iBasso PB5 Osprey

The PB5 Osprey is a balanced Korg NuTube portable headphone amplifier made by iBasso and is comes out at the same time as their new D16 1-Bit portable DAC. Since I already owned the DX320 Max, I only purchased the PB5 and used an ownership discount to get the product at a lower price than the $1599 retail price of this unit, which sets out to compete with Cayin's C9 NuTube portable amp and others.

I reviewed the C9 a while back and enjoyed its smooth and enjoyable sound, but disliked how bulky it was. The PB5 is a bit smaller and much lightweight. It's actually surprisingly lightweight, especially without its included green leather wrap-around case. This makes it more portable and more transportable, although it is still thick.

Dimensionally, its very similar in size to the DX320 and DX300 MAX units, in all aspects, and shares similar width and length as the Fiio M15S, but double it's height. This makes it easily stackable with either of the DAPs I use.




The PB5 comes with a bounty of accessories, including 2 sets of 4.4mm to 4.4mm balanced connectors for input/output connection from your source to the amp, the green leather case, a carrying bag, and USB cable. The PB5 cables have a very unique bullet-looking connector shape that I've only seen iBasso use and make and makes it stand out.

There is a downside though, as the larger shape of the connector head makes it sometimes difficult to turn the adjacent volume knob. The knob takes on the same stepped attenutation feature of the 320 Max here, which I found quite annoying. It's slightly less annoying here for whatever reason, but it still cuts in and out when changing volume and can also be stuck between steps causing imbalance.





The other annoyance I found is that USB Charging only works with standard USB-A to USB-C cables. USB-C to USB-C cables and power delivery or even quick charge sometimes don't work. It likes to get old skool power feed. It also will automatically shut-off charging as soon as the battery levels max out, and then start to drain battery even while charging. I'm sure this is a feature to prolong battery life, but I also wish there was a way to bypass the battery altogether, or have some sort of smarter feature here to keep it plugged in and going.

You can, however, unplug and replug the cable after it's started to drain and it'll start charging again. To combat this feature, I ended up buying an in-line on/off switch for my USB-C cable so I can just toggle it on/off to start charging again without having to replug it each time.




The other thing to note on this device is that it is balanced input only! That means you'll need to use a source that is capable of feeding it sound via 4.4mm cable, whether that be using the included 4.4 to 4.4 cables or by an adapter for XLR to 4.4mm or some other connector of your choice.

It does, however, output music to both 4.4mm balanced and standard 3.5mm stereo, so you can use this with any headphones or in-ear monitors of your choosing, or output it to an amplifier, powered speakers, or whatever you want.

Sound​

When I first put this unit on, I was met with an overly bassy and partially muddy sound. It was overwhelmingly focused on the low end, but after about an hour, the Korg NuTubes seemed to have stabilized and it was much cleaner sounding, but still featured a deep subbass boost from what I normally listen to. That was the part that definitely stood out quickly when paired with my neutral monitors, the Subtonic Storm IEM.

It was so much of an effect that I had to measure it to make sure I wasn't just fooling myself. After measuring it on the Storm, the Dunu Zen, and the Tin T5S, I discovered that there was a bass shelf added to the system that started around 100Hz down to subbass regions, with about a 3-5dB addition.







This, coincidentally, was what I had been toying with on my Subtonic Storm recently -- seeing what a small bass shelf addition in the 20-60Hz region would do and I ended up liking the results quite a lot and had been playing with this EQ lately. With the iBasso PB5, I no longer had to mess with software EQ, as it made almost the exact same EQ naturally.

It's not just a bass quantity boost though. The amplifer also added a more 3D holographic space and with a touch of additional upper treble air. These are things that weren't measurable in a simple FR graph, but it was something I heard in comparison to just listening to either my iBasso DX320 Max or Fiio M15S without the amp connected.

Adding this extra layer of bass and imaging may not work with all headphones and IEMs. With the already bassier Sony MDR-MV1 headphones, I didn't necessarily feel like it added much, and only made the bass levels more than necessary. With the Unique Melody MEST MK3 IEMs, I felt the darker tonality was heightened with the PB5 amp and it's already forward soundstage made smaller for some reason.

It does shine with more balanced and neutral-tuned products though, and I absolutely love the extra fun-factor it adds to the Storm and the Dunu Zen. These two products really benefit with this amplifier, given them an extra oomph to bassy tracks, and smoothing out the treble even more.

Final Thoughts​

The PB5 was an amp I wanted to test out at a discounted price and I've been very happy I've tried it. It's been a wonderful new addition to my semi-portable audio chain and one I take with me to work everyday to use. It's a perfect pairing with the Subtonic Storm and the Dunu Zen IEMs, and is definitely something I recommend to check out if you are interested in adding a more punchy and dynamic amplifier flavor to your current audio chain.

The PB5 Osprey is iBasso's answer to the question of the best possible portable amplifier with analog tube magic. Of course, no "real" electrode tubes are used here, as these would be too large in a portable device and their waste heat would also be too high. Instead, so-called nutubes are used, two of them in fact. These type 6P1 nutubes are manufactured by Korg and Noritake and are already in their sixth generation. Similar to a vacuum tube, a nutube has an anode grid structure and works exactly like a triode vacuum tube. Similarly, it also produces the same characteristic rich overtones, the unique warm tube sound with excellent linearity. But compared to a conventional vacuum tube, a Nutube requires considerably less energy, is significantly smaller and lasts up to 30,000 operating hours.



The PB5 is a real powerhouse. With an output of 2x 1.7W @32 Ohm provided by a total of 6 rechargeable batteries, it can drive almost any pair of headphones. Even my Hifiman Susvara! The 24 position stepped attenuator already used in the DX320 MAX Ti is used for volume control. This incorporates the so-called stepped attenuators. By using high-precision resistors, the difference between the channels can be limited to +/- 0.1 dB, which is significantly better than with conventional potentiometers. A total of 24 steps are possible. The behavior of such a setup means that turning the knob initially cuts off the sound, creates a channel imbalance or distorts it until it is "locked" into the next step. The advantage of the PB5 compared to the DX320 MAX Ti is the clearly more perceptible rasterization up to the next step. This means that you almost never reach a position where the left and right channels drift apart. This is something I would have liked to see on the DX320 MAX Ti. The OPAMP is an OPA827 from Texas Instruments, known for its superb performance and very analog sound.



As with the DX320 MAX Ti, the 6 batteries can be replaced very easily. The housing is simply unscrewed with 2 screws and the batteries can be replaced directly. There is a USB-C port on the back for charging the amp. No QC or PD chargers are supported, only standard USB chargers with 5V. The PB5 is fully charged after around 2.5 hours and then runs for around 10 hours. In addition to a power switch, there is also a switch on the back to switch between low and high gain. On the front is the 4.4mm input as well as a 4.4mm and 3.5mm output for headphones and IEMs.

The PB5 is supplied in a high-quality box that is appropriate for the price. This contains the PB5, which in turn is stored in a leather pouch. Another velour bag contains a USB connection cable and 2 high-quality 4.4 mm cables in two different lengths. From the outside, the cables look like a mixture of pure silver and copper. The plugs are very classy and solid and contribute to the high-quality overall impression.



The PB5 itself is protected by a green leather case. The workmanship of the PB5 is very good and the design with the glass panel on the top, which offers a direct view of the illuminated nutubes, is very successful in my opinion. I would have liked to see a more matching design to the DX320 MAX Ti, i.e. silver with golden controls and a brown leather case. However, the PB5 has not only been developed specifically for use with the DX320 MAX Ti, but can also be used with any DAP that offers a 4.4 mm lineout. With the DX320 MAX Ti you just have to be careful not to use the highest gain level 4, as this can lead to clipping. I only ever use gain stage 1 on the MAX, as this gives me a much wider control range, especially when using very sensitive IEMs.



To test the PB5, I naturally use my reference, the DX320 MAX Ti, as a playback device. I tested several headphones such as the brand new Austrian Audio Composer, Focal Utopia 2022 and Susvara. For the IEMs, I used the Radon6, Xenon6, Trifecta and Raven. The PB5 is absolutely dead silent with all headphones. Only with the Trifecta can you hear a minimal hiss in the background, but this no longer plays a role when the music starts. It is important to expose the PB5 to a longer burn-in phase. I recommend at least 100 hours, preferably 150 hours. At the beginning it still sounds a bit constricted and the bass only opens up after about 30 hours and then slowly reveals its full potential. The nutubes in particular apparently need this longer break-in phase.

The difference in sound between direct connection to the MAX Ti and PB5 is immense. There is no need to make any major comparisons. You can hear it directly. The MAX Ti has the characteristics of a transistor amplifier with a very clean, transparent, fast and very resolving sound. The PB5 conjures up richer timbres with more texture and body, especially in the mid-range. The already very holographic imaging of the MAX Ti gains even more breadth and depth here.




Personally, I attest to the MAX Ti's very neutral reproduction. The PB5 takes this reproduction two steps into warmer realms and emphasizes the bass range a little more, which is more energetic and not quite as soft. My first test for the PB5 was the Austrian Audio Composer. And from the very first second, I just thought that can't be right. It fits like a glove! The very analytical Composer, which can sound very bright depending on the source, gets the necessary foundation in the mids with the silkier highs through the PB5 and sounds more musical, more involved, better than I've ever heard it before! Even my stationary system with the Niimbus US4 has to admit defeat to the mobile chain with the PB5 here. The Composer needs a tube, absolutely. You have to hear it. It's like a Utopia on stereoids. But in my opinion, the Utopia 2022 also gains in musicality with the PB5. And above all the necessary soundstaging, which is a class above, because it's bigger. If I set the MAX Ti to gain level 3 and the PB5 to high gain, even driving my Susvara is no problem. And the nutubes also fits really good on the Hifiman. Fantastic timbres, terrific soundstaging and a bass range with real punch. The 1.7W is good for the Susvara and the PB5 is therefore also somewhat more powerful than the MAX Ti. On the Niimbus US4 + D90 III, the Susvara plays a little less colorful but with even more control, especially in the bass range. Nevertheless, the PB5 is the best mobile amp on which I have heard the Susvara, great!

The Empire Ears Raven is also an absolute dream combination. A very technical IEM in terms of character with terrific bass, lightning-fast transients and a resolution like no other. The mids are very transparent, as the texture of the Raven is somewhat slimmer, and the PB5 gives it a real boost in the mid-range, so that everything sounds a little more contoured with more fullness and directness. I thought a Radon6 could also do with a little more warmth. And this also benefits from the PB5. With the now more analog-sounding mids, it approaches its brother, the Xenon6, in this area.

Does everything sound better with the PB5? In my opinion, the very warm and bass-heavy IEMs are not the best choice, depending on the source. A Xenon6 that already sounds very warm and very analog with a neutral source like the MAX Ti gets an overall picture that is too colorful for my taste with the PB5. The Trifecta, which I love for its full-bodied reproduction of every single sound element, gets another boost in the bass with the PB5, which is a bit too much of a good thing for me and makes the speed too bouncy. But I think you have to weigh things up depending on the source. The MAX Ti with its ROHM DACs doesn't exactly play on the very bright side. I think if you use a brighter DAP such as the FiiO M17 in combination with the PB5, the Xenon6 and Trifecta might be a better fit.

In my opinion, iBasso has created a really ingenious portable amplifier with a lot of power with the PB5. The price of 1500 USD is certainly no bargain. But if you are addicted to the tube sound and want to upgrade your DAP with a lineout, this is the right device for you. The only important thing is that the DAP also has a "real" lineout. In other words, a direct output from the DAC section and not one that also runs through the amplifier section. Because this so-called "double amping" leads to a loss of sound. Now I'm all the more excited to hear how a stack of the new D16 Taipan (1-bit discrete DAC) + DX320 MAX Ti + PB5 sounds!