Quote:
Originally Posted by b0ck3n
Ti Kan's example of running the 114 dB/V, 32 Ohm PX100 headphones with 5x gain is anything but "real world".
It's as real as it gets, my friend.
The PX100's quoted "114dB" sensitivity is for 1Vrms (according to
the PX100 manual and other web sources). This is a little unusual. The typical convention for headphone sensitivity specification is dB/mW, not dB/Vrms. So the PX100's sensitivity figure is not directly comparable to most other headphone specs. In fact on the same amp, at the same volume knob position, the PX100 is only a little louder than the HD600.
For 1Vrms into 32 ohms, the power being delivered is a bit over 31mW. To get 1mW into 32 ohms, you need only 0.18Vrms. Doing a conversion from one to the other. the PX100's sensitivity becomes 99dB/mW. I mention all this to show that it's not the most efficient portable headphones compared to many others. Yet it is an immensely popular model so it's appropriate to use as an example.
How much gain is needed? You must take into consideration the sensitivity of the headphones, the output level of the source,
and add enough headroom to account for recordings that are low in volume so that you don't run out of volume pot travel. Many recordings are almost constantly pegged near the 0dBFS line and dynamically compressed. These recordings will sound very loud (and bad). But there are also recordings that are recorded very low in level (some from the late 70s and 80s for example), maybe 10-15dB below 0dBFS. Thus, it is prudent to select an amp gain level above and beyond what's needed to amplify a 0dBFS signal to compensate.
Let's say we want to have 15dB of additional headroom. And we assume we want to be able to have unclipped peaks of 114dB (just take Sennheiser's spec as an example because it's a reasonable number and happens to require a nice clean 1Vrms). To add 15dB extra headroom we need to swing 5.6Vrms. Note that we're not actually going to run 5.6Vrms into the headphones, we would set the volume control to whatever listening level we want. Let's then assume a y2 DAC as the source with 1.4Vrms output @0dBFS. The gain we want is then 5.6V/1.4V = 4x. If we increase the peak dB SPL requirement to 120dB then we need a gain of 8x.
This illustrates that a "Mini³ with gain of 5x driving the PX100" case I mentioned is not outside "real world" usage, and is in fact quite a nice setting. On average recordings (not too loud, not too soft), my volume pot position is usually a little over half way, but on really soft recordings I can turn it up to satisfactory levels without running out of pot travel.