A question about preamplifiers and headphones

I have a NAD C162 preamplifier (I also have NAD C272 power amplifier and a NAD CD/SACD M5 player). I wonder if I can purchase Sennheiser HD 598 headphones and plug them into the headphone plug on my preamplifier, or whether I might need to purchase a separate headphone amplifier in order to listen to these headphones. I have received contradictory advice about this matter. I would love not to have to purchase a dedicated headphone amplifier. I mainly listen to classical music in my own living room.
It will function, but I suspect that it won't be optimal sonically, and conceivably also in terms of the peak volume levels that can be reached cleanly (which may be an especially significant consideration for well recorded minimally compressed classical symphonic music).

The relevant specs on the headphones are:

Impedance: 50 ohms
SPL: 112 db (1 kHz, 1 Vrms)

Those are fine. However, the specs on the C162 headphone output are:

"Phones: 100 ohms" (it's not clear if that means output impedance, or recommended load impedance, or minimum recommended load impedance).

"Phones: 190 mv into 8 ohms" (it's not clear why they are specifying output into 8 ohms if the output impedance or recommended load impedance is 100 ohms or greater).

Regardless of how the 100 ohm figure is interpreted, it suggests that driving 50 ohm headphones with the C162 will not be optimal sonically. Although the degree of compromise MIGHT be small, depending on how the impedance of the phones and the output impedance of the preamp's headphone output vary as a function of frequency.

-- Al
dedicated headphone amp is recommended regardless of headphone sensitivity or impedance. you can start as low as from little fiio and feel huge benefit.
My tube headphone amp (from Ebay..."Bravo" 12AU7 brand, although there are various versions out there) from China took a week to get here and cost around forty bucks shipped. The power supply died and they shipped me a new supply free of charge right away. Luckily I have an extra set of outputs on my preamp for it and it all works really well...drives a pair of Grados easily with tons of headroom. I had no idea this thing would be so good and all I added was a supposedly better tube.
I have a very similar setup with the NAD C372 integrated and Sennheiser HD580s. The impedance mismatch is even worse with the integrated (220 ohm output impedance, if I'm remembering right) to the 300 ohm impedance of my Senns. But the combination sounds surprisingly good. I also considered buying a headphone amp (and owned a nice tube headphone amp in the past) but have been very satisfied with the NAD's performance for as often as I use headphones. Like me, you could always leave a headphone amp as a future option and enjoy what you have meanwhile--a pretty happy medium!

One note if you haven't absolutely decided on headphones yet: I used to own the Senn HD555/595 (prior models to the current HD598) and they are not as neutral or highly resolving as the HD580/600/650. Also, with the NAD my HD555/595 had grainy treble and huge mid-bass bump that my HD580 does not have--the HD580/600/650 is a much better match.

That said, the HD580/600/650 are a class above the lower Senn lines, and if you need to keep the headphone budget down you can pick up the HD580 for ~$130 used and the HD600 for ~$220. The HD580 and 600 have the same drivers; I've owned both and am very happy with the 580 now. Hope this helps.

Check this out, too, and search Head-Fi for NAD and Sennheiser:

The headphone output is usually an afterthought in the circuit...usually a poor quality chip. I am using an Ayre preamp into a dedicated amp for my 650's with astonishingly great sound.
Thanks for your advice. I did find a good deal on Sennheiser HD 600, and I'm now listening with them through my NAD C162 preamplifier (to classical music). It sounds quite good to me. Do you think a dedicated headphone amplifier would make much actual difference? I'm loving the sound as it is. Perhaps I'm just not that sophisticated.

Congrats on the purchase! Glad to hear you're enjoying the Senns. It sounds like you're a man doing well.
The HD600's are forgiving with poor amplification, but it
doesn't mean they don't benefit from a proper headphone amp.
They scale well with a dedicated one.