I have a pair of Sennheiser HD 570 (I believe 64 ohm), and a pair of Superlux HD 681 (32 ohm) headphones.
Both work fine with my old solid state NAD preamplifier that has a standard 1/4" female headphone jack. However, I've just purchased a vintage Conrad Johnson PV-7 tube preamp that does not have a headphone jack. The Conrad Johnson preamp does have a SECOND standard (RCA) OUTPUT, so it is capable of driving 2 power amplifiers at the same time.
I figured I could just use one of those cheap "Y" Adapters (2X Male RCA to 1X Female 3.5 Mini Stereo) to hook the phones to that second Conrad Johnson preamp output mentioned above.
But . . . the sound is just wrong, and I don't want to damage my preamp or headphones. I could be mistaken, but I don't believe the preamp is made to drive either 32 or 64 ohms from these RCA outputs.
I'm wondering what circuity is used in factory installed headphone jacks to accommodate the wide range of from 32 (or fewer) - 300 ohms that various brands and models of headphones present to the jack?
Yes, I know I should buy a headphone amp, but I just spent my upgrade budget upgrading my Musical Fidelity X10v3 tube buffer. Maybe I can use the tube buffer to impedance match?