The LM4562, which was later rebranded as the LME49720, is a very high precision opamp. It provides stellar performance (low THD, even into a low-impedance load, low noise, reasonably low DC offset, etc.) at a very competitive price. The OPA1612 does provide lower noise and lower distortion, though not meaningfully lower. It is also considerably more expensive. Furthermore, the OPA1612 contains an internal compensation circuit, which results in a phase wobble in the open-loop frequency response of the opamp. This makes the OPA1612 harder to use - especially in a composite amplifier, such as the output stage of the HPA-1.
In a composite amplifier, a precise amplifier is used to provide error correction on a less precise amp. In case of the HPA-1, the output stage is a 300 MHz, high output current, current-feedback line driver. It actually provides quite low THD on its own, but its DC precision needs improvement for it to be used as a headphone driver. The LM4562 takes care of that and further reduces the THD of the output stage. Using the LM4562 in a composite loop with the line driver also drastically lowers the output impedance of the amp. These things combined is what drives the performance of the HPA-1.
Also - in the interest of full disclosure - I do have a warm spot in my heart for the National Semiconductor parts. I used to work for National Semiconductor. I worked for three years in their Precision Amplifiers Group (designed the LMP2021) before I was reorganized into their Precision Timing Group, where I worked on the LMK03000, LMK04000, LMK04800, LMX2581, and some other parts. I was with National/TI for a little over a decade.
That said, I like the old Burr-Brown guys and their work too. The OPA16xx are from the former Burr-Brown (gobbled up by Texas Instruments in 1999; TI gobbled up National in 2012).
Anyway… At the <-120 dB THD level, things like resistor materials matter. I use ±0.5% and ±0.1% tolerance metal film resistors to ensure that the resistors do not add any distortion of their own. Similarly, the one capacitor in the signal path is a Nichicon Muse UES-series bipolar capacitance, which offers exceptionally low distortion.
The ALPS RK271-series “Blue Velvet” volume pot was chosen for a few reasons:
- It’s very well known and well-regarded in audiophile circles
- It provides a good feel, which leads many to associate the HPA-1 with quality (as they should based on its measured performance)
- It offers excellent channel matching
- It is available (albeit as a custom order item from ALPS) in a low-impedance version (lower noise)
The main drawback of the RK271-series pot is that it does not offer any way to mechanically secure the pot to the PCB (aside from the six pins that connect the electrical signals). This means the pot has to be secured mechanically somehow or the use of the pot will cause the solder joints to fail over time. Thus, I added an internal sub-panel or bracket to hold the pot and to provide stress relief on the 1/4" output jack.
Neutrik recommends that the output jack is supported by a panel or bracket, but many manufacturers go without it (as you can see in the various tear-downs of amps). You can see the bracket in the picture below.
The switches are of high quality and sealed so they cannot become contaminated over time. The switches are not in the signal path - all the signal switching is accomplished by relays. They should last a lifetime.
The relays used for signal switching have gold-plated switch contacts. They were selected for their ability to reliably switch low-level signals. The relays are low sulphide types and are sealed, which means that the risk of switch contamination is extremely low. Unfortunately, that also means that the HPA-1 contains about $30 worth of relays! They’re decidedly non-cheap!
Even at the $899 price point, I cannot justify the design and setup cost of a custom designed chassis, so I went with the nicest chassis I could get for a reasonable sum of money. Even at the relatively low quantities (QTY = 100 per order), they’re still surprisingly expensive. That said, even with its “upper class military” ruggedness (thanks @lost33), the chassis is pretty darn nice. The surface finish is consistent and smooth. The front and rear panel have a slight matte texture, which presents really well. The internal length of the chassis is accurate to within ±0.18 mm (which is amazing!), though the manufacturer commented that they’re usually more precise than that. 
The PCB is mounted on standoffs that are pressed into machined blind recesses in the aluminum bottom. This ensures that the chassis is grounded (it’s a safety thing).
Regarding the features: The feature set was developed from a market analysis I conducted last year. I had about 100 respondents indicate their preferences and expectations. To my surprise, and contrary to my expectations, a vast majority did not want a 4-pin XLR output, but did want an XLR input.
Going with a 1/4" output only also made it feasible to squeeze the amp into the current chassis. Had I needed to support both 1/4" and 4-pin XLR outputs, the chassis would have needed to be larger (-> higher cost, higher cost of packing materials, higher shipping cost, etc.) And, honestly, there aren’t really any meaningful technical advantages of the 4-pin output. Sure. You can get higher output power, IF the output driver supports it. And you get to skip a few mΩ of shared ground impedance. But that’s it.
Now from a marketing perspective, it’s another story. The 4-pin connector is large and in charge and screams “this is a serious amp” more so than the 1/4" connector. And should I choose to release an even-higher-end amp than the HPA-1, it will certainly have a 4-pin output (and be in a larger, heavier, and prettier box, etc.).
I really wanted to hit the sweet spot on the price/performance ratio with the HPA-1, so I chose to spend money where it would provide the most benefit: Circuit board, electronic components, and circuit board assembly. The raw board is manufactured in Ontario, Canada (just outside of Toronto). I buy genuine parts from authorized distributors and deliver them and the circuit boards directly to my assembly house, located a 15-minute drive from my house in Calgary, Canada. They do amazing work.
The chassis is nice but not outlandish. Similarly, the packaging is nice but not outlandish. I pack the HPA-1 in a 200-lb rated white cardboard box. The HPA-1 is braced by two foam end caps. The white box then goes into a plain cardboard box (with proper foam bracing) for shipping.
I hope I’ve struck a good balance between an “engineer’s amp” and an “all-marketing” amp. Time will tell.
You can find the HPA-1 here: https://www.tomchr.com/products/hpa-1
Note that I updated the page last week, so it now shows all the performance measurements in the “Performance Graphs” tab rather than as a downloadable report.
Tom