August 8th, 2012 -- by Chris Martens
I first encountered Ray Samuel’s SR-71B in prototype form at a Can-Jam event back in 2010, and was fascinated by the little amp from the start (even though it was just a naked circuit board at the time). The concept for the product struck me, then and now, as being amazingly ambitious, in that it called for a very high performance portable amp that would offer sophisticated, fully balanced circuitry from input to output, plus sufficient output to drive just about any headphone on the market (save for electrostatic ‘phones, of course). In order to give the preceding sentence its proper weight and context, it helps to carefully consider two facts. First, most headphone amps with fully balanced circuitry are relatively large and expensive (think four-figure price tags). Second, despite what their manufacturers may claim, most desktop amps fail to do a truly credible job of driving “just about any headphone on the market,” either because they are too noisy to drive high-sensitivity ‘phones well or insufficiently powerful to drive truly difficult loads. Is it possible, then, for a portable amp to succeed where many full-size desktop amps have failed?
The answer, in the case of the SR-71B ($650), is “Yes”, though with a few caveats, which we’ll discuss in this review. $650 is a lot of money to pay for a portable amp, but arguably a small price to pay for a headphone amp—regardless of configuration—that promises do as many things well as this one does. And speaking of small, let’s note that the SR-71B is truly tiny, as in about the size of a deck of playing cards. So, the key questions we need to answer in this review involve sound-quality (how good is it, really?) and the amp’s ability to deliver on the promise of being able to drive nearly any headphone on the market.
To answer both questions, we evaluated the SR-71B by using it with a variety of source components, some with balanced-outputs, and with a wide range of earphones and headphones. At the risk of getting ahead of ourselves, let us cut to the chase and tell you right up front that we think the SR-71B builds a very strong case for being one of the two finest portable headphone amplifiers currently available. To learn why that’s our assessment, read on.
- Fully-balanced, “quad mono” circuitry. The SR-71B was the first portable amplifier in the world to offer fully balanced circuitry.
- Circuit implemented via extremely high quality balanced output op amps, supported by premium quality resistors, capacitors, etc. The amp features an ALPS volume control.
- Switch selectable single-ended and balanced stereo inputs.
The single-ended inputs are implemented via a 3.5mm input jack, while the balanced inputs are implemented via a miniature, square-shaped, four-pin connector.
A number of specialty cable makers offer stereo XLR-to-miniature 4-pin adapter cables that make it easy to connect high-end home or pro-sound source components to the SR-71B.
A special phase-splitter circuit within the SR-71B ensures that, even when single-ended source components are, the fully balanced headphone outputs will still be properly driven.
- Switch selectable left and right-channel master gain switches, with settings form low, medium, and high gain.
- This feature lets users adjust the overall sensitivity of the amp to match the sensitivity of their earphones and headphones.
- For example, you might use the “Low” or “Medium” gain settings with high-sensitivity custom-fit in-ear monitors, but the “High” gain setting with very hard-to-drive full-size headphones.
- Single-ended and balanced stereo outputs (implemented, respectively, via a 3.5mm mini-jack and the aforementioned miniature 4-pin balanced connectors). Whenever the amplifier is powered up, both the single-ended and balanced outputs are live. By design the amp provides circuitry that converts single-ended input signals into balanced signals, ensuring that the full benefits of the amplifier circuit are always brought into play.
- Built-in quad Lithium-ion battery packs. The battery packs provide 18V peak-to-peak swings in single-ended mode and 32V peak-to-peak swings in double-ended mode. According to Ray Samuels, the SR-71B’s maximum voltage swing capabilities are roughly twice those of any other fully balanced portable amplifier.
- Wall wart-type battery charger with charging status lights (“red” means “charging”, while “green” means “fully charged.”
- Available in satin black and matte silver finishes.