The issue with the volume knob is a real pain... they want you to keep it at 12 o'clock and use your pre-amp for volume control. I have a new DAC on order with PEQ and a nice volume knob which I hope will free from needing to use the one on the MHA200.
Simply put, this doesn't work either. I tested and measured McIntosh's proposed way to control the volume of this amplifier. Let me show you the actual readings I got when I held the MHA200's volume knob at 12 o'clock. My DAC is capable of controlling the level of its balanced outputs in the range 0-16 Vrms. So, I measured the MHA200 headphone outputs with the following fixed inputs: 2 Vrms, 4 Vrms, 16 Vrms.
The results are as follows:
Load switch - Output amplitudes @ Inputs 2 Vrms, 4 Vrms, 16 Vrms.
32 Ohm - 0.031 Vrms, 0.064 Vrms, 0.195 Vrms.
120 Ohm - 0.055 Vrms, 0.114 Vrms, 0.345 Vrms.
250 Ohm - 0.087 Vrms, 0.177 Vrms, 0.535 Vrms.
600 Ohm - 0.131 Vrms, 0.265 Vrms, 0.800 Vrms.
All results were obtained with a load of 300 Ohm.
Now, the impedance of my Sennheiser HD650 headphones is about 333 Ohm. I set the Load switch to 250 Ohm.
With an input voltage of
2 Vrms, I got an output amplitude of
0.087 Vrms from the MHA200.
This is equivalent to
0.02 mW of power delivered to the headphones, and equals
82 dB SPL loudness.
With an input voltage of
4 Vrms, I got an output amplitude of
0.177 Vrms from the MHA200.
This is equivalent to
0.09 mW of power delivered to the headphones, and equals
88 dB SPL loudness.
With an input voltage of
16 Vrms (!), I got an output amplitude of
0.535 Vrms from the MHA200.
This is equivalent to
0.86 mW of power delivered to the headphones, and equals
98 dB SPL loudness.
I prefer my headphones to have a maximum SPL of 106 dB. This requires 6 mW of power delivered to the headphones. This is equivalent to 1.412 Vrms output amplitudes from the headphone amplifier. As you can see, this cannot be achieved with any position of the Load switch or with any input signals from the DAC, with the volume control set to the 12 o'clock position.
