Got the Hifiman EF5 recently. $399 from Audio Advisor. I tried it with both my K601 and a borrowed Hifiman HE5LE.
The EF5 is a hybrid amp. 12AU7 input stage, and OP275 output stage. Apparently the main reason for its existence is to drive Hifiman's orthodynamic headphones such as the HE5 and HE5LE. Hifiman knew that most headphone amps don't have the power to drive these fairly inefficient orthodynamic phones.
One thing that struck me immediately about the EF5 is the incredible dynamics, the effortless power in transients. Far beyond any other amp I've used, it made the K601 very lively and sound incredibly extended, full of presence.
However, there was an irritating and fatiguing quality in the highs of the K601. I tried putting 120 ohm resistors in series with the output of the EF5. I suspect the EF5 has a near zero output impedance, but the K601 is specified to work with the IEC standard of 120 ohm output resistance.
With the series resistor, the fatigue disappeared! It still doesn't have the smoothest highs, but at least there is very little fatigue. I can listen to it cranked up forever.
The EF5 is a bit "grayed-out" in instrumental timbre and "feeling quality" compared to the main amp I've been using, the DNA Sonett. The Sonett is a SET and I think SET's are the king of "feeling quality."
But I'm thinking of having Igor at K Works Audio do a custom mod of the EF5. Igor has worked with this amp before, and he says it's an incredible value. For $399 you get some exotic parts and an exotic design in the power supply. But, the amp also has cheap parts. Therefore, it's a good base for a mod. Igor has some tricks up his sleeve--- he uses a particular opamp in most applications, and has some special combinations of bypass caps. In his DAC, the SuperBerry, the result is so good that you would never guess a tube-lover like me would end up loving an opamp-based DAC.
So, about the HE5LE headphone--- I actually don't like it as much as the K601. The K601 has a more open and expanded headstage and seems more dynamic to me. That sense of effortless power in the transients---well, I get more of it in the K601. I also get more timbral color in the K601.
Oh, one note---I haven't really let the HE5LE burn-in yet.That could change everything.
Mike
