I have previously written about changing my views on the impact of a good headphone amp, and the THX AAA 789 and later the Benchmark HPA4 (with DAC3B).
But I needed to set up two rooms. I was happy with the Benchmark stack and could have replicated that, but in the interest of science I decided to try another system. Chord gets most attention for their DACs, but Rob Watts also has an interesting perspective on driving headphones: he wants to minimize the number of active stages and the parts count, so he makes the DAC analog output stage itself capable of driving a headphone, with no separate amp stage. Intriguing. So I decided to get a Chord Hugo TT2.
Interestingly, the TT2 and DAC3B+HPA4 are comparable: DAC + headphone amp, both ambitious, both in the $5,000 range. And interestingly, the companies have taken very different approaches: Benchmark separates the DAC and the headphone anp, Chord integrates them tightly; Benchmark uses a standard ESS DAC chip and surrounds it with careful engineering, Chord uses custom code in an FPGA; Chord’s digital implementation avoids substrate noise so they don’t need differential amplifiers to cancel the common-mode noise, they can use a single stage running single-ended, if you absolutely need or want a balanced output they add a second amp stage but Rob discourages it, says the extra stage reduces transparency.
So we have two systems, comparable in goals and price, opposite in every aspect of design and implementation. I found both deeply satisfying.
(Chord’s DAC design is certainly interesting too, especially in combination with the M-Scaler which doubles the price, but I won’t discuss that here, this is a headphone amp discussion.)
Headphone amps face tough challenges, because headphones differ in impedance and sensitivity much more than speakers, sometimes by orders of magnitude. People focus on power, but that’s not the whole story, some cans require heavy current, some high voltage, some have widely varying impedance which can disturb a poorly designed amp (see Benchmark’s paper https://benchmarkmedia.com/blogs/application_notes/12838141-headphone-amplifiers-part-1). I have been a long-term Audeze user and recently got the new LCD-4z, which has lower impedance (15 ohm) than their classical LCD cans (LCD-4 is 200 ohm)—the 4z can be driven by the battery-powered Hugo 2 so I can listen in the garden, very cool, and the sound is outstanding. But happy as I am with the LCDs, especially with their robust bass suitable for my favorite acoustic jazz music, I wanted to explore a different style, and I chose the Hifiman Susvara, more airy and light, more like electrostatics, but still with robust bass. But the Susvara is extremely difficult to drive: listed at 83 dB (and 60 ohm), while the LCD-4z is 98 dB (and 15 ohm).
Both amps handle the LCD-4z well, of course. With the Susvara, I found the Benchmark worked well. But amp power ratings and headphone impedance is tricky. I found I preferred the HPA4 volume control 20-25 dB higher than with the 4z, in spite of the official 15 dB efficiency difference. But this makes sense: the volume control is actually managing voltage, not power, and because power is voltage-squared/R and the specified sensitivity is relative to power, when we quadruple the impedance (from 15 ohm to 60 ohm) we need to double the voltage to keep the same power, and double the voltage is 6dB, so we need to crank up the volume 21 dB (15 + 6). With the Benchmark I play the Susvara at -4 dB, but not to worry, it goes to +15.
With the TT2 it was more complicated: the single-ended output is marginal with the Susvara, it can get loud enough but doesn’t control the bass well, gets sloppy. But the balanced output has significantly more power (18W vs. 7.3W @ 8 ohms) and works well with the Susvara. (The TT2 has supercaps which help with heavy short-term current needs.)
(Btw, the THX AAA 789 which started this whole thing can’t drive the Susvara adequately.)
Both amps, and both headphones, are fabulous. I may be fooling myself by playing louder than normal, no way to calibrate the sound level, but so be it, the fact that both amps and both headphones can play very loudly without stress, without the slightest distortion — that’s where the whole amp story is justified.
Which is better? This is not a product review, I’m not a reviewer, and why would you care about my opinions? This is a discussion of interesting technologies in an important area. Two interesting but different headphone amp designs, both DAC-amp combinations are excellent, both headphones are excellent, all four combinations work well.