I often get asked, when can you just use the outputs on your CD player or Preamp etc to drive a pair of headphones?
There are several issues that need to be addressed in answering this question and they will be difficult to address completely without circuit knowledge for every piece of equipment asked about. However, in general:
If the unit has op amp or discrete SS output amplifiers and the stated output impedance is 100 ohms or less, then higher impedance 'phones like the Sennheisers and Beyerdynamics can be driven with little problem. Grados can be driven, but bass performance, in particular, will suffer.
As a rule of thumb, bass damping will suffer noticeably when the output impedance of the driving device is more than 1/2 to 1/3 of the 'phones impedance. So for 300 ohm 'phones, you really want to stay below 150 ohms if you are a bass freak. Non-bass freaks like myself are happy with the sound of a tube preamp with a 300 ohm output impedance driving 300 ohm 'phones, though.
Most line powered equipment is going to power its analog circuitry from +-12v or +-15v supplies, so output voltage limitation isn't a factor for 'phones, except for the the likes of the AKG K1000.
Here is also a very good source for calculating the question, "Will this headphone output be able to drive my cans"
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Hope this Helps
When can you just use the outputs on your CD player or Preamp etc to drive a pair of headphones?
