[Samueru Sama]

Well with the HD800 being rated for 102dB/V, 2Vrms is still good for 102+6dB = 108dB, which is still plenty loud for most situations. And again most people's complaints of the HD800's sound have nothing to do with the driving power available behind them. For example they'd say that the bass sounds weak or loose even though the impedance of the HD800 peaks at the bass and for a given variable load headphone the frequencies at which the impedance peaks are easiest to drive. Lack of understanding of amp / headphone interaction will lead most of those same people to cite said low frequency impedance peak as the reason for the loose / weak bass they hear. Heck they claim loose / weak bass as the result of any kind of underdriving when statistically underdriving headphones says nothing either way about frequency response changes, as illustrated by this sample argument and my rebuttal:
castleofargh made good analogies of how power does and does not relate to driving a pair of headphones properly. Here's a simpler analogy to how much sense it makes to always ask for more power from a given amp: http://tvtropes.org/pmwiki/pmwiki.php/Main/TimTaylorTechnology

To make things more fun, amplification problems are often the LAST thing I look at when faced with a sound quality problem but the FIRST thing for most anybody else around here.

 
There's actually a video on one of the tutorials on r/headphones that literary implies that you have to do the power calculations dB/mW at the impedance peak of the headphones, a while back I sent feedback asking them to fix that and to this day the video is still there. xd
 
I would think that 108 dB SPL is loud enough for all situations, at least with my ears, I prefer that people measure their max dB SPL with high DR music, and in my case I've found that I don't go higher than 102 dB SPL.

 

[zareliman]

  It's a power supply voltage issue.  If the DAP or DAC is meant to run or charge on USB power, that's single pole 5V.  Without some internal voltage multiplier (often a noise source), the maximum RMS voltage you can ever get is just under 2V (actually about 1.7V), and that assumes you can peak at the full supply rails (usually not).  If the system can run on a higher supply voltage, and you have a means of doing that cleanly, you can clip at much higher voltages.  Back in the day when I designed stuff we ran bipolar 15V supply rails, and clipped well above 8V rms.  Today's lower supply voltages dictate maximum output before clipping...all stages.   With a DSP EQ you have to scale gain anyway, because most music will hit a peak within a few dB of 0dBFS.  If you apply gain EQ digitally there must be scaling applied or you can go above 0dBFS quite easily.  
 
The only places you'll ever see 16V is a pro-audio balanced line output or a power amp. 

There's plenty of USB powered devices that can output 2V line. It's pretty standard to see usb powered dac/amps claiming 200mW at 32 Ohms, which would mean 2.45V RMS. Some will offer up to 5.6V for the headphone output (around 120mW for 300 ohm).

 

[pinnahertz]

 
There's plenty of USB powered devices that can output 2V line. It's pretty standard to see usb powered dac/amps claiming 200mW at 32 Ohms, which would mean 2.45V RMS. Some will offer up to 5.6V for the headphone output (around 120mW for 300 ohm).

I'd be happy to look a few of those over, if you'd like to quote a few examples. 
 
Note, in my post, I said "Without some internal voltage multiplier...".  

 

[zareliman]

  I'd be happy to look a few of those over, if you'd like to quote a few examples. 
 
Note, in my post, I said "Without some internal voltage multiplier...".  

Yeah I noticed the "internal voltage multiplier" bit, I don't think I have seen that. I think most usb-dac boards work with 2V RMS line (high output impedance). From that the manufacturer can either chop it for a lower impedance HP output or draw more power from the USB (or even external power source) for a higher power HP output.

http://hifimediy.com/DACs/ready-made-dacs
All the hifimeDIY dacs claim 2V line output, even the ones for smartphones. The don't have potentiometers so a headphone also will receive this kind of voltage (maybe lower due to damping).

The well known Fiio E10k outputs 200mW at 32 ohm (that's more than 2V)
http://fiio.net/en/products/27/parameters

I can't recall the 5.6V ones, but I think it was a mistake since 2V RMS is exactly 5.6V Peak-to-peak.

 

[pinnahertz]

  Yeah I noticed the "internal voltage multiplier" bit, I don't think I have seen that. I think most usb-dac boards work with 2V RMS line (high output impedance). From that the manufacturer can either chop it for a lower impedance HP output or draw more power from the USB (or even external power source) for a higher power HP output.

http://hifimediy.com/DACs/ready-made-dacs
All the hifimeDIY dacs claim 2V line output, even the ones for smartphones. The don't have potentiometers so a headphone also will receive this kind of voltage (maybe lower due to damping).

The well known Fiio E10k outputs 200mW at 32 ohm (that's more than 2V)
http://fiio.net/en/products/27/parameters

I can't recall the 5.6V ones, but I think it was a mistake since 2V RMS is exactly 5.6V Peak-to-peak.

http://hifimediy.com/DACs/ready-made-dacs
All the hifimeDIY dacs claim 2V line output, even the ones for smartphones. The don't have potentiometers so a headphone also will receive this kind of voltage (maybe lower due to damping).
 
They also state "no dc blocking capacitors", meaning they're using an internal bipolar supply.  If it's running off 5V USB power, there's an internal switching bipolar converter to accomplish that.  It's also quite power/current limited, capable of under 1V with a 32 ohm load. 
 
Yes, I know the E10k, also uses an internal switching voltage converter of some kind.  As does anything exceeding 1.7V rms output.  
 
Two things to be careful of: the internal switching voltage converters affect the minimum output impedance of the amp (like the hifime) .  It's a cost thing...higher current from the converter drives the cost up, actually way up.  And the maximum RMS output voltage must also include a THD figure.  Many amps and DACs conveniently skip that.  Once you don't spec THD and max output it could be a maximum RMS output at 10% THD and easily do 2V with no voltage converter on USB only.  The little T-power amplifiers do that all the time.  But it's not a real figure unless you really like 10% THD from hard clipping.
 
If you look at a really well thought out design you'll see some real consideration for the power supply side, not just a cheap voltage converter.  The discussion of the power side of the O2 provides quite a bit of insight. 

 

[zareliman]

Two things to be careful of: the internal switching voltage converters affect the minimum output impedance of the amp (like the hifime) .  It's a cost thing...higher current from the converter drives the cost up, actually way up.  And the maximum RMS output voltage must also include a THD figure.  Many amps and DACs conveniently skip that.  Once you don't spec THD and max output it could be a maximum RMS output at 10% THD and easily do 2V with no voltage converter on USB only.  The little T-power amplifiers do that all the time.  But it's not a real figure unless you really like 10% THD from hard clipping.  
If you look at a really well thought out design you'll see some real consideration for the power supply side, not just a cheap voltage converter.  The discussion of the power side of the O2 provides quite a bit of insight. 

Mind if I ask some questions ?

Would you say the main source of Harmonic distortion in amps is clipping ?
I was looking at some power amps (TDA7492 and TPA3116) and they seem to have wildly varying numbers. Higher power usually goes accompanied by higher THD numbers, but also the power supply seems to play a role (the circuits can work with varying input voltage specified from 12 to 24V or so).

Using a higher voltage power supply will increase the maximum volume or just reduce clipping/THD at high volumes ?

 

[pinnahertz]

  Mind if I ask some questions ?

Would you say the main source of Harmonic distortion in amps is clipping ?
I was looking at some power amps (TDA7492 and TPA3116) and they seem to have wildly varying numbers. Higher power usually goes accompanied by higher THD numbers, but also the power supply seems to play a role (the circuits can work with varying input voltage specified from 12 to 24V or so).

Using a higher voltage power supply will increase the maximum volume or just reduce clipping/THD at high volumes ?

Clipping is the biggie, but not the only cause of distortion.  It's a sudden onset of distortion that increases rapidly as level goes up. Other mechanisms are more subtle. 
 
You have to look at a THD vs output curve to see what's going on.  There should be a curve family with THD vs output for various supply voltages and currents.  Should be in a data sheet. 
 
Generally for a chip amp like that supply voltage doesn't change the circuit gain, just the maximum undistorted output.  
 
My experience with T-power amps highlighted that the stated maximum power output figures are for distortion levels that nobody would actually find acceptable.  For example, look at this one's specs.  See what's missing?  The specified load!  I own one of these, measured it extensively.  The 20W THD figure is "optimistic' at best, I got more like under 10W at that THD level, with their included 24V power supply.  The 40W figures are for obscene amounts of THD, but of course, it will do 40W rms that way. 
 
edit: Oh yeah, when you figure the maximum possible output voltage based on the DC input, it should only do about 9 clean watts into 8 ohms without some internal voltage conversion...which in this case, isn't there.

 

[zareliman]

  Clipping is the biggie, but not the only cause of distortion.  It's a sudden onset of distortion that increases rapidly as level goes up. Other mechanisms are more subtle. 
 
You have to look at a THD vs output curve to see what's going on.  There should be a curve family with THD vs output for various supply voltages and currents.  Should be in a data sheet. 
 
Generally for a chip amp like that supply voltage doesn't change the circuit gain, just the maximum undistorted output.  
 
My experience with T-power amps highlighted that the stated maximum power output figures are for distortion levels that nobody would actually find acceptable.  For example, look at this one's specs.  See what's missing?  The specified load!  I own one of these, measured it extensively.  The 20W THD figure is "optimistic' at best, I got more like under 10W at that THD level, with their included 24V power supply.  The 40W figures are for obscene amounts of THD, but of course, it will do 40W rms that way. 

 
Class D Power amplifier TDA7492
Output Power(Each Channel@ VS = 25V)
50W/Channel(6Ω, 10%,THD+N)
40W/Channel(6Ω, 1%, THD+N)
40W/Channel(8Ω, 10% THD+N)
38W/Channel(4Ω, 1% THD+N)
Efficiency - >90% @ Full Power(RL = 4Ω)

So I should take that this unit starts clipping at 38/40W and 50W is just noise (that's supposed to be measured with their 24V supply, although the unit takes 8 to 26V).
 

 

[pinnahertz]

   
Class D Power amplifier TDA7492
Output Power(Each Channel@ VS = 25V)
50W/Channel(6Ω, 10%,THD+N)
40W/Channel(6Ω, 1%, THD+N)
40W/Channel(8Ω, 10% THD+N)
38W/Channel(4Ω, 1% THD+N)
Efficiency - >90% @ Full Power(RL = 4Ω)

So I should take that this unit starts clipping at 38/40W and 50W is just noise (that's supposed to be measured with their 24V supply, although the unit takes 8 to 26V).
 

Kind of.  The amp clips at output voltages ranging from 12.3Vrms (4 ohms, 1%) to 15.5Vrms (6 ohms, 1%).  Actually, it starts clipping a bit below that.  The 50W figure is probably about correct, but who wants 10% THD? Even on peaks?  The fact that it clips at different voltages shows a current limit somewhere, possibly the power supply, but I haven't read the entire spec sheet.  The supply voltage range just shows the amp will work between those voltages, obviously with reduced output capability at lower voltages.  This isn't unusual or unique to Class D/T-power amps, it applies to just about any amp, any class, any topology.   The difference with these chips is they can be much more efficient, thus the amp can be smaller and lighter (and cheaper), but they also want to make it look fantastic, so they quote power output figures that make no sense in the real world. And people do buy them for that.  Most other amps exhibit the same effect of output clipping voltage being a function of load and power, but they usually also spec full power at some THD level below 1%...and mean it. 

 

[zareliman]

  Kind of.  The amp clips at output voltages ranging from 12.3Vrms (4 ohms, 1%) to 15.5Vrms (6 ohms, 1%).  Actually, it starts clipping a bit below that.  The 50W figure is probably about correct, but who wants 10% THD? Even on peaks?  The fact that it clips at different voltages shows a current limit somewhere, possibly the power supply, but I haven't read the entire spec sheet.  The supply voltage range just shows the amp will work between those voltages, obviously with reduced output capability at lower voltages.  This isn't unusual or unique to Class D/T-power amps, it applies to just about any amp, any class, any topology.   The difference with these chips is they can be much more efficient, thus the amp can be smaller and lighter (and cheaper), but they also want to make it look fantastic, so they quote power output figures that make no sense in the real world. And people do buy them for that.  Most other amps exhibit the same effect of output clipping voltage being a function of load and power, but they usually also spec full power at some THD level below 1%...and mean it. 

So this makes sense with something I learned a long time ago: that you always want to buy an amp to use around 60% to 75% of it's capacity (depending on the specs headroom) since the distortion kind of ramps beyond some point (and only a few decent manufacturers limit their product specs in that threshold). And anyway, since loudness is logarithmic, the extra volume you get from that last 25% of power is not much and absolutely not worth the exponentially increasing distortion.
To me the spec sheet looks much better with low THD at decent power than high THD at slightly higher power, I guess the average consumer behaves in a different fashion.

 

[castleofargh]

Originally Posted by zareliman /img/forum/go_quote.gif
 
So this makes sense with something I learned a long time ago: that you always want to buy an amp to use around 60% to 75% of it's capacity (depending on the specs headroom) since the distortion kind of ramps beyond some point (and only a few decent manufacturers limit their product specs in that threshold). And anyway, since loudness is logarithmic, the extra volume you get from that last 25% of power is not much and absolutely not worth the exponentially increasing distortion.
To me the spec sheet looks much better with low THD at decent power than high THD at slightly higher power, I guess the average consumer behaves in a different fashion.

the good old rules of thumb are only that. when you really know nothing, it's not a bad idea as it really only tels you to have some matter of headroom for whatever reason. but what if the gain setting is set so that 100% on the volume knob is still way below 1% disto? then all you care about is getting loud enough for your comfort and the 60% rule means nothing.
I guess some people would also say as you do for some slew rate ideas, the same way some will always think balanced is better because it's double the voltage and slew rate(which is really just another "it goes to eleven" joke to me. double, sure but double what?).
as mentioned by pinnahertz, knowing the amp design (and understanding it) goes a long way, or at least having some graph with disto over increasing voltage like the one on the top right here http://www.innerfidelity.com/images/ViolectricV281.pdf   if you reach some voltage or current limit into some load, that kind of graph should tell it as it is.