A couple of years ago I discovered a one-man audio company in New Jersey called K Works (Igor Kuznetsoff). I purchased some of his products, and discovered they are really, really great for my tastes.
These days I use headphones exclusively. In the past I have owned several headphone amps. I've heard and owned several capacitor-coupled SET designs, including the Zen Little Buddy and the Wheatfield HA-2. However I really fell in love with SET when I heard a classic implementation of it, the DNA Sonett. (By classic I mean relatively simple design and transformer-coupled output.)
But always interested in “the new,” I asked Igor at K Works if he had ever heard a headphone amp that really grabbed him. For another customer, he did a recent custom modification of the Hifiman EF5 and it worked out very well. Let me explain what the EF5 is. It's a hybrid amp with a 12AU7 input stage, and a solid-state output stage that uses an op amp and some discrete transistors. It's VERY high-power for a headphone amp (probably several watts) and that is by design: Hifiman produced this amp to drive its inefficient orthodynamic headphones such as the HE6LE. It drives the K1000 easily. But the amp works just fine with common-variety dynamic headphones.
Igor knows I LOVE good dynamics, and he said the stock EF5 had explosive dynamics. It's only $400 from Audio Advisor, so I ordered one. Using it with the AKG K601 headphone, the crispness and definition of transients (like leading edges of instrumental attack) were stunning. Instruments and inner voices were clearly separated. But, it was a bit over-eager, presenting transients with too much emphasis, and could get very raucous in big orchestral climaxes. It also was weak in musical emotion compared to my DNA Sonett (the classic SET I mentioned above).
But, Igor explained that it was a really good value, with high-quality parts and sophisticated power regulation. It just had a few weaknesses in parts and design that could be easily remedied.
So I took the plunge and commissioned Igor to mod it. Mainly he planned to upgrade the power supply, the capacitors, the op amps, and internal wires. He didn't put in a fancy volume pot (there's hardly any room in the chassis). He probably did other stuff too. Basically I trusted him to do the most important things. The mod was $1600 (so plus the $400 for the amp, total cost was $2000), although I hesitate to mention this number because this sort of job can be extremely variable in cost and I don't want anyone to assume this is what he would charge for a similar mod in the future.
I got the finished EF5 a few weeks ago, and I've tried it with both the AKG K601 and K701. (As a side note, I discovered that the K601 needs a little series impedance with the output to take the edge off its tendency to fatiguing highs—about 20 to 30 ohms removes that fatiguing quality beautifully well.)
This amp just explodes with musical feeling. It presents details and dynamics with high resolution, but it does more than that. To the extent it magnifies the resolution, it also magnifies the musical feeling. If, on a particular recording, it has ten times the perceived resolution of other amps I've heard—then it also has ten times the musical feeling of other amps I've heard.
This is not such an obvious point. I've heard lots of equipment that is detailed but without much musical feeling. Say I doing an experiment by comparing two devices, A and B. And I decide that B has more resolution, but not more musical feeling. This is common. In fact that's my opinion of the comparison between the K601 and K701—the K701 has something like four times the resolution and imaging capability, but no extra musical feeling. Maybe even less musical feeling.
My modded EF5 presents musical feeling with absolute clarity. Tragic music is intensely tragic, joyful music is intensely joyful. There's also a type of musical feeling that doesn't really fit categories like “tragic” or “joyful”--it's the passion that one senses in the swelling and ebbing of the dynamics. In the modded EF5, this passion is so intense that I couldn't possibly miss it.
If you were 90% in a coma and just minutes away from death, but somehow I got you to listen to passionate music on this EF5, and just a tiny bit of that sonic signal reached your consciousness, you would feel it. It's that clear.
In describing the passion in the dynamics, the technical term “microdynamics” is useful. This term describes how clearly the equipment presents small dynamic changes. Small dynamic changes may be small, but in classical music (and other types) they are a powerful source of musical feeling. It's not enough that my amp tells me analytically the loudness level is changing—I must FEEL the effect of this change. And that's what the modded EF5 does.
It also has good treble extension, so that instruments are much higher-resolution. Say you're listening to a string quartet. You can hear all the subtle modulations of attack and tonal quality. But I have to say again—you FEEL these modulations. It's not enough to hear them. One must feel them.
It has good bass extension, detail, and especially IMPACT and POWER. You can't really feel headphone bass in your body, but you can potentially tell how deep and powerful a drum is, or hear the nuances in tonal quality on a stringed bass.
It makes the musicians sound like better musicians. That's a litmus test for me. If listening to a familiar recording on a new device (be it amp, DAC, power conditioner, etc.) makes me think, “I didn't realize before how good the musicians are,” then I know I've found something special.
In the photo below, you see the power supply and amp. The stock volume knob was tiny and had nearly invisible markings, so I had K Works replace it with a skirted knob and clear markings. The little box at the end of the gold cable adds variable series resistance to the output, which turns out to be critical for getting the best sound from AKG headphones. The amp main box is sitting on pneumatic isolation feet (not part of the stock amp).
A view of the internals. At the back you can see a strip of ERS paper. Caps have been upgraded and bypassed.
