Pros: Small, fully balanced, surprisingly powerful, rechargeable battery, silent background with IEM, very high gain available for low voltage sources.
Cons: No room for single ended output for those that have both SE and balanced headphones, & cannot be adapted to single ended output either.
RSA F-35 Lightning
HISTORY: My first portable headphone amplifier was the Ray Samuels Tomahawk in 2007. I needed it because my Shure E4c were not very efficient and needed more power to wake them up or energize them, and my Sennheiser HD600 were much more demanding and needed significantly more power than my iPod could supply - without the amplifier the HD600 felt dull and bland, while the E4c lacked impact and dynamics. An amplifier transformed them both, and thus began my Head-Fi journey.
Since then I have grown to appreciate what a good amplifier can do for my listening enjoyment, especially with full size headphones designed for home use. I have owned and heard dozens of different portable amplifiers, and even went on to do a review of 13 different portable DAC/amps a few years ago before I settled on my favorite pairing of Pico DAC, DACport, and Audioengine D3 as sources + the RSA SR-71b and Pico Slim as my two main portable amps). I would use the Pico slim with just a small iPod Nano, or the Pico DAC+Slim with an iPad camera connection kit, the DACport or more recently the D3 for most headphones on my Macbook, with the SR-71b added into the mix for the more demanding full-size headphones. Most of the other portable gear has fallen by the wayside. I don't recall the last time I used my Meier 3MOVE or Headsix, iBasso D4, or Practical Devices XM5. And many others that were not as unique have been sold a long time ago.
Portable headphones often sound good without an amplifier when used with a decent portable music player, but even then it was easy to observe that adding an amplifier to most portable gear would provide an upgrade in sound quality, even in cases where the extra power was not needed. A lot of gear has a nicer sounding line-out than the headphone out jack, so an improvement (quality) could be realized by adding an amp, even if you didn't need more power (quantity). Only in the past couple of years have we begun to see portable DAPs that have both excellent sound quality and power output levels; but these can cost as much as $1000 or more, and may require additional time to convert your music into the proper format when you already have it all loaded onto your iPhone or iPad in Apple Lossless format.
The challenge with many portable amplifiers is that they often run at 5 volts and didn't have enough power to do justice to full-size non-portable headphones such as HE-500 and LCD-2 orthodynamics. And they may not have enough voltage swing to properly drive the high impedance HD600 and HD800. They still improved things over the headphone out of an iPod, but they didn't rival desktop amplifiers. When traveling or mobile, it was difficult to get the full desktop rig experience in a small travel bag or backpack. A few years ago only an occasional portable amp could manage to do that, such as the Meier Corda 3Move, Meier Stepdance, or balanced RSA Protector. When these portable amps came out and could drive these difficult headphones it was a revelation, albeit with some compromises here and there.
When the RSA SR-71b came out it blew everything portable out of the water when using the balanced output. Although the 1/8" jack was not quite as detailed, open, airy, or spacious as the RSA P-51, the balanced output was noticeably ahead of the P-51 in power and punch with a similar level of transparency. Single ended is very good, but not exceptional like the balanced output. Via the balanced output I considered the SR-71b to have sound quality on par with some more expensive desktop amps like my maxed Woo WA6, ALO Amphora, CEntrance DACmini and HiFi-M8, or Nuforce HAP-100. And amazingly the SR-71b sounded better with my HD800, while packing more punch with my HE-500 and LCD-2. To have more power coming out of a portable amplifier than some well respected desktop amps was unheard of, and to match the sound quality was just as impressive.
Over the years of ownership of the SR-71b I found that I was not using the 1/8" output at all, which meant wasted space on the front panel. Ray Samuels apparently also found that many people using portable amps with 1/8" outputs were mostly using portable headphones, and that most people using the SR-71b were using full-size balanced headphones. So he developed the F-35 Lightning that was smaller and more convenient to carry, while packing a similar punch to the larger SR-71b with the majority of popular full-sized headphones.
FEATURES: The F-35 lightning gives up the 1/8" jack that I wasn't using, which allows it to be lighter and about 2/3 or 3/4 the size of the SR-71b. It still offers the glowing red power switch, balanced output, three levels of gain (3,6,21), single ended or balanced inputs, built-in rechargeable battery, and a 3 year warranty (1 year warranty on the battery). The charger is also 8.4v instead of 16.8 volts. I have never tested the battery life, but I have used both amps all day long and not had to plug them back in to charge before I was done listening for the day.
The Lightning is a dual-mono design, and in balanced mode the right channel and left channel are completely independent. The single ended input uses a phase splitter to convert the signal into a balanced signal, and there is very little difference in sound with the two inputs if the source is just as good via either SE or balanced.
GEAR: I used an iPhone 5S and Macbook Retina Pro as music transports. I used an Apple camera connection kit with Pico DAC or the CEntrance HiFi-M8 as sources for the iPhone 5s digital audio. For the Macbook I used the PS Audio Perfectwave DAC Mk2 or Stello DA100 Signature for their balanced outputs, plus the Audioengine D3 or CEntrance DACport as portable laptop 24/96 sources to feed into the F-35 Lightning. I used an ALO jumbo Cryo Silver mini-mini interconnect for single ended input from the portable DACs, and a Moon-Audio Silver Dragon balanced input cable from the balanced desktops DACs.
I used several headphones for testing, including JH Audio JH16 Pro Freqphase with a balanced TWag II cable, HD600 with a balanced APureSound V3 cable, HD800 with a balanced Moon-Audio Black Dragon cable, HiFiMan HE-500 and Audeze LCD-2 rev2 with a Silver Dragon cable, and HE-6 with a stock HiFiMan PCC-OCC cable.
SOUND: The sound quality is quite good by any standards, portable or desktop. With your eyes closed you simply would not know that you are listening to a portable headphone amplifier. With IEM the Lightning has a completely black background with no hiss, and good channel balance at low volumes in low gain. In medium gain the left channel comes in a little sooner than the right, but good balance is still achieved at fairly low volumes on the volume pot.
When compared to it's more expensive sibling, the SR-71b, the F-35 Lightning is only slightly more forward sounding but never in your face or too upfront. The micro-detail, ambience and air is just as good, so a large venue or large space is not lost on the listener - only the position relative to the stage changes between the two amps. The F-35 might be closer to the front row seats than 5-10 rows back like the SR-71b or ZDT desktop amps. This makes the F-35 sound slightly more involving and vivid at times, and sometimes more intimate too.
But it never pushes you up onto the stage, and never sounds closed in with any headphone - the Lightning typically offers a good sense of depth, imaging and separation. The soundstage is only as large as what the headphones you're using can offer, but even when switching from the HD800 to the HE-500 or LCD-2 rev2 the soundstage never becomes too forward or artificially small. You only hear that slight change in position to the musicians when you switch back and forth between this amp and the SR-71b or EC ZDT desktop amp; and without a direct comparison you might not ever know what you are missing, which is not much at all.
The bass seems to have slightly more mid-bass bloom and a slightly warmer lower midrange than my SR-71b, but the sound never becomes thick or troublesome. This is especially beneficial for headphones that have more of a tilt towards the high end like HD800, to give the instruments the proper weight and feel that they deserve. I've found it difficult to enjoy the HD800 with my Woo WA6 when it's got the right tubes installed for everything else, because the combination doesn't help the instruments feel solid and present. I feel the same way with the DACmini, where the HD800 can sometimes feel more fatiguing in addition to lacking some weight. The HiFi-M8 is a definite improvement with HD800 over the DACmini, but still not optimal like the Lightning.
But I'm listening to the HD800 right now via F-35 Lightning and balanced Perfectwave DAC, and I don't think anyone would ever complain that the HD800 are bass-lite or rolled off with this amplifier. Sure, the HD800 still don't have the impact of the LCD-2, but the HiFiMan EF6 is my only amp that can get the HD800 closer to the LCD-2's bass. The reproduction of string bass in jazz music is very fast with great texture, and with electronic music it throbs and surges as it should. With Infected Mushroom the bass reproduction from my HD800 is just as strong via the Lightning as it is with my Eddie Current ZDT desktop amplifier! Switching to the LCD-2 brings the bass output to a whole new level - my vision is rattled and blurs by the time I can hear distortion as I crank it up to dangerous volume levels.
The midrange sounds rich, vibrant and life-like. I don't think I would change anything there. There is no suck-out anywhere, and it handles male and female vocals equally well. Pianos are very dynamic and realistic without ever sounding hollow, and saxophones sound rich and breathy.
The treble response with the F-35 Lightning seems to be just as refined as with the SR-71b and is never sibilant. Nor does it make the HD800 sound etched or fatiguing as many amps are prone to do. The Lightning renders cymbals and snares with a realistic shimmering timbre and tone, with good extension. Guitars and strings are crisp and detailed, and electric guitars are crunchy and scratchy when called for. Neither amplifier could be called bright either, and in fact the F-35's treble is very slightly muted in presentation when compared with the DACmini or HiFi-M8. However, I would also never call the F-35 dark sounding or rolled off - it sounds good with HD800 and LCD-2 alike, although with slightly less treble presence than my DACmini or Eddie Current ZDT desktop amplifiers. Again, something you wouldn't have considered on it's own without having done a direct comparison, and a simple switch of tubes to a TungSol 5751 will bring the ZDT's treble in line with the F-35. So, there is no fault there either.
POWER: Good bass impact is often associated with amps that offer higher power output, and while the F-35 lightning cannot drive the very inefficient HE-6 to the same high volumes as the SR-71b, it does to drive the HD800, LCD-2 and HE-500 to insane volumes before hearing any distortion. Maximum volume can be reached in medium gain with a strong balanced source, and high gain is only needed when using a weak source like an iPod line-out. Overall it's maybe only about 3-4 dB (maybe 5) less powerful than my SR-71b or ZDT before the LCD-2 start to show distortion. That makes the Lightning quite a bit louder than the Pico Slim, or even the RSA Tomahawk, P-51, and Protector that I used to own. The Lightning will cause permanent hearing damage if you don't monitor your listening volumes and duration properly - I'm pretty sure that I briefly hit 120 dB several times with each headphone, although I tend to listen at less than 90dB (I have a Radio Shack dB meter that I've used in the past).
SUMMARY: The fact that I can even compare the Lightning to my $2700 ZDT side-by-side and still enjoy it this much is a testament to how good this little amplifier is. Does it sound like a $2700 amp? No. But neither is it put to shame by the ZDT when driven by a high-end balanced DAC. This is an amazing portable amplifier.
This is an amplifier that I can pair with a high-end source and full-size headphones, and it scales up to perform at a level you'd expect from a more expensive desktop amplifier. Even when paired with my new HiFi-M8 it offers slightly more power and increased synergy with HD800 in a small portable package. And it sounds quite good with a budget portable 24/96 USB DAC/amp such as the D3 or DACport as the source (used as a pre-amp). The DACport and Audioengine D3 sound fantastic for the price, and they go everywhere my Macbook goes, but adding the Lightning between the DAC and the headphones takes them to a whole new level with full-size headphones and IEM alike.
If a person already has a nice portable amplifier with single ended output but find that they don't have enough power for their more demanding headphones, I would skip the SR-71b and go for the less expensive F-35 Lightning with a re-cable or re-termination of the stock cable, assuming that you aren't crazy enough to be driving the HiFiMan HE-6 in a portable rig. I just don't see the benefit to a portable HE-6 over the HE-500 or LCD-2 rev2, but you can still enjoy the HE-6 at moderate volumes with the Lightning if you must (i.e. louder than average). Moon-Audio makes a wide variety of cables compatible with the Lightning's balanced output, including short adapters to convert the Kobiconn 4-pin jack to a mini-XLR, 4-pin full size XLR, or Dual 3-pin balanced jacks if that's what you use on your headphones. Other cable makers like Ted The Cable Pro are following suite. I think it costs about $150 shipped for a three foot balanced Silver Dragon input cable.
If a person wants to save money on a desktop rig because they also need to be portable, pairing the Lightning with a good quality DAC or DAP is a reasonable way to go as well. If you need a portable DAC to feed into the Lightning, the tiny Audioengine D3 is hard to beat, and it can drive most headphones after you've run down the battery on the Lightning. And if you need a portable amp that has good synergy with the HD800 I would highly recommend the Lightning, although it's just as good with phones like HD600, HE-500 and LCD-2. It can be difficult to find the right amp for the HD800 that also sounds good with everything else, without making the HD800 fatiguing after long listening sessions. My Pico Slim sounds great with the HD800, but it doesn't have the voltage swing to do them justice.
The only person who might benefit from the SR-71b over the F-35 Lightning, other than the insane HE-6 user, is someone who needs to drive single ended portable headphones but cannot afford both a single ended amp and the F-35. However, there are affordable single ended amps that will sound a little better than the SR-71b in single ended mode - so the Lightning plus a smaller single ended amp would be more rewarding for most audiophiles, although a little more expensive than the SR-71b alone.
I rated the Lightning as a 10/10 for Value and Quality. I gave it a 9/10 for Audio Quality, but would have picked 9.5 vs other portable amps if that was an option. I also gave it a 9/10 for Design, partly because the headphone cable blocks access to the gain switches when it's plugged in - I would have liked to see the output jack closer to the side of the face-plate so that the gain switches could be next to the power switch. This would have also made it easier to turn it on and off when a headphone is plugged in, if you have fat fingers. Likewise, the input switch and single ended input are covered by the balanced input cable, so you can't leave it connected to both sources and switch back and forth. I understand that some features are dictated by the requirements of the PCB layout on an amp this small, and there are limited choices when implementing the design, so it didn't get a 7 or 8 for that reason.
(I'll come back later and post some photos of the F-35 Lightning next to the iPhone, D3, SR-71b, PIco Slim and HiFi-M8)