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Raw frequency response plots show the headphone having some difficulties sealing on the head causing the bass response to vary some when moved from position to position. These do measure more like on-ear cans in that regard. I suspect the seal will be better on real ears and I did hear bass extension as modestly better than measured.

Though a little wavy and uneven, the compensated response plot shows a mild warm tilt overall. There is a little discontinuity between 200Hz and 500Hz, but the run-up to…

Nelson Pass's Lab has produced an impeccable headphone amplifier with more than enough power, that's quiet, solid, and very clean. Several other reviewers have reflected on this excellent amplifier, but I want to weigh in with my unique sonic perspective and also measurements of its performance.

The Pass's sculpted front panel is very pretty and makes a distinctive show piece; there's no need to hide this baby. The rest of the…

I. The Genesis
I've been preparing to write this article for many years! In order to make this investigation, I needed to create a laboratory-grade distortion processor, which I've just completed designing and building. Let me introduce you to Bob's Blender (Fig 1). In a moment we'll discover its features and controls.1 But first a little background: The underlying topologies in analog amplifiers, preamps, and equalizers have been evolving for decades. At first there were only tubes: triodes, tetrodes and pentodes. Then, of…

Sennheiser HE 60 headphones: Open-back design. Transducer principle: electrostatic, push-pull. Frequency response: 12Hz–65kHz, –10dB; 18Hz–55kHz, –6dB; diffuse-field equalized. THD: <0.1%.
Weight: 0.6lb. (260gm, less cable).
Serial number of unit reviewed: not available.
Price: $1690 with HEV 70 amplifier (1995); no longer available (2020).

Sennheiser HEV 70 amplifier: Frequency response: 10Hz–20kHz ±1dB. S/N ratio: >90dB. THD: <0.01% (1kHz). Maximum output voltage: 400V. Input sensitivity: 250mV. Input impedance: 10k ohms. Bias…

Both the Sennheiser and the Stax headphones were evaluated over the same system. Their respective amplifiers were driven directly from a Mark Levinson No.35 D/A converter. A Krell KPS-20i CD player was used as a transport, and alternately from its own analog outputs. The digital link, when used, was a Kimber AGDL coax; the analog interconnects were Cardas Hexlink Five Series.—Thomas J. Norton

Stax SRM-727II ($2200)
This is an all solid-state amp with a zero-feedback output stage. From the Stax USA website:

Ray Samuels Audio A-10 Thunderbolt ($6500)
Bristling with features, the Thunderbolt is both an electrostatic headphone amp and a pre-amp. This amp is an all-tube design for the audio section with a separate solid-state power supply, and uses matched pairs of 5687 tubes for the headphones, with matched 6SN7s used for the analog outputs of the pre-amp. Two balanced inputs, three unbalanced inputs, and a tape and pre-amp output (both with balanced and unbalanced connections) grace the rear of the main electronics unit, which can rest atop or aside the separate power supply.

At…

Woo Audio WES (starting at $4990)
The Woo WES is the largest amp of the group sporting a quad of EL34 output tubes fed by a quad of 6SL7 driver tubes on the headamp, which can sit atop or aside the matching chassis of the main power supply. Two 12AU7 tubes mounted on the rear of the headphone amp perform phase splitting duties for the single unbalanced input pair. A front panel knob can select between two balanced inputs and the lone single ended feed. The amp I auditioned was the stock amp without any of the upgrades.

I must say the Jack Woo has wooed me towards tuby sounds…

Cavalli Audio Liquid Lightning (~$5000)
The amp reviewed here is the first version of the amp, which is now undergoing a second revision with an expected release in April of 2013. Alex tells me the new amp is largely a cosmetic revision and will be in a machined aluminum case similar to his Liquid Glass amplifier. This is an all solid-state design using MOSFETs as the power output devices. The amp has one each balanced and unbalanced input with a rear panel switch for selection.

The Liquid Lightning was, by far, the most difficult amp to get a grip on. Maybe it was because I…