11-Jul-2019, 22:08
This thread has been rolling around in my head for some days now as I tried to make sense of some of the comments plus my own experience with amps which, as was pointed out above, related to class AB amps which behave differently. In the end 4 things seemed to stick with me:
1- @ "Jean-Marie" 's comment that we can know the peak output of a Devialet and his quote from the ADH section of the config file showing the settings for Voltage and Puissance at 0 dB;
2- @RebelMan 's comment that Devialet's have no dynamic headroom,
3- @thumb5 's comments in various places about crest factor.
4- I seem to remember reading a post in which one of the Devialet team was quoted as having said that his perfect amplifier would have infinite power. I haven't been able to find the quote on a quick and rather cursory search so I'm sorry I can't give attributions to the poster or the Devialet team member.
Anyway, what ended up coming out of all of those things rolling around in my head was the following and it may sound surprising. It may also be wrong but I think that logically it does hold together and is consistent with all 4 of the things I listed above. Feel free to call me an idiot if you wish but give an explanation of where you think I'm going wrong.
My conclusions are this:
- We can ignore crest factor. The reason for that is that we're working with a digital device, peak output occurs at 0 dB and the highest level for a digital signal is also 0 dB. Yes, crest factor exists in music but the signal at the peak of the crest can't be encoded at a higher level than 0 dB.
- When we set the volume level we tend to do it based on how loud the music sounds to us at that moment but what we're really doing is setting the output level for a signal strength of 0 dB. If we set volume to -20 dB, the maximum peak in the music will occur 20 dB lower than the amp's maximum output. Since the peak signal level is O db, setting the volume to 0 dB will result in the amp delivering peak output for the highest signal level which can be encoded. We don't need dynamic headroom to handle peaks, the amplifier is basically designed to be capable of peak output continuously which is why it has no dynamic headroom. If things aren't loud enough for us with musical peaks of 0 dB level at a volume setting of 0 dB we need a bigger amp.
- Provided we don't exceed a volume setting of 0 dB, the amp shouldn't clip, at least in theory. In practice I guess it may because things like speakers which present a different load to the amp and things like SAM may make a bit of a difference and result in the amp "running out of steam" a bit below a volume setting of 0 dB.
- Why have a volume control which can set levels above 0 dB if the above is true. Some musical content is mastered with its peak level lower than 0 dB. If the peak level is mastered at -5 dB, you would need a volume setting of +5 dB to reach peak output of the amp and people should be able to reach peak output. Some test content is mastered at levels like -20 dB and people using that test material may need to play that content at the amp's peak output level for their test. There's some good reasons for having a volume control which can go above 0 dB but you would only want to go above a volume setting of 0 dB for signals mastered with a peak level lower than 0 dB
- If the ideal amp is one with infinite power, then an ideal amp will have no dynamic headroom because it will always be capable of delivering as much power as needed for as long as it's needed so it will be capable of infinite power continuously if required and, just based on the meaning of infinite, we can't have "more than infinite" so dynamic headroom for an amp which has infinite power simply doesn't exist, dynamic headroom will never be required.
The Devialet designers seem to have taken that idea of an amp with infinite power and delivered amps which essentially behave the way such an amp would behave but with a limited power output. We get the amp's rated power for as long as we need it but we can't get any more than it is capable of. Peak output is peak output, that defines the level that musical transients (the top of the crest) can reach, and we can stop worrying about crest factor and dynamic headroom provided we don't exceed peak power output. A volume setting of 0 dB is the best guide we have to when that is likely to occur but if you're using SAM and/or have a speaker which can present a difficult load you may want to keep your volume setting a few dB below 0 dB.
That's what I ended up coming to for my conclusions and I have to admit they surprised me. I ended up thinking about the amp's power delivery capability in a very different way to how I thought about such things before this thread. Like I said, i could be wrong.
1- @ "Jean-Marie" 's comment that we can know the peak output of a Devialet and his quote from the ADH section of the config file showing the settings for Voltage and Puissance at 0 dB;
2- @RebelMan 's comment that Devialet's have no dynamic headroom,
3- @thumb5 's comments in various places about crest factor.
4- I seem to remember reading a post in which one of the Devialet team was quoted as having said that his perfect amplifier would have infinite power. I haven't been able to find the quote on a quick and rather cursory search so I'm sorry I can't give attributions to the poster or the Devialet team member.
Anyway, what ended up coming out of all of those things rolling around in my head was the following and it may sound surprising. It may also be wrong but I think that logically it does hold together and is consistent with all 4 of the things I listed above. Feel free to call me an idiot if you wish but give an explanation of where you think I'm going wrong.
My conclusions are this:
- We can ignore crest factor. The reason for that is that we're working with a digital device, peak output occurs at 0 dB and the highest level for a digital signal is also 0 dB. Yes, crest factor exists in music but the signal at the peak of the crest can't be encoded at a higher level than 0 dB.
- When we set the volume level we tend to do it based on how loud the music sounds to us at that moment but what we're really doing is setting the output level for a signal strength of 0 dB. If we set volume to -20 dB, the maximum peak in the music will occur 20 dB lower than the amp's maximum output. Since the peak signal level is O db, setting the volume to 0 dB will result in the amp delivering peak output for the highest signal level which can be encoded. We don't need dynamic headroom to handle peaks, the amplifier is basically designed to be capable of peak output continuously which is why it has no dynamic headroom. If things aren't loud enough for us with musical peaks of 0 dB level at a volume setting of 0 dB we need a bigger amp.
- Provided we don't exceed a volume setting of 0 dB, the amp shouldn't clip, at least in theory. In practice I guess it may because things like speakers which present a different load to the amp and things like SAM may make a bit of a difference and result in the amp "running out of steam" a bit below a volume setting of 0 dB.
- Why have a volume control which can set levels above 0 dB if the above is true. Some musical content is mastered with its peak level lower than 0 dB. If the peak level is mastered at -5 dB, you would need a volume setting of +5 dB to reach peak output of the amp and people should be able to reach peak output. Some test content is mastered at levels like -20 dB and people using that test material may need to play that content at the amp's peak output level for their test. There's some good reasons for having a volume control which can go above 0 dB but you would only want to go above a volume setting of 0 dB for signals mastered with a peak level lower than 0 dB
- If the ideal amp is one with infinite power, then an ideal amp will have no dynamic headroom because it will always be capable of delivering as much power as needed for as long as it's needed so it will be capable of infinite power continuously if required and, just based on the meaning of infinite, we can't have "more than infinite" so dynamic headroom for an amp which has infinite power simply doesn't exist, dynamic headroom will never be required.
The Devialet designers seem to have taken that idea of an amp with infinite power and delivered amps which essentially behave the way such an amp would behave but with a limited power output. We get the amp's rated power for as long as we need it but we can't get any more than it is capable of. Peak output is peak output, that defines the level that musical transients (the top of the crest) can reach, and we can stop worrying about crest factor and dynamic headroom provided we don't exceed peak power output. A volume setting of 0 dB is the best guide we have to when that is likely to occur but if you're using SAM and/or have a speaker which can present a difficult load you may want to keep your volume setting a few dB below 0 dB.
That's what I ended up coming to for my conclusions and I have to admit they surprised me. I ended up thinking about the amp's power delivery capability in a very different way to how I thought about such things before this thread. Like I said, i could be wrong.
Roon Nucleus+, Devilalet Expert 140 Pro CI, Focal Sopra 2, PS Audio P12, Keces P8 LPS, Uptone Audio EtherREGEN with optical fibre link to my router, Shunyata Alpha NR and Sigma NR power cables, Shunyata Sigma ethernet cables, Shunyata Alpha V2 speaker cables, Grand Prix Audio Monaco rack, RealTRAPS acoustic treatment.
Brisbane, Qld, Australia
