Differing volume levels between voltage matched sources

[virtua]

Hi guys,

So I've been using an Asus Xonar DGX soundcard for a very long time, and I've become very familiar and comfortable with some volume settings on there that I generally use day to day. However, I'd just gotten a Meizu Hifi DAC, TempoTec Sonata HD Pro, Apple USBC to 3.5mm dongle to replace it. I'd read into voltage matching headphone outputs in order to match levels between different devices. So I went out and bought a Digital Multimeter (Uni-T UT131A which has a stated range of 200mV~250V and accuracy of ±(0.7%+3)) and started testing the different dongles using the millivolts AC mode and documented the voltage levels at different windows volumes (between Apple USBC to 3.5mm, Xonar DGX and Meizu Hifi DAC) using a 0dbFS sine wave at 1khz through Audacity. However, something that I've noticed which is strange, is that between the voltage matched sources, I can perceive some are just clearly louder than others - now I can't say this was a double blind test and that it is free from placebo - I don't have a switcher and I'm aware when I switch the devices, but between the voltage matched Apple USB-C to 3.5mm adapter vs the Asus Xonar DGX - the Apple dongle is clearly much louder at reasonably closely matched levels (within a decibel) using the same headphones.

I've considered that it might be the output impedance of the Xonar DGX, while I haven't tested it myself I've heard that it's 10 ohms. Plugging all the relevant numbers from my headphones and sources/voltages into this calculator still doesn't seem like it would make the difference it really seems to have in volume.

I've considered a few reasons why this isn't working as intended, 1. The resolution/accuracy of my multimeter is affecting results. 2. Because I'm testing voltages open (with a 3.5 male to male) instead of under load, it may be affecting the voltage readouts between devices. 3. The output impedance may be affecting the frequency response of my headphones (at 10 ohms it only has a 6:1 damping factor) or perhaps this Xonar DGX is not flat (Just a thought, I can't prove this though).

Is there anything that I'm missing here? Any help would be greatly appreciated

 

[AnalogSteph]

I'd suggest:

1. Verify actual output impedance. The output you are using may not be the one featuring the 10 ohms. Maybe that's just the front panel headphone out, and the rear output has more like 100 ohms. If you do not happen to have the odd suitable resistor floating around, a Y splitter cable may be a worthwhile investment.
2. Do your level checks using the exact same sound API for playback as your player software. Ideally, export your test tone and play it like you would all your other music. I've seen differences up to 3 dB.

 

[virtua]

I'd suggest:

1. Verify actual output impedance. The output you are using may not be the one featuring the 10 ohms. Maybe that's just the front panel headphone out, and the rear output has more like 100 ohms. If you do not happen to have the odd suitable resistor floating around, a Y splitter cable may be a worthwhile investment.
2. Do your level checks using the exact same sound API for playback as your player software. Ideally, export your test tone and play it like you would all your other music. I've seen differences up to 3 dB.

Hi Steph, so I went into the menu of my Asus Xonar DGX and found that I was actually using the speaker mode instead of the headphone mode (10 ohm), I'd read that the speaker mode was 110 ohm output impedance. I plugged some numbers in and now my perceived volume matched results are aligning a lot closer to my measured decibel matched results when taking into consideration the newly found output impedance which leads me to believe I think I have the right numbers now.

Thanks for your help !