Quote:
Originally Posted by jcx
you really should learn to do basic amp/headphone #s
read http://headwize.com/articles/hearing_art.htm twice, once to avoid hearing damage from listening too long at too high an Average Loudness and the again to find the very high peak SPL of live music which your amp/headphone combination should be able to reproduce without clipping - most people are surprised by how much Headroom is needed
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http://forum.lowyat.net/lofiversion/...p/t315593.html
the LDM+ apparently uses this op amp:
http://datasheets.maxim-ic.com/en/ds...A-MAX9724B.pdf
if you can read the data sheet you can get a very good idea of the LDM+ probable performance limit
while potentially not as dire as I initially thought because the maxim op amp uses a sw cap DC-DC supply inverter that doubles the available supply voltage, you need to work the details to get a reasonable performance estimate
despite the doubled supply voltage the story for driving Grados remains rather poor
looking at the 3V “OUTPUT POWER vs. LOAD RESISTANCE” graph it is clear to an engineer that the max9724 is in fact current limiting when trying to drive a 32 Ohm load like the Grado
taking ~27 mW at 32 Ohm load from the graph a fair assessment would multiply by 1.2**2 to estimate the 3.6 V supply behavior => 39 mW into 32 Ohms at 10% distortion, equivalent to 1.25 Vrms or 3.5 Vpp
so despite the doubled supply the max9724 is current limiting at a level that gives the same power limit as a iPod Shuffle using a split 3.6 V supply into 32 Ohms
Conclusion, my statement that the LDM+ is likely no better driving Grados than a Shuffle seems to still stand on fundamental technical grounds
Grados are generally 98 dB spl @ 1 Vrms, 1.25 Vrms ~= +2 dB re 1 Vrms
98 + 2 = 100 dB spl at clipping driving a Grado from a supply split 3.6 V battery amp like the iPod, and as a coincidence due to the max9724 current limit, the same number applies to the LDM+
The LDM+ would give higher power and desired dynamic headroom with higher impedance cans, the high impedance HD600/650 will actually be driven to higher spl levels than the Grados
But the best impedance match for the LDM+ is for 70-120 Ohm headphones
with 32 Ohms, the LDM+ 100 dB at clipping with the Grados doesn’t make it in my book
Ohsa 8 hr exposure limit is 90 dB, many people may listen at this average level (even if they probably shouldn’t)
Dynamic music can easily have +20 dB peaks, 90 + 20 = 110 dB
check out where 110dB is on the Headwize article chart of musical sources
The simple technical spec based conclusion is that a LDM+ will not drive a Grado to realistic spl, it will clip, possibly frequently
I can’t see advocating buying an Amplifier that can’t potentially drive the headphones to at least 110 dB without clipping
No you don’t need the capability often or even at all with some genres of “music” – much pop is already compressed to death, some with only 6 dB peak to average level will deafen you at average loudness that never clips even a LDM+ with Grados
http://en.wikipedia.org/wiki/Loudness_war
but it aint audiophile source,
LDM+ with Grados don't rate as a audiophile system - since it don't have HeadRoom