This is a review and detailed measurements of Sennheiser HDV-820 DAC and Balanced Headphone Amplifier. It is on kind loan from a member. The HDV-820 costs US $2,400 with Prime shipping from Amazon. Settle down! I know you can't afford it but maybe it is good.While the enclosure itself is not that unique, Sennheiser nails the feel and look at the controls:Usability is not quite there though. I literally had my thumb pinched between the volume control and input selector. In reverse, trying to change the inputs, your fingers may hit the volume control.Other than that, the volume control, selector and especially the power button with its white LED ring are quite nice. The light pipes for the input indicators bleeds to adjacent labels which is unfortunate.As you see, you have every type of headphone output and then some. Back panel shows a lot of inputs and outputs as well:Nice to see analog inputs on a DAC+Amp combo.Lacking is AES digital input in this price range.There are no gain controls. Just turn up the volume and it will keep getting louder. This likely will hurt its signal to noise ratio.As usual, we start with our dashboard with XLR Output:XLR output seems to be tied to headphone out as it could go as high as 11 volts output! I dialed it down to 4 volt for above measurement. Distortion+noise is in the competent range:Signal to noise ratio is not as good as it should be for this price range:Jitter is very good:Multitone test shows low intermodulation distortion:But when we measure that relative to level, we see the clear signature of ESS DAC chip:We have the classic "ESS IMD Hump" and higher noise in general.The headphone amplifier works with analog input and output. But I thought I measure it in combo with the DAC first:We have good bit of power but disappointing distortion and noise level. Above is with the volume turned down. The higher the volume, the higher the noise level. They should have implemented a gain setting.For these tests I fed the HD-820 with XLR Input (in the back -- don't confuse this with XLR Headphone out). I started with 1/4 Headphone output:This is not good. There are a lot of amps that do better:Signal to noise ratio is nothing to write home about:Expect to hear some hiss with sensitive IEMs and headphones.Frequency response is dead flat and hence excellent:But this doesn't mean you get flat response from your headphones because of this:45 Ohm impedance means that unless your headphone has a flat response (e.g. planar magnetics), its frequency response is going to change. This may make it sound better or worse. It also means lots of power will be wasted into low impedance headphones. We will see this effect shortly.Here is power vs distortion into 300 ohm load:Wow. The gap in noise and distortion is quite massive relative to our state-of-the-art headphone amps we can get today for as little as US $99. We do have lots of power though so dynamic range won't be an issue.Issue you will have with low impedance loads like 33 ohm:This is just bad for a desktop headphone amplifier.Comparing 1/4 "unbalanced" headphone output to "balanced" XLR Output we get:I was surprised how much cleaner XLR output was. But then it got distortion even more than 1/4 headphone out. You do get four times more power with XLR output which is always good to have.Sweeping the input frequency and measuring distortion and noise we see quite a rise:Distortion rises to almost 0.2% at 20 kHz! This is crazy high for any amp let alone an expensive one.I started my testing with the 25 Ohm MrSpeakers Ether CX headphones. That was a mistake. The HDV-820 produced the worst sound I have heard from them by far. It was distorted, lacked bass and dynamics and overall screwed up. Yes, that is a technical term. It is the opposite of not screwed up.The situation drastically changed with Sennheiser HD-650 using 1/4 headphone jack. There was tons of power and dynamics. And whatever change the HDV-820 made in my short listening, seemed to be a positive.Per our recent policy, I am testing devices for a few minutes until they get warm to see how they behave on distortion+noise. This is the HDV-820:Yes, you are seeing what I am seeing. The channel in blue had a mind of its own just sitting there. Performance degraded as much as 5 dB on its own. There was no correlation with temperature during this 25 minute test. Probably not audible but sure demonstrated lack of good engineering.At the US $2,400 price point I expect perfection. Sadly are not remotely there with Sennheiser HDV-820. The DAC is fine but stability is not. The headphone amp with its high impedance and hence low power/higher distortion makes this amp suitable for a small set of headphones. With many headphones you are going to experience a different sound than the headphone is meant to produce.I think Sennheiser should take out the amp, license a THX amp and then they would have something proper. Even then, US $2,400 is a lot of money.Needless to say, I can't recommend the Sennheiser HDV-820.------------As always, questions, comments, recommendations, etc. are welcome.We probably not going to get sun for another 5 months here. Depression will likely set in soon and heaven knows you don't want me depressed. The only solution to it in my book is lots of money in my pocket. So pleaseas much as you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/