Few basic measurements done with Xonar U7 and Focusrite Solo 3.
First measurements were done with my Picoscope that showed a
35mV DC voltage on output. This is a respectable value for an amplifier that is able to push over 30V RMS on 8-Ohms speakers. I've seen headamps with 20-30mV of DC.
The below measurements were done without vs. with speakers connected, to show the influence of the 3-ways speakers for the output stage and the measurements values differences. The voltage used was around the value of 4.5V RMS, so a 5 W was used as reference here, as Amir suggested in his own power amps reviews. However, increasing the volume will lower the THD+N and increase the final SINAD. External DAC used was Conductor V2+ on DAC-Out (PRE-Out is too noisy due to the internal pre-amp chip).
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External DAC used on CD-input, CD-DIRECT pressed, no speakers connected, 1 KHz
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External DAC used on CD-input, CD-DIRECT pressed, real 4-Ohms speakers connected, 1 KHz
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Built-in DAC (COAX-input), CD-DIRECT pressed, no speakers connected, 1 KHz
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Built-in DAC (COAX input), CD-DIRECT pressed, real 4-Ohms speakers connected, 1 KHz
Seems that the Conductor V2+ has a better DAC inside than the A-S701, because with speakers connected I was able to get a THD+N of 0.0046% vs. 0.0071%.
Usually, when speakers are connected, the low-end of the audio band is more impacted, so below tests are with 50 Hz sines.
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External DAC used on CD-input, CD-DIRECT pressed, no speakers connected, 50 Hz
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External DAC used on CD-input, CD-DIRECT pressed, real 4-Ohms speakers connected, 50 Hz
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Built-in DAC (COAX-input), CD-DIRECT pressed, no speakers connected, 50 Hz
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Built-in DAC (COAX input), CD-DIRECT pressed, real 4-Ohms speakers connected, 50 Hz
Again, the external DAC connected to CD-input was measuring a bit better than the built-in one for 50 Hz test signals as well, with a THD+N of 0.0052% vs. 0.0080%.
So, for 5 Watts of power on 4-Ohms speakers connected and external DAC playing sines I got a THD+N of
0.0046% with 1 KHz and
0.0052% with 50 Hz.