[quote name='Aldem' date='Nov 4 2014, 01:03' post='879462']
Let's use the AKG K701/702/712 (same specs) in this example.
AKG specs
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Sensitivity headphones 105 dB SPL/V
Max. Input Power 200 mW
Rated Impedance 62 Ohms
Asus Xonar Essence STX
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Output Signal-to-Noise Ratio (A-Weighted) (Front-out) : 124 dB
Output Signal-to-Noise Ratio (A-Weighted) (Headphone-out) : 117 dB
Input Signal-to-Noise Ratio (A-Weighted) : 118 dB
Output THD+N at 1kHz (Front-out) : >0.0003 %(-110 dB)
Output THD+N at 1kHz (Headphone-out) : >0.001 %(-100 dB)
Frequency Response (-3dB, 16bit/44.1KHz input) : 10 Hz to 90 KHz
Output/Input Full-Scale Voltage : Line output (RCA, 3.5) : 2 Vrms (5.65 Vp-p)
Output impedance: ~11 Ohms
O2+ODAC
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DAC PERFORMANCE
Frequency Response +/- 0.1 dB (10 hz – 19 Khz 24/44)
THD+N 100 hz 0 dBFS 0.0029%
THD+N 20 hz –1 dBFS 0.003%
THD+N 10 Khz –1 dBFS 0.003%
IMD CCIF 19/20 Khz –3 dBFS 0.0011%
IMD SMPTE –1 dBFS 0.0004%
Noise A-Weighted dBu 24/44 -102.8 dBu
Dynamic Range > 110 dB A-Weighted
Linearity Error -90 dBFS 24/44 0.0 dB
Crosstalk 0 dBFS Line Out 100K -93.5 dB
USB Jitter 11025 hz J-test 24/44 Excellent
Maximum Output Line Out 100K 2.0 VRMS
Distortion < 0.005%
DATA SUPPORT
Audio Formats 16/44, 16/48, 16/96, 24/44, 24/48, 24/96
Interface USB, Audio Class 1
Native Driver OS Support Windows XP & Later, OS X x86, Linux
DIMENSIONS
Case Dimensions (mm) 108.50 x 80.00 x 29.50
Case Dimensions (in) 4.27 x 3.15 x 1.16
Weight 11oz
AMPLIFIER PERFORMANCE
Frequency Response +/-0.1 dB (20Hz-20kHz)
THD 1 Khz 150 Ohms 0.0016%
IMD CCIF 15 Ohms 0.001%
IMD SMPTE 0.002%
Noise (ref 400 mV) -105 dB
Max Output (33 Ohms) 613 mW
Max Output (150 Ohms) 355 mW
Max Output (600 Ohms) 88 mW
Output Impedance 0.54 ohms
Crosstalk (15 ohms) -65 dB
Channel Balance (50% volume) 0.6 dB
CONFIGURATION
Gain 2.5x and 6.5x*
Volume Potentiometer Taper Alps 15A or 3B
CONNECTIVITY
Analog Output 3.5mm or 6.35mm*
Analog Input 3.5mm
Digital Input Mini-USB
DAC Line Output 3.5mm or RCA**
Power Input 14-20VAC
* Denotes Customization Option
For this question, which specs are important to look at ?
From the Xonar:
Output/Input Full-Scale Voltage : Line output (RCA, 3.5) : 2 Vrms (5.65 Vp-p)
Output impedance: ~11 Ohms
From the O2:
Max Output (33 Ohms) 613 mW
Max Output (150 Ohms) 355 mW
Max Output (600 Ohms) 88 mW
Output Impedance 0.54 ohms
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But the Max output numbers are not in terms of voltage.
I've done the arithmetic and obtain:
[code]
Power Impedance Voltage
0.633 33 4.57
0.355 150 7.30
0.088 600 7.27
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The specs of the headphones are:
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Sensitivity headphones 105 dB SPL/V
Max. Input Power 200 mW
Rated Impedance 62 Ohms
and converting that into volts:
200 mw into 62 ohms = 3.52 volts
3.52 volts is about 11 dB over 1 volt, so the max SPL rating of the phones is 116 dB
The Xonar puts out 5.5 v p-p which is 2 volts. 2 volts is 6 dB over 1 volt, so it will drive the headphones to about 111 dB.
IMO either amp is more than sufficient.