iBasso PB5 Osprey NuTube Portable Headphone Amplifier Review

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.


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 more difficult to drive 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.