[petwoip]

Here's my setup: I have a JEC TC-716 stereo source selector (audio switcher). This switcher has 4 pair of RCA jacks and 2 stereo 3.5mm jacks. I use all four RCA jacks, one from my computer (which has a Realtek ALC889 audio chip), one from another computer in the room with the same specs, and one from a Wii. The final RCA jack goes to a Swan M10 2.1 speaker system. Usually I have the audio go from my computer to the speakers, but other configurations work just fine. In fact, it's possible to have the sound from my computer and the Wii playing simultaneously, each at half the normal volume.
 
Anyway, today I plugged my Sennheiser HD 518 headphones into one of the 3.5mm jacks and the volume is about half of what it would be if plugged directly into my computer. I have to compensate for this volume loss by turning my computer volume to 80/100 versus the usual 40/100. Also, I made sure that the switcher is only active for the computer and the headphones, so I know there is no interference with another device. The audio quality seems to be ok, but I don't completely trust my ears and it's very difficult to do a blind test because switching the setup between trials takes too long.
 
So what I want to know is, why is the volume lower for the headphones when connected to a switcher, and would a headphone amp ensure high volume and quality? I'm not very knowledgeable in this area, but I'd appreciate a technical answer in order to learn how these things work.

 

[stv014]

Since the switch box allows more than one source to be selected at once, it probably connects them to its output through resistors, which is not an issue for line inputs, but makes headphones (that have much lower impedance) quieter and also sound somewhat different because of the added source impedance.

 

[petwoip]

Ok, and just so I understand, a headphone amp would be useful because it provides enough voltage to counter the increase in source impedance, preserving the source? Would it make sense to get an amp that is plugged into the wall instead of a portable amp?

 

[stv014]

Quote:

Ok, and just so I understand, a headphone amp would be useful because it provides enough voltage to counter the increase in source impedance, preserving the source?

 
With the amp, the impedance of the switch box would not affect the headphones, since it has its own, ideally much lower output impedance. That is, it does not just amplify voltage, but also current, and it should have a high (kΩ range) input impedance and low (usually less than 100 Ω, but often less than 1 Ω) output impedance. The voltage drop from a non-zero source impedance is by a ratio of load_impedance / (load_impedance + source_impedance). So, if the switch box uses 100 Ω resistors, your headphones have 50 Ω impedance, and the input impedance of the amp is 10 kΩ, then connecting the headphones directly to the switch box reduces the voltage by a factor of 3 (50 / 150), but on the amp input it is only by about 1% (10000 / 10100).
 
Quote:

Would it make sense to get an amp that is plugged into the wall instead of a portable amp?

 
Yes, if you find that convenient. You just need to be careful to avoid ground loops with computer sources and AC powered amplifiers. This mostly affects larger amps with a 3-prong AC connector, while wall wart powered amps (e.g. the O2) are normally not grounded and not affected by such issues.

 

[petwoip]

Thanks for the help! This is making a lot more sense now.