I plugged my IEMs into my new Framework and noticed they were sounding way bassier than usual. After ruling out OS audio processing, I tried measuring the output impedance to see if that was the culprit. I played a sine wave at a lower level (to not overload the amp) and measured the output unloaded and across a 30-ohm load.
Correct me if I’m wrong, but
50mv unloaded
18mv loaded with 30 ohm resistor
30 * (50/18-1) = 53.3 ohms output impedance.
Honestly pretty disappointing. I was hoping I wouldn’t have to sacrifice a whole USB port just to get a usable audio output for my cheap IEMs.
For those that don’t know why, any headphone with a low, non-flat impedance curve will have an altered frequency response (it’ll sound different) due to the changing ratio between the amplifier’s output impedance and the load impedance at different frequencies. This primarily includes a large variety of IEMs that use balanced armature drivers, or have multiple drivers with a crossover. A good headphone amplifier should have output impedance below 1 ohm.
Update: Just borrowed the FW13 AMD I got for my dad this Christmas to measure the headphone jack, and got the same result.
Although I guess it wouldn’t help with your desire for high-quality audio unless you could package a decent DAC into the expansion card, too. At first I was more thinking about the Framework 16 which doesn’t have an aux port at all.
I’m certainly not expecting high power output in this form factor, but my baseline for acceptable headphone output has been the Apple USB-C to 3.5mm adapter. It does 1v into 32 ohm without trouble, and has output impedance of 0.3 ohms for about $10.
This would be cool for FW16 owners as well, since it would free up an expansion slot.
That’s only the actual headphone jack from what I can tell, not the audio codec itself…
Although there are a few additional components on that board including what looks like transistors and a whole ribbon cable connecting it to the main board so I’m not sure if it’s entirely hopeless. Perhaps someone more knowledgeable on circuits can chime in; I don’t seem to see the part on the schematic.
Interesting topic. I did the output impedance measurement and can confirm 50ohm. I then took apart my FW13 AMD traced it out, off the audio board and right next to the realtek there are two 47.4ohm resistors.
Not sure how flexible the ALC295 is but these resistors are definitely external of the IC and could be changed. Maybe Framework could improve this on future mainboards by lowering the value.
They are even pretty big ones so changing them should be relatively easy. Headphone jack is working well enough for my ears so I am certainly not going to do that but following this out of pure curiosity.
