-db volume meaning

Discussion in 'Archived Threads 2001-2004' started by Michael Varacin, Aug 19, 2002.

  1. Michael Varacin

    Michael Varacin Stunt Coordinator

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    Why do most high end recievers list the volume as -db? What is the 0 decibal level based on?


    Michael.
     
  2. JohnGoggan

    JohnGoggan Stunt Coordinator

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    I asked this as a newbie a few days ago myself -- was confusing me also. [​IMG]
    From what I understand, some receivers allow you to set a specific volume as "0" -- and then you adjust from there. In general, people set the "reference volume" to be 0 on their receiver -- so that they know exactly where it is all the time (and then generally listen below reference volume -- so in the -db range).
    See the FAQ on calibrating your setup to find the reference volume level (and exactly what it is) for more details.
    - John...
     
  3. Vince Maskeeper

    Vince Maskeeper Producer

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    It depends on the unit, but I can give a basic background.

    The idea of 0, for "Full Scale" would simply mean that this is the full potential level being passed from the source to the amplifier, without additional boost or cut at the preamp stage.

    It is a gain scale for audio. If you would like to think of it in a percentage scale: 0db means 100%.

    In a preamp device, any setting over this 0db number means it's boosting the relative level of the signal, everything below means it is cutting the relative level of the signal.

    Everything in the audio world is measured on a similar 0 DB "full scale" scale. 0 meaning 100% of the potential.

    -vince
     
  4. Wayne A. Pflughaupt

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  5. Vince Maskeeper

    Vince Maskeeper Producer

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    Indeed- however I didn't directly state this in my post because it often prompts an additional question from those unfamiliar: "If 0 is maximum gain- how come my receiver can got to +6 or +10?"
    So, it is often a situation where the explanation of the concept of signal flow comes into play. The best basic explaination is that the level of "output" (SPL) from the system is the combination of:
    Level Of Input > Level of Gain At Preamp Stage > Amplifier Output.
    A reduction in the level of input to the preamp (say a DVD or CD which is mastered at a lower level overall), will obviously then result in a lower output. Thus, some receivers have the ability to push their preamp stage a little harder, to "overdrive" lower level inputs. An input coming in at 80% can be boosted to 100% by pushing the preamp volume over 0.
    But this idea of 0db level = 100% of potential propigates throughout the signal chain. If you realize that each element in the signal chain operates on this same principle- you begin to see that it serves as a universal scale by which to work with audio signal.
    Here's a rough signal flow example, simplified:
    A 0db signal from your source (100%), passed through a 0db setting at your preamp (100%) will be passed into the amplifier stage at its maximum expected level (100%), and thus passed out its output at the maximum output wattage (100%).
    A 80% signal from your source, passed through a 0db setting at your preamp (100%) will be passed into the amplifier stage at 80%, and thus passed as output at 80%.
    A 80% signal from your source, passed through a 50% setting at your preamp will be reduced by half and passed into the amplifier stage at 40% (half of the orginal 80% from the source), and thus passed out the amplifier at 40% of maximum wattage.
    So, long story short- this internal db scale is a measure of percentage of maximum-- with the 0 level being the 100%maximum gain (as Wayne suggested above).
    -Vince
     
  6. Kevin*R

    Kevin*R Agent

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    Volume selector dB's are based on the idea of a normalization of the selected magnitude, that's to say db's are proportional to "selected magnitude"/"max magnitude". Since mathematically log10(1) = 0dB we know full volume will always be 0dB. The beauty here is the receiver wattage falls out of the euation.
     
  7. John Kotches

    John Kotches Cinematographer

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    Ummm guys, you're overlooking a few pertinent details here.
    First off, for THX equipment, assuming calibration with a -30dB FS signal, 0dB means you are playing back at reference level. Many (if not all) components with THX badging allow for levels above reference. This would be a relative volume scale. It's relative to a predefined and calibrated level.
    If however, your product is on an absolute scale, than 0dB is maximum output. This is how some gear displays output.
    It is a serious misconception to say that 0dB on an absolute scaled product equates to full output without clipping. That is a function of numerous details, including speaker sensitivity, range of channel trims needed for even speaker level calibration, impedance of speakers and the amplifier (or amplifier sections) ability to deliver current into the load.
    Regards,
     
  8. Wayne A. Pflughaupt

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    Vince, you wanna take this one? [​IMG]
     
  9. John Kotches

    John Kotches Cinematographer

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    Wayne,

    Not to be rude, but there's nothing for Vince to "take" here.

    THX products with relative volume scales can (and do) exceed 0dB, since this only signifies level relative to reference, not an absolute output level. Check out something like B&Ks Reference 30, an Anthem AVM-20 as examples of preamps which have a maximum output level of +10dB as measured vs Reference.

    OTOH, some products utilize absolute scales vs. their maximum outputs, such as the Sunfire Theater Grand-III, Integra DTR-6.3, and the Sherwood Newcastle R-963 as examples where 0dB signifies maximum output.

    Regards,
     
  10. Kevin*R

    Kevin*R Agent

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    No sweat, for THX change the denominator of the equation to use reference voltage instead of max voltage. For sanity however the reference had better remain constant for the dB comparisons to be relative.
    In the end 0dB still results in a unity ratio. [​IMG]
     
  11. John Kotches

    John Kotches Cinematographer

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    Kevin,

    Yes, one has to assume that the THX logo gives us at least that, so that -3dB is 1/2 reference voltage, -10dB is 1/10th referene voltage etc, etc, etc.

    There will always be some small margin for error, depending on accuracy of tracking of the volume control itself.

    Regards,
     
  12. Vince Maskeeper

    Vince Maskeeper Producer

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    John- I'm unsure if I have misunderstood some of your posts, as some of the points seem to be contridictory: depending on how certain terms are read, I could take it to either agree or disagree with my points at various passages. Nonetheless, I have done my best to respond to what I interpreted to be your meanings- these are not necessarily directed at you, but rather using your quoted passages in an attempt to clarify what both you and I are saying. If I have misread or misunderstood any of your passages, I apologize in advance- and I would urge you to clarify your point (as it's obvious from Wayne's reply above that maybe more than one of us misunderstood what you're trying to say).


    First off, for THX equipment, assuming calibration with a -30dB FS signal, 0dB means you are playing back at reference level. Many (if not all) components with THX badging allow for levels above reference. This would be a relative volume scale. It's relative to a predefined and calibrated level.


    This is relatively misleading, and depending on how you meant some of the terms, it could be considered false. The 0db position, THX or no, doesn't necessarily mean "ref output", unless specifically calibrated to be so. The volume position in relation to output are not directly related-- or to say it a better way, one can not always determine one from the other. Zero position can equal ref if you calibrate that way- (and, as long as you measured output correctly, you could reach that calibration using -20dbfs or even -10dbfs tones, not only -30dbfs)--- but this 0 position could just as easily NOT equate to ref ouput level:


    As I'm known to use, the prime example of this is to imagine your current home theater system in madison square garden. 0 on your volume scale might equate to ref output when calibrated in your home, but will mean something quite different when asked to power a larger space like the Garden.

    0 will likely not equal ref level using your system in Madison Square Garden, but 0db will still mean 100% gain in terms of the preamp stage... even if this 100% gain flowing from preamp to amp cannot provide you with 105db SPL peak in Madison Square Garden


    So, to say that 0 = "Ref Level" is misleading. What's worse, assuming a -30dbfs test signal would not this statement any more or less accurate. If you use a -30dbfs signal input, ref level is wherever you get 75db of measured SPL output. If you use a -20dbfs signal input, ref level is wherever you get 85db of measured SPL output. If you use a -10dbfs signal input, ref level is wherever you get 95db of output. If you use a 0dbfs signal input, ref level is wherever you get 105db of measured SPL output.

    Regardless of the level of ref tone, and regardless of THX certification- if you calibrate your system to reach 105db peak output with the volume knob at the 0 position, then that will be ref for you. However- you could have easily calibrated it to hit 105db output at a volume position of -7 or +2 or at a volume knob position you refer to as "home base"- the name is absolutely meaningless, as long as the output is calibrated using proper tones and you achieve the target output SPL.


    Of course, since the equipment has no way of knowing the size, acoustic condition, speaker sensitivity, relative humidity or overall shape of your intended listening room, it would be impossible for ref level to be "predefined" (at 0 or any other volume position) as you suggested.


    If however, your product is on an absolute scale, than 0dB is maximum output. This is how some gear displays output.


    Once again, misleading- and depending on the reading, false. 0db isn't necessarily a "maximum" in terms of output (neither from the system SPL persepctive, or from the preamp stage perspective)- as many preamp devices can push a signal beyond the level of 0, as I outlined above- resulting in making up net gain.

    The 0db reading is not a display of overall output- rather (as explained above) it is related directly to the preamp gain stage [or at least should be, and is if the equipment is designed correctly]. The overall output level will be influenced by the amount of input level, the amount of amplifier wattage, the impedance of the speakers, the sensitivity of the speakers, and the properties of the space. The little number on the volume knob has no control over any of these elements, and so could never be a measure of output.

    A good way to imagine this whole thing is to see a receiver as 2 discrete elements: a preamp device and an amplifier device. For ease of explanation- assume the amplifier section has no controls: it simply amplifies whatever signal it is given, up to it's maximum input (this is not how all work- but for ease of thought, it should be good enough). If the amplifier gets a signal at 100% of its input level, it will give you 100% of its output level. If it gets 90% of its input level, it will give you 90% of its output level...

    So, if the amp can do nothing but amplify the signal it receives: the duty of regulation of level the amp is getting falls on the preamp device. Since the amp and preamp will be forever mated in this case (in the case of a receiver where they are built together)- it would make sense to build the output stage of the preamp to match the input stage of the amplifier.

    So, they basically create it so that when the preamp is provided with a signal at "full scale" (100% level) coming in from an input source (like a cd player for example), it provides "full scale" output (100%) to the amplifier when the preamp volume is in the 0 position: and thus the amplifier provides 100% output to your speakers (the amount of wattage this 100% is will be influenced by impedance, and the amount of "output SPL" the speakers then produce are a matter of sensitivity).

    This means that in signal flow, when given a 100% signal coming in- the preamp's 0 position provides the max 100% signal out to the amplifier.

    However- in the case that you provide an input signal that is well below that maximum "full scale" 100% amount- the preamp has some gain headroom above 0 to boost the input signal in order to achieve a net gain. So an input source with a signal at 90% to the preamp can actually be boosted by the preamp to provide 100% to the amplifier.

    If you provide an input signal at 100% to the preamp, and then push the preamp to levels above 0, it will pass more than 100% to the amplifier and cause clipping at the amp input stage.


    If you think of it as water pressure, the preamp stage regulates the water allowed to flow from the source into the next stage. When placed at the 0 position, all this means is that the full relative pressure provided to the preamp will be passed along to the next stage (input of 100% of full scale will be passed on as 100% of full scale, signal coming in at 80% of full, will be passed out at 80% of full).

    At a preamp volume position less than zero- the level will be reduced (input at 100% can be reduced to 85% and passed to the amplifier- netting you 85% of the available output of the amplifier)--

    And at a preamp volume level above 0, the signal will be boosted above what the preamp was receiving. This might be fine if the signal the preamp received was peaking at less than the maximum (like an 80% input signal boosted by the preamp to 100%), but if the input signal was coming into the preamp at max- the additional boost above 0 at the preamp stage will result in the preamp supplying excessive level to the amplifier stage (100% input level could be boosted to 120% by the preamp and passed to the amplifier).

    This is a bit difficult to explain in text- but the basic premise is as I explained it above: the 0 position exists as a "max" level in the audio world: it it serves as sort of a neutral "no boost/no cut" in relative gain percentage.


    It is a serious misconception to say that 0dB on an absolute scaled product equates to full output without clipping. That is a function of numerous details, including speaker sensitivity, range of channel trims needed for even speaker level calibration, impedance of speakers and the amplifier (or amplifier sections) ability to deliver current into the load.


    Well- you are correct, but the concepts of clipping are a bit more complicated.

    It is not necessarily "a serious misconception" to say 0 equates full output without clipping, but it might be an oversimplification. 0 does equate to full output without clipping in the preamp output stage...

    0db on the receiver volume, coupled with no boost at the individual speaker levels (i.e. all speaker levels are 0 or below)- would likely equal no chance of clipping at the preamp stage, provided the equipment is designed correctly. But of course it is often a bit more complicated than that.

    However- the issue of speaker sensitivity and impedance you introduced above would have little impact on the issue of "clipping" in this type of case, or at least not directly. These two factors will impact overall wattage and SPL output, no doubt, but amplifiers are not traditionally able to be "clipped" at the output stage.

    The issue with the real concept of "clipping" usually comes in the preamp signal path... most often the preamp device is pushed beyond its limits, pushing signal and overloading the amplifier inputs -- causing distortion. This would not be dependent directly on speaker impedance nor sensitivity (in fact, it would happen if there were no speakers connected at all).

    You could argue that the lack of output (SPL volume) which would be influenced from impedance or speaker sensitivity issues would CAUSE a person to overdrive the preamp and thus create distortion at the amplifier... but it would be an indirect cause. If a person kept their preamp device at or below 0, I would say that "clipping" would be unlikely. If they place their preamp at 0 and don't like the amount of SPL output they get, they likely need a more powerful amplifier or more sensitive speakers...

    If they have an issue of bottoming out their speakers due to excessive wattage (also an issue of sensitivity or impedance)- I don't think of this as traditional "clipping"- rather as exceeding x-max on their speaker.

    Again- this idea gets deep into the idea of how a signal path works, and what opportunities exist for "clipping" (in the source, at the source converters, at the source output, at the preamp input, in the preamp conversion or processing, at the preamp output, at the amplifier input). Usually I hesitate to delve into these realms, as eyes tend to glaze over-- but I feel compelled when confusion arises from simplified posts like the one I made above.


    In the most basic sense, if you have a source, preamp and amplifier are matched in input/output voltage and impedance:

    You feed a 100% full scale tone from the source into the preamp, place the preamp at 0 gain, and then pass the signal onto the amplifier: the source will produce a 100% full scale tone, the preamp will pass this tone to the amplifier at the amplifier at 100% of its input level.

    0 gain position will give you the same % scale in, same % scale out (assuming each element in the chain are all operating at the same relative voltage level).


    THX products with relative volume scales can (and do) exceed 0dB, since this only signifies level relative to reference, not an absolute output level. Check out something like B&Ks Reference 30, an Anthem AVM-20 as examples of preamps which have a maximum output level of +10dB as measured vs Reference.


    But it's not necessarily a direct relative to SPL "ref level": rather it is a measure of internal gain percentage at the preamp.

    If you calibrate it so 0 position also equals ref output in your listening space- great-- but it doesn't necessarily have to...

    Out of the box, 0 position on a preamp device will likely not provide ref output in all situations. It is not associated with an exact SPL ouput level, which "ref level" is an exact SPL level.

    A prime example: preamp at 0 connected to a 25 watt output amplifier will provide a very different amount of SPL from the exact same preamp at 0 connected to a 4000 watt amplifier.

    If you were to setup your home system with 10,000 watt amps on each speaker- it would likely take a position of only minus 50 on the volume knob to reach "ref level" SPL output in your listening space. However the amplifier (and the preamp) would only be operating at a fraction of their maximum ability. In this case it is glaringly obvious that the 0 position doesn't equal "ref level" (in this example it would be -50), but instead it obviously represents an absolute scale of gain of the preamp.

    In this extreme 10,000 watt amplifier example, the +10 gain setting on the Anthem AVM-20 you used as an example would obviously not be +10db as measured vs Reference, rather it would end up being 60db over "reference output" (which was -50) in this configuration. However, even in this unlikely configuration, it would still be an absolute scale of gain potential within the preamp.

    We need to clearly delineate between internal gain structure, and output level (and ref level is a measure of output level). While the two are related, they are not absolute to one another. Two identical preamp configurations with different amplifiers will provide different output spl while maintaining the same gain position.

    To put it another way- the volume position is an absolute measure of internal gain scale, completely independant of output. You and I could have identical preamps, and at a volume position of 0db they would both be at 100% of their scale- however if I bought a 2 watt amp and you bought a 2000 watt amp to go with our respective preamps, I'd get almost no output at all (let along "ref level" while you were enjoying the movie... Despite our vastly different output conclusions, our preamps would be operating at their full potential...


    0 position, as I said above- means passing 100% of relative input level to output level: maximum. Can the preamp device exceed max? Certainly- you can force it to boost the incoming signal to 125% or more of what it gets in input. However if what it is getting in input is full scale- this increase beyond 0 will likely cause clipping at the amplifier stage...

    -V
     
  13. Vince Maskeeper

    Vince Maskeeper Producer

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    As a footnote to the above, I wanted to state that the idea is absolute, but sometimes the actual process is not. Some products operate on a similar gain system, but do not necessarily denote the 0 position as the max.

    Some actually make the 0 position slightly boosted- to make their product appear more powerful. In these products, the same concept above applies, but the 100% input = 100% output position might actually fall at a position below 0 on the gain knob.

    Some other products make the 0 position slightly cut- to insure that the average user will not damage their speakers. In these products, the same concept above applies, but the 100% input = 100% output position might actually fall slightly above the 0 position on the display.

    The post above takes for granted that the products were designed correctly, which thankfully more and more HT products are actually doing correctly. I am assuming from Allusions made in John's posts that THX has decided on a different % for 0 on the gain knob, but I'm not aware of the change.

    Regardless, the point remains that the volume gain position in the preamp stage is not really a measure of "output" exactly (and thus volume knob position could never be equated with a specific spl output until calibrated in your space): rather is simply a measure of gain passed to the next stage. If designed correctly, this gain scale will reach 100% of the relative input level (Passing out in output the relative level it receives in input) at 0 position.

    -Vince
     
  14. John Kotches

    John Kotches Cinematographer

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    Vince,
    Yes, you have completely misunderstood my posts. I sent you a PM and have now calmed down sufficiently to respond in what I would consider to be a rational fashion.
    Let's start with this post of mine, emphasis added for making the point clearer
    On THX Equipment, which employs -30dBFS calibration tones and locks the volume control to 0dB automatically, then there's only one assumption here. The user has read the manual and calibrates to 75dB for all speakers. That's the extent of it. Adhering to the established standards, yields repeatable results.
    That's the point of standardization in the first place.
    And that's enough for now. The rest of this will have to wait until I get a few hours sleep. Viva la Time Zone differences.
    Regards,
     
  15. Vince Maskeeper

    Vince Maskeeper Producer

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    It wasn't so much a "reading past" on my part- but maybe a misinterpretation.
    I think the confusion is/was arising with the term "predefined". In your post you say, quite obviously, that a system in a new environment (MSG) must be "re-calibrated" to provide ref output... which to me seems to be the opposite of "predefined".
    I took "predefined" in your post to mean that you felt the 0 position on the preamp device is manufactured to provide ref output in any and all environments, which is not necessarily true. 0 doesn't necessarily mean ref output, unless specifically configured to do so. There for it is not "based" on this ref, as the original poster asked.
    Let me try another analogy which might clear up this confusion:
    Let's pretend for just a moment that this is not the HTF, but rather the "Automobile Association Forum". A gentleman named Michael comes on our forum and posts a message like this one:
    "I just bought a high end car, and noticed that the accelerator pedal has a small plug below it that stops the accelerator from being pushed all the way down to the floor. I wonder, what is this "stop" position based on?"
    Now, how I read it, the analogy of what you posted initially above would be:
    "Well Michael, if you have a Ford car and you calibrate the engine properly, this stop point will be equal at peak to the speed limit (65 miles per hour)."
    Now, while what was said here is entirely true, it has nothing to do with the question asked. The stop point of the accelerator is NOT "based on" the speed limit by its very nature. It's not set to equal the speed limit as it comes from the factory. Rather it can be calibrated so that position on the accelerator equals 65mph of engine output in your particular configuration on a particular road. But this is not what it is "based" upon.
    In addition, this is only true if the user calibrates this position to equal this speed. As such it isn't the "nature" of this position, rather is a user "option".
    Now, in the same analogy, what I wrote above would translate to be:
    "Michael, this stop position on your accelerator is designed as the maximum acceleration level your engine is designed to allow.
    The accelerator pedal is a scale ranging from 0% acceleration (your foot off the pedal) to 100% acceleration (when your pedal hits the stop plug). Technically speaking you could remove the plug and push the engine to 125% of acceleration scale (which might be helpful on small hills for example)- but you run the risk of damaging your engine."
    Now, again what is said here is all factually correct- but is also a better answer as to what the scale is "based upon".
    While it is absolutely true, in both the Ref Output Level example and the Speed Limit example that the system can be calibrated to yield a specific output based upon a specific input-- it is false to say that this is the "basis" for the system, or that one "equals" the other.
    In rereading your post above- what you said was true: If you calibrate your system to yield ref output level at the 0 position, it will yield ref output level at the 0 position. But this is like saying "the system is calibrated for ref output as long as it is calibrated for ref output": it's a truism.
    But this is a chosen user configuration element- not what the system is based upon.
    I apologize for the confusion- in rereading your posts, I see now that I was simply misreading as I took many of your circular conditionals to read as absolutes.
    -V
     
  16. Vince Maskeeper

    Vince Maskeeper Producer

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    I wanted to point out one small thing about this.

    Although I haven't directly tested these particular products at a voltage level, what you will likely find is that they are based on the same gain system to voltage output. I don't think the gain structure on the THX items represent a relative to ref, rather based on a voltage level.

    In your outlining them, you showed products which operate on a -inf to 0db scale (Sunfire Theater Grand-III for example) and show that in contrast to ones which operate on a -inf to +10 scale (Anthem AVM-20 in your example)-- but chances are they are more similar than different.

    Now, it is likely that if you provided a full scale signal to each of these, and placed both at 0db on the gain scale, you would yield the same voltage output. The Anthem would simply have an addition 10db of ability to extend above this level, while the Sunfire would not. The Anthem maximum voltage output would exceed the Sunfire, but I'd; guess that at 0db they'd be real close.

    Now, in your post, you refer to the +10 on the Anthem as a "maximum", which is technically correct as +10 position would provide the absolute maximum this preamp could provide in voltage: however the gain scale in this item would range from approx 0% to 200%, with 0 likely representing the same "100% fs output" and same voltage that it represents in the Carver.

    So, in the case of the Anthem, it might be better to describe 0db as full (100%) with all ranges above 0 to be a boost (greater than 100% gain).

    It would be wrong to look at the +10 position on the Anthem as analogous to the 0db position on the Carver, just because it was the maximum each could produce. Instead, it would be important to consider the voltage output each would produce to an attached amplifier at a give db point and determine how close they are to each other at 0...

    I would love to have some of these laying around to test them. Does anyone know what the "standard" input sensitivity for "Home Theater" amplifiers is? Since professional amplifiers range all over the place (.775v, 1.0v, 1.23v, 1.4v), I wonder if the home items are more standardized.

    Anyway, I think that was an additional source of confusion on this issue.

    Thanks for reading.

    Vince
     
  17. John Kotches

    John Kotches Cinematographer

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    Vince....

    Man do I hate it when my browser crashes in the middle of typing a post....

    Let's discuss this some more.

    Assumption 1: The Anthem (as an example) is calibrated to 75dB with internal test tones.

    Assumption 2: The Sunfire was calibrated with the volume control at -10dB.

    Assumption 3: Clipping is not an issue across the board.

    In this case, 0dB on the Anthem would give you 105dB at the listening position.

    The Sunfire would give you 115dB at the listening position.

    105dB != 115dB.

    Now you might be able to calibrate the Sunfire so that 0dB would equal reference level, assuming you had sufficient channel trim range available. I certainly don't with my speakers.

    Also, in discussing the 0dB and additional gain on an "absolute" type volume control...... Regardless of whether the preamp stage is physically capable of higher output, you can't set the volume control any higher, so leftover gain is "wasted" in a manner of speaking.

    Sorry for the very delayed reply, it's been a weird day for me.

    Regards,
     

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