! 😉
What I want to find out is if it's possible to have a NFB capacitor with close no none influence in the sound. I'm disappointed that adjusting the RC constants didn't make the capacitor disappear. Maybe if I use a really big value?
Other thing I'd like to find out is if DC coupling the amplifier kills the dynamics? A trade off between dynamics (AC coupled) and fine detail (DC coupled)? My listening tests weren't conclusive about this...
I also realise we always have a great deal of caps in a power amp and they are good thing and unavoidable. I love PSU with lots of them! 😉
What I want to find out is if it's possible to have a NFB capacitor with close no none influence in the sound. I'm disappointed that adjusting the RC constants didn't make the capacitor disappear. Maybe if I use a really big value?
Other thing I'd like to find out is if DC coupling the amplifier kills the dynamics? A trade off between dynamics (AC coupled) and fine detail (DC coupled)? My listening tests weren't conclusive about this...
signal coupling Electrolytics do have measurable distortion - but still low order, low levels compared to most dynamic loudspeakers
but I am skeptical of too simple a subjective correlation - particularly if not demonstrated double blind with level matching, ability for fast switching - not by "memory" with a soldering iron in your hand - such uncontrolled "just listen" "tests" are simply useless for informing engineering except in the grossest errors of % high distortion or dB frequency response changes
read Sy's Testing article at linear audio http://www.linearaudio.net/index.php?option=com_content&view=category&layout=blog&id=39&Itemid=64
but I am skeptical of too simple a subjective correlation - particularly if not demonstrated double blind with level matching, ability for fast switching - not by "memory" with a soldering iron in your hand - such uncontrolled "just listen" "tests" are simply useless for informing engineering except in the grossest errors of % high distortion or dB frequency response changes
read Sy's Testing article at linear audio http://www.linearaudio.net/index.php?option=com_content&view=category&layout=blog&id=39&Itemid=64
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That's interesting reading, thanks for the links jcx 🙂
I'm aware of the importance of blind tests, that's why I come to the forum asking if I'm imagining things. And if I'm not, then I'd like to know the objective explication for the differences in sound.
I believe the difference between cap and no cap is real even without a proper blind test.
Dynamics, I'm not so sure. It's here I should do a proper blind test. Is there any objective reason for a fully DC coupled amp have lesser dynamics than it's AC coupled version?
I'm aware of the importance of blind tests, that's why I come to the forum asking if I'm imagining things. And if I'm not, then I'd like to know the objective explication for the differences in sound.
I believe the difference between cap and no cap is real even without a proper blind test.
Dynamics, I'm not so sure. It's here I should do a proper blind test. Is there any objective reason for a fully DC coupled amp have lesser dynamics than it's AC coupled version?
😀 Humans pretty well All harbor 'beliefs', often derived only from anecdotes and or teachings...typically by those with an agenda.
Faith indeed moves mountains.
Factual or even credible is rarely a requirement 😉.
Witness the world around us
Faith indeed moves mountains.
Factual or even credible is rarely a requirement 😉.
Witness the world around us
Don't be fooled into thinking that because a cap is inside the signal path or feedback loop rather than beside, it is more important than the one(s) that shapes your DC servo: in order to eliminate DC or VLF, you have to use a number of poles, and lousy pole components have more or less the same kind of effect, wherever they are located.
The ones in the DC servo will come with a vengeance: some of their undesirable features will be amplified, and the nasties from the servo will also be coupled in some way, even if they are attenuated (by regular passives, of course).
There is no magic bullet: DC servoes are convenient but they do nothing for the quality of reproduction, quite the opposite in fact. Which is probably why they are sonically preferred to simple passives: it is similar to the tube sound effect. The result is more colourful, less boring...
If you want DC-servo advantages without the opamp and its "features", you can use multiple constants feedbacks.
In this example (using opamp rather than PA for convenience), the settling time would be governed by the time constant of R1 and C1, in this case 100 seconds, which would be absolutely unbearable for practical purposes.
A second, much faster loop is added, but it is normally isolated from the regular FB path by back to back diodes.
Here, the time constant is only 470ms, which allows for a shortened stabilizing time.
The sim shows the situation towards the end of the stabilizing process: the current through the diodes is progressively decreasing to ~0. The signal is a maximum amplitude 10Hz waveform, normally near the maximum possible stress.
This is just an example, the constants can be multiple and adapted, etc.
At least, here you see clearly where the non-linearities are susceptible to come from.
For the rest, I can only recommend you make proper A-B-X tests: you will probably find that most of the folk lore is unnecessary, and simplify your life as a consequence...
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Hang on a second.
The OP asked about DC coupled vs. AC coupled in the title and then went on to talk about 1) "NFB capacitor" and 2) making this cap "invisible" via other means.
Presumably he is talking about the (usually electrolytic) cap at the inverting input to the typical diff pair at the input of the amp? The purpose of which is to raise the NFB at very low freqs so that at DC the amp has unity gain, thus preventing excess gain causing DC offset...
There is the "other" NFB cap that might be there to drop the HF gain down to unity at some frequency, aka "stability cap".
So which one are we talking about?
Or are we talking about a cap at the input to ground, or a cap in series with the input (DC blocking)?
How about a cap in series with the output?
Also there are and have been amps designed and built that are DC stable without the use of a servo or a DC blocking cap at the input.
Thought it might be good to clear up which caps are being adjusted to do what?
Imho, "dynamics" are a subjective perception that can't be related directly to any one or combination of factors. It's more of a system wide subjective impression, also known as "jump factor" and "slam". Otoh there are all sorts of things that you can do to a given amp that may alter the perceived subjective impression that may or may not be difficult to measure, but even having been measured may not be correlated back to be useful to alter the sound of another amp at another time, or even be clear as to why or how that particular amp appeared to change character.
The OP asked about DC coupled vs. AC coupled in the title and then went on to talk about 1) "NFB capacitor" and 2) making this cap "invisible" via other means.
Presumably he is talking about the (usually electrolytic) cap at the inverting input to the typical diff pair at the input of the amp? The purpose of which is to raise the NFB at very low freqs so that at DC the amp has unity gain, thus preventing excess gain causing DC offset...
There is the "other" NFB cap that might be there to drop the HF gain down to unity at some frequency, aka "stability cap".
So which one are we talking about?
Or are we talking about a cap at the input to ground, or a cap in series with the input (DC blocking)?
How about a cap in series with the output?
Also there are and have been amps designed and built that are DC stable without the use of a servo or a DC blocking cap at the input.
Thought it might be good to clear up which caps are being adjusted to do what?
Imho, "dynamics" are a subjective perception that can't be related directly to any one or combination of factors. It's more of a system wide subjective impression, also known as "jump factor" and "slam". Otoh there are all sorts of things that you can do to a given amp that may alter the perceived subjective impression that may or may not be difficult to measure, but even having been measured may not be correlated back to be useful to alter the sound of another amp at another time, or even be clear as to why or how that particular amp appeared to change character.
May I safely conclude that there's no objective evidence that DC coupling an amplifier results in lesser dynamics?
Thank you all for your answers.
Thank you all for your answers.
That's a safe conclusion.
Objectively measuring the dynamics will prove that changing the NFB DC blocking cap to a short circuit produces no change in the dynamics, when the AC impedance of the DC blocking cap is virtually zero at all the frequencies that the amplifier is designed to handle.
That is quite different from proving that there is no subjective difference between cap and no cap in the NFB DC blocking position.
This second is almost impossible to disprove.
Does that help with proving that removing the DC blocking cap improves the measurable performance of the amplifier?
Thank you, Andrew! 🙂
It goes in line with my subjective perception, that dynamics stay the same. I've been told otherwise, that dynamics are worst without the NFB capacitor. I'me happy to know there's objective support to my perception. 🙂
It goes in line with my subjective perception, that dynamics stay the same. I've been told otherwise, that dynamics are worst without the NFB capacitor. I'me happy to know there's objective support to my perception. 🙂
In my understanding, it comes back to ensuring the signals applied to the amplifier can be properly handled by the amplifier.
This is taken care of by the passive input filtering.
What gets past the input filter must not cause the amplifier to misbehave.
That then results in good sound quality.
This is taken care of by the passive input filtering.
What gets past the input filter must not cause the amplifier to misbehave.
That then results in good sound quality.
Technically, if it is a low distortion amplifier that means that the amp produces very little different than the input, just raised in amplitude at the output. So, there ought to be no difference no matter if it is AC or DC coupled.
Differences happen due to other factors in real world situations, including how a given amp handles the reactive elements of the speaker load, and how the amp behaves at and near clipping, as well as the subjective sonic perception of the spectra of harmonic distortion (assuming it is audible). The power supply plays a role, since that's what we are actually listening to. 😀
But why a given amp sounds subjectively better, or even different, on a given speaker than another, when using reasonably well built and designed amps? Don't think anyone has a clue on that. Not yet.
Differences happen due to other factors in real world situations, including how a given amp handles the reactive elements of the speaker load, and how the amp behaves at and near clipping, as well as the subjective sonic perception of the spectra of harmonic distortion (assuming it is audible). The power supply plays a role, since that's what we are actually listening to. 😀
But why a given amp sounds subjectively better, or even different, on a given speaker than another, when using reasonably well built and designed amps? Don't think anyone has a clue on that. Not yet.
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