Hi, I’m a complete newbie having just got into this wonderful hobby - sorry for the basic question.
I’m running a Cambridge Evo 150 from Roon Rock on a Nuc and want to get a decent set of headphones such as the Sennheiser HD800S.
Do I need a seperate headphone amp, and if so, why, and what will it give me?
The amp already has a headphone output, so you don’t need a separate one.
IME, HD800 (and the “s” version) can be a little harsh and bright with certain amps. I don’t have experience with the Evo 150 so can’t speak to whether it would be up tp the task or not. I personally went through many amps until I found one or two that gave me what I was looking for, YMMV. It will depend on how you like the HD800s with the built in amp.
A concern would be that it looks like the Evo 150 only has a 3.5mm jack so may be a weak point for connecting the HD800s. Unless you have a cable terminated in 3.5. I believe the HD800s usually comes with 6.5mm and Balanced 4.4mm cable. I own the HD800 not the “s” version and bought a balanced XLR cable for it…
I would imagine the 3.5mm headphone socket on the evo is more of a convenience than for serious listening. I had a quick look in the manual and the output spec isn’t specified.
The 800s, like the 800, are known to be on the bright side. Not sure how class D is going to help tame that. Tubes will help tame it some. High Impedance Sennheiser generally sound better when properly driven on a tube Amp in my experience. I’ve never had the 800s but have had a trusty HD650 for some years and it’s a much better listen on a tube Amp, as are other dynamic high Impedance cans generally.
If you are prepared to drop the cash on a pair of cans at the price of the 800s then be prepared to drop some on a suitable dedicated head Amp to drive them properly. No point in buying a sandwich without the filling is there?
However, the evo doesn’t have line out / tape loop so am unsure as to how you can connect a dedicated headphone Amp to it.
Only way to confirm if the 800s are suitable on the evo is to try i guess.
You can get a 6.35 to 3.5 adapter.
The headphones are quite revealing so you can try them with your amp using an adapter such as this
Just noticed that you have not bought the headphones yet.
If the sound seems fine to you then all good.
I have had a look at the manual for your amp and it has a pre out rca connection so you could try an external headphone amp and see how much it improves the sound for yourself.
Have you got a local dealer to try some out?
No idea what your budget is so hard to say.
Running a headphone amp off of a pre-out really is not recommended unless you are trying to fry your gear or ears.
Or both.
Plenty of headphone amps have rca inputs and volume control, which if hd800s are in the mix are reasonable to budget for and if that takes Op budget too far then spend money on less expensive headphones and an amp combo to suit.
Other amps use plenty of different options, optical,usb, etc.
I would say to op to check on his local dealer if he has one.
I fail to understand how OP can connect a headphone amp to the evo.
They will have 2 gain stages in play if they run it from the pre out connection - that is designed to control a power amp, not another pre amp.
Unless the speaker are disconnected from the evo then there will be output on both amps.
Even in that scenario the volume on the evo would have to be managed so as not to overload the input on the headamp.
The short story here is that OP has an amp that is not designed to run an additional headphone amp on. It has no RCA tape / monitor loop.
Each to their own but I would advise the OP not to run a headamp from a pre out connection.
In general, it is recommended to invest in the transducers (speakers and headphones) first, as they are the weakest link, so to speak. Since you’ll be doing home listening, you should go for higher impedance headphones.
It will be fine until you decide it isn’t because upgrade-itis kicks in.
If you think the 800 is too bright you can tame it with roon DSP without resorting to trying some mythical amp matching.
There’s a lot of nonsense talked about headphones and amps. Try it first then see if it lacks anything compared to your speaker setup.
I put a slight bass shelf uplift on my 800 and as I’m over 60 don’t notice the supposed brightness due to my aging ears.
As in a things audio your mileage may vary 
Thanks for all the comments.
I think what I’ll do is talk to my local ‘bricks & mortar’ store, try a pair and see how I get on.
Not sure he has listened to them yet to know if he feels they are bright or not but as he is seeking a dealer to try for himself then I am sure he will know which he prefers.
I would say, don’t decide after a 15 min listen as what initially sounds impressive might get less so after an hour or so.
Headphones also need to be comfortable so what suits one person might not another.
If I were a “complete newbie”, I’d be a bit nervous going into an audio dealership. Make sure you stick to the plan, get only headphones and stay within budget.
Well the headphones are circa £1500 in U.K. so will have plenty to choose, but you can’t buy headphones without trying them for fit in all senses of the word.
No point having great reviews for headphones that to you sound terrible or are too uncomfortable after 20 mins
You don’t need one but could possibly benefit from a dedicated headphone amp. I’d say try the HD800S at a dealer with different amps and try them at home on your EVO.
I have had the HD800S and stilll have the HD600 running a Feliks Audio Echo MKII tube amp and there is a huge difference between the Echo MKII and the headphone out of my CXA81.
I never tried before buying.
That is up to you.
I always do for every piece of hifi I have ever purchased.
There are many headphones that I listened to that I would never in a million years want to have even if they were gratis . Some were just uncomfortable to wear even after a short while, too heavy and clamping force too high.
I did have to return a pair of Sennheiser because of clamping, but that was it. Being able to return is essential to online shopping.
Just make sure you can return them for a full refund if you don’t like them. I ordered my Focal Clear from headphones.com and they have a one-year return policy. Fortunately, I love them.
To the OP, you might want to use your headphones in a different location. If so, you can use a Raspberry Pi 4 running RoPieee to connect to your network with ethernet or WIFI. Then, you need a headphone amp/DAC to connect to the RPi4. My Chord Mojo 2 is the best I have ever listened with.
https://headphones.com/products/focal-clear-over-ear-open-back-headphones
