These days, pro audio's end products often live in the "Headphones World." The home stereos of the past have been supplanted by the mobile devices of today. Fewer listeners have full-size speaker systems, and much listening is done through various grades of "personal monitoring," be it earbuds or headphones of varying quality levels. The key takeaway for an audio professional: A final master needs to sound good in the Headphones World. To accomplish this goal, excellent headphone monitoring in the studio is key.

All too often, the headphone output on a recording interface is an afterthought. And forget about plugging ‘phones into the mini-socket on your computer, because then you are at the mercy of the built-to-dirt-cheap-price sound interface on the motherboard. Some high-quality DACs, like those from Benchmark Media Systems, have superb and powerful headphone amps built in, but that's somewhat rare. The common practice for a headphone output on a USB interface, DAW, or analog mixer is to use a pre-packaged Texas Instruments (or generic) headphone amp chip. These can actually be useful because they are about the quality level of better consumer-level headphone drivers, but they're not refined enough to trust for recording, mixing, or mastering.

Radial Engineering, the Canadian company famous for its direct boxes, 500-series modules and frames, and other problem-solving products, bought the Hafler brand from Rockford in 2013, and more recently acquired Dynaco, David Hafler's famous brand from the 1950s. Those who know consumer audio history will recognize both names. David Hafler (1919-2003) was a pioneering audio entrepreneur. With partner Herb Keroes, Hafler started out in 1950 with the Acrosound line of output transformers for tube amplifiers, designed for the then-revolutionary Ultra-Linear output circuit topology (Google that term for details). In 1955, Hafler partnered with engineer Ed Laurent to form the Dyna Company to produce audio components (preamplifiers, power amplifiers, and eventually tuners), sold assembled and as kits. Many audiophiles of certain ages have had first- hand exposure to Dynaco Stereo 70 (ST-70) tube or Stereo 120 (ST-120) solid-state amps. To this day, Dynaco amps retain value — even wreckers with intact transformers. Several companies still produce replacement circuit boards, and a vintage ST-70 with intact iron can be made to sound very clean and modern with an upgraded power supply and driver circuit. The main knock on the original Dynaco was that the products were built to a common man's price-point, and thus the power supplies tended to be saggy and the drive circuits tended to be more focused on a low parts- count than such things as steady tube bias and optimal drive of the output tubes and transformers. That said, Radial Engineering's history of Dynaco — included among the documentation of its Hafler products — claims more than 350,000 ST-70 amps were sold.

Dynaco was sold to Tyco in 1966. David Hafler continued on briefly at Dynaco, and then worked for cartridge maker Ortofon for a few years. In 1977, he founded the David Hafler Company, which he sold to Rockford in 1987. Audio pros may recall the Hafler 9505 power amp. Designed by Rockford engineer Jim Strickland, it was a heavy-duty but musical amp favored for driving big soffit-mounted monitors in the big-studio heyday.

Bringing history up to the present day, Radial's relaunch of the Hafler brand is through a line of heavy-duty small-profile "accessory" devices for the pro and high-end consumer audio markets — four flavors of phono preamps and two headphone amplifiers. The latter are the subject of this review.

Both of Hafler's headphone amps include a feature that will be very handy to some audio engineers, particularly for mixing and live-recording using headphones. Called the Focus control, it is described by Radial Engineering President Peter Janis this way: "The Focus is a matrix that blends the two inputs together and then remixes them to the center. This creates a bigger middle, which simulates listening to speakers in a room." By comparing a mix, or even a two-mic live recording setup, with the Focus control switched in and out, an engineer can optimize his or her end product for both speakers and headphones. The control range, to my ears, veers from "a little more center" to "quasi-mono" with most of the sound energy in the middle. Because of the input and output setup of both amps, it's possible to take an unbalanced feed from the headphone amps, with the focus control engaged, and, for instance, run a separate 2-mix with a different-sized center. I'm not sure how useful that would be in the real world, but the possibility is there to run a separate headphones-only mix that moves some of the sound energy between the ears, and keep the wider spread for a speakers-only mix.

I asked Janis about the target market for these amplifiers. He said: "Our primary target for these is the recording engineer. Today, with the incredible proliferation of headphones, you can no longer mix on speakers alone. You need to hear your mix through headphones. And with so much production happening in smaller home-based studios, being able to work quietly is a necessity. This is why we developed the Focus control. This matrix simulates the effect of listening to music in a room."

Hafler's two headphone amps are very different circuits and are aimed at different users. The HA15 is an all-discrete (no integrated circuits) solid-state amp with what Radial calls "quasi-Class-A output." It produces 20 dB of gain, maximum output power of 3 V RMS, and can drive headphone impedances of 8–400 Ω. Janis says both amps can actually drive headphones up to 1000 Ω, but "because different headphones have different impedance curves, we find that headphones over 80 Ω appear to be the most linear and have the least listening artifacts. In our testing, we have found that, quite often, headphones of different brands that are rated at the same impedance will produce dramatically different sound quality. Air and forwardness seem to be better with most brands of higher impedance headphones. We are still learning!"

The HA75 is one of the more deluxe standalone headphone amps in the market. Janis said, for the "majority" of the voltage amplification, it uses a 12AX7 tube with B+ voltage of 150 V — his point being that this isn't a "starved tube" circuit intended only to add "warmth" through harmonic distortion. The current amplification and output stage is all-discrete solid-state. Maximum output is 5 V RMS, and gain is 30 dB. Input is front-panel switchable between XLR (balanced) and RCA (unbalanced), allowing for two sources. There is a front- panel control for feedback around the tube stage. Keep it on the left side of the knob's rotation, and there is more distortion-controlling feedback, resulting in a cleaner sound. Turn it clockwise, and lower feedback results in varying levels of "warm," "phat" harmonic distortion. This may or may not be desirable for a professional audio engineer, but remember that Radial also aims the Hafler brand at audiophiles, with Hafler advertisements becoming commonplace in high-end audio magazines. In fact, Janis told me that a version of the HA75 with a built-in DAC, called the HA75D, is headed for production. It's worth noting that the HA75 also has unbalanced line outputs, so one could use the feedback control as a sound effect on the 2-bus or final mix, or on individual tracks. As always, Radial can't help but design in some hidden utility!

To check out both Hafler headphone amps, I drove their balanced (XLR) inputs from my Lynx Hilo interface [Tape Op #90] and listened to a variety of CD and high-resolution files. I also drove the amps' unbalanced inputs from a Cambridge Audio DacMagic XS USB interface connected to my laptop. I listened through both Audio-Technica ATH-M50 [#66, #113] and Sennheiser HD-650 [#43] headphones. Both amps were capable of driving the 600 Ω HD-650 as loud as I wanted to listen, but I'll say from the outset that the HA75 provided a deeper and more detailed sound quality at both the lowest and highest comfortable listening volumes. With the ATH-M50, the sound quality was closer between the Hafler amps, particularly when the feedback setting was switched out on the HA75. I found the Focus controls to be pretty darn close. (In other words, "10 o'clock" on both amps produced about the same stereo image, with a slightly enhanced center.) For pure listening pleasure, I liked customizing the Focus control, especially for older stereo recordings where stuff is hard- panned right and left and can be unpleasant when heard through headphones.

Compared to the headphone amp built into my Hilo, I much preferred how both Hafler amps sounded driving my headphones. I noticed a fuller sound at lower levels, and super clear and clean sound as loud as I wanted to drive them. The Hilo doesn't drive the Sennheiser HD-650 very well; it's designed for lower-impedance cans.

The built-in headphone amp of my Benchmark DAC2 HGC converter [Tape Op #97, #111] gave the Haflers a run for the money, sound-wise. But the Haflers, being purpose-built, win on features. It was informative, though, to compare the Benchmark when both Hafler amps had their Focus controls (and Feedback on the HA75) bypassed via the front-panel switches. It took some time to exactly adjust output levels for what sounded like a proper A/B/C comparison, using the Benchmark as the DAC and feeding the Haflers from the Benchmark's two unbalanced outputs. But once I had what sounded like the same levels driving the ‘phones, no matter what socket I plugged into, I heard a little bit of "flavor" from each different amp — not exactly the same sound qualities. I would describe them this way: "super-clean" but thinnest out of the Benchmark; "clean and fast" with "a little meat" out of the HA15; and "more colored but in a pleasing way, bigger without annoying exaggeration" out of the HA75. The differences were more pronounced through the Sennheiser HD-650, but I heard the same qualities through the Audio- Technica ATH-M50.

I think the HA15 hits the sweet spot for many pro-audio uses. The Focus control is a useful innovation, and the Radial- style build quality is tough enough for any situation where you'd dare take a microphone or computer. The high-quality, clean and powererful amplifier will be a step up for most setups, and you will likely get better mixes that play nicely in a wider range of listening environments.

The HA75 is a really nice piece of gear. I think, at that price point, and considering there is a semi-fragile tube inside, it might not be ideal for live recording or other on-the-go uses (it's also about twice as large and twice as heavy as the HA15). But as a headphone-playback system with sound-sculpting controls to enhance listening enjoyment, it's quite the thing. The sound quality is clean enough when the feedback control is bypassed to make it a reliable monitoring tool. I think the version with the built-in DAC will find a wider market. That said, I will miss listening to the review unit after it's shipped back to Radial. It's one of the nicest-sounding headphone amps I've ever heard.

One final note on the history of Hafler: Radial Engineering recently showed a prototype of the Dynaco ST-70 tube amplifier, a complete redesign that includes none of the original built-to-low-cost compromises (but also features a much higher price). Janis says more Dynaco-branded products are in the pipeline. It's nice to see David Hafler's legacy in capable hands, and a familiar brand return to the world of pro audio.

Tape Op is a bi-monthly magazine devoted to the art of record making.

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