I’m quite familiar with the Wolfson WM8740 in the HM-650. The piece of equipment that’s stayed with me more than anything other than my AKG K271 is my Gamma2, which uses the WM8741. The typical Wolfson warmth stays true with the HM-650. I noticed a slight bit of roll off at the harmonics with the HM-650, which kind of makes the “Vintage” switch, which lowers the treble by 3dB, a bit too much for most headphones.
It sounds kind of silly, but the HM-650 actually sounds like the Sennheiser HD650 (oh, wow I just noticed that) of players. The treble is a bit subdued, but the midrange is lush and forward and the bass is slightly elevated.
Treble, as I’ve stated, is a tiny bit rolled off, as it is in many WM874X implementations, but it’s tasteful in an OTL way. It does not sacrifice detail and is in fact a good bit more natural than the FIIO X5, which I felt sounded slightly robotic. I’ve yet to try the iBasso DX90, but before anyone asks, if it is SABRE implementation is anything like most others, it’d be a different signature—the DX90 likely has more energy and presence whereas the HM-650 tries to meld the treble to be as inoffensive as possible and integrate it with the rest of the sound.
The midrange is unquestionably forward, making it a joy for most headphones lacking slightly in mids (though don’t expect miracles; they couldn’t save the V-MODA M100) but a bit too much for something like the Sennheiser HD650 and, at times, the OPPO PM2. Timbre is excellent, as expected from something with an amp module that seems to have much thought put into it, easily besting the X5 and even, from my very brief experience, the Astell and Kern AK100 MKII—I wish I could add more impressions about the AK, but I only heard it briefly and only remember being pretty disappointed with the midrange, which sounded artificial.
Bass is where the HM-650 does strangely well for a WM8740 implementation. A great number of WM8740-based players and DACs I’ve heard struggle with bass, blurring it slightly. But the HM-650 had surprisingly capable separation thanks to its fantastic amp section. Not only is the bass well-executed, it manages to be this good while still being slightly elevated. I wouldn’t call it a bass head’s dream, but it’s easily the most engaging DAP I’ve used.
Now comes the bad part: with the balanced card, using any IEM was a like a game of roulette. The HM-650 has so much gain, and the pot has such drastic changes in volume that sensitive IEMs are basically unusable. I’m sure the regular card or better yet, the IEM card is a much better choice.
But the HM-650 is an absolute treat with planars. It drove my ZMF Blackwood with ease and headroom to spare. Of course, the HM-650 doesn’t do the job as well as my EHHA, but it’s shockingly good for a DAP. While the X5 struggled with even the HD650, I actually could drive the HD650 on low gain.
I’m at a bit of a loss as to how to describe the HM-650. It sounds fantastic—for “only” $100 more it’s a solid step up from the FIIO X5. The only problem is that its battery life is kind of pathetic and makes it hard to recommend as a portable DAP, not that its size makes it very portable anyway. Though it is much more manageable size-wise than a DAP/amp stack, which is what I believe Hifiman aims to replace with the HM series. I actually use the HM-650 paired with the Blackwood (usually with the charger in my backpack) as a library setup with great success. But I did occasionally get called a “hipster with a cassette player.”