[spixel]

Headphone A
42 ohm impedance
sensitivity 100mV = 98db
 

Headphone B
18 ohm impedance
sensitivity 1Vrms = 113db
 
Headphone C
Unknown
 

When connected to source A (desktop pc onboard) headphone A is by far the loudest, followed by C and  B the quietest.

When connected to source B (mobile phone), headphone A still the loudest, followed by headphone B and C the quietest.


How can headphone B be much quieter than A when connected to one source, but only a bit quieter when connected to another?

 

[MindsMirror]

The source's output impedance can explain this. Source B may have higher output impedance than source A. A high output impedance will reduce the loudness of a low impedance headphone more than it will a high impedance headphone. Headphone B has a relatively low impedance, so the effect of the source output impedance will be relatively high. If headphone C is relatively high impedance, the source's output impedance will have relatively little affect on it.

 

[spixel]

Would the sound quality be affected at all by the high output impedance or just the volume? Is there any affordable amp with low output impedance that I could use with my pc to better suit low impedance headphones?

 

[cjl]

Sound quality can be affected, depending on how reactive of a load the headphones present (how much their impedance varies with frequency). As for amps, I don't know what you consider affordable, but I run an O2/ODAC with my computer due to atrocious amounts of noise and insufficient power from the onboard sound jack, and they do a fantastic job.

 

[MindsMirror]

The Magni 2 is another popular choice. Like the O2, it has a gain switch and low output impedance which is good for sensitive and low impedance headphones.

 

[castleofargh]

  Headphone A
42 ohm impedance
sensitivity 100mV = 98db
 

Headphone B
18 ohm impedance
sensitivity 1Vrms = 113db
 
Headphone C
Unknown
 

When connected to source A (desktop pc onboard) headphone A is by far the loudest, followed by C and  B the quietest.

When connected to source B (mobile phone), headphone A still the loudest, followed by headphone B and C the quietest.


How can headphone B be much quieter than A when connected to one source, but only a bit quieter when connected to another?

you need to give us more information about the IEMs, but as suggested it's most likely a change in signature from non flat impedance that may be quite significant and make you feel like the loudness was impacted(well it was, just not everywhere). if you give us the models and maybe the models of your sources, we may be able to tell you more(if measurements are available).