Phatlab Sassy II Review

Phatlab Sassy II

Greetings audio fans. Today we’re looking at the Phatlab Sassy II portable headphone amplifier. Portable HiFi is having a resurgence in popularity of late as can be seen by the current furious competition in the DAP and headphone market.

  • Beautifully constructed from premium materials and components
  • Tubes!
  • Pristine, clear, powerful sound
  • Excellent battery life
  • Massive driving power
  • Comes at a premium price
Phatlab Audio website

While earphones and IEMs remain the best option for most when they’re out and about there are a lot of people using DAPs around the home so as not to be tied to their listening chair or desk. One’s home is also the ideal place for full-sized headphones but again people don’t always want to be tied to their desktop setups to enjoy music around the house and the majority of DAPs don’t have enough grunt to power high impedance or hard to drive headphones. The solution? A portable amplifier to connect to your DAP or even mobile phone so you can lie back on the sofa or watch the sunset from your balcony whilst enjoying your favourite music.

Phatlab’s Sassy II is a single-ended triode/solid-state hybrid portable headphone amplifier. Phew, that’s a mouthful! So why tubes and not pure digital amplification? Among all the approaches, single-ended triode tube topology has unique characteristics of amplification.

It produces rich 2nd harmonics, which is not possible in any solid-state amplifier. Since the single-ended triode topology is so simple, it will not introduce IMD (intermodulation distortion), feedback phase shift and all kinds of arguable drawbacks found in solid-state amplifiers. It maintains the clear, purer, and warmer (debatable due to 2nd harmonics) sound.

I met Mr Lin, the CEO of Phatlab Audio in person at CanJam Singapore 2017. We chatted over some coffee about the Sassy II, portable amplifiers in general and the future of portable audio. It was immediately obvious that he is very well versed and passionate on the subjects of electrical engineering and audio.

About Phatlab

Based in Taiwan, Phatlab was founded in 2015 by a husband and wife team. The company’s CEO and lead designer was a previous employee of ENIGMAcoustics and worked on the legendary hybrid dynamic and electrostatic Dharma D1000 headphone. This led him to recognize that the future of the audio industry is in headphones and so he set out on his own to create Phatlab.

Right from the beginning, Phatlab aimed high at the enthusiast market with the Phantasy, a pure class-A single-ended triode portable headphone amplifier and the original Sassy hybrid amplifier. The Sassy has now been superseded by the Sassy II and Sassy GT (Gold Tuned) amplifiers.

Disclaimer: This sample was sent to me for the purpose of an honest review. All opinions and observations here are my own based on my experience with the product. I have no affiliation with the company and do not benefit financially from this review. The Sassy II has an MSRP of $699 and is available for purchase from Jaben stores. For those in North America, you can contact Mr Michael Brown of RSM Communications and Final Audio US via email at


The Phatlab Sassy II portable headphone amplifier comes in a plain black box with gold print on the top. On the front side is a magnetic flap that lifts up to open up the box. Inside there is a “Getting started” booklet sitting on a layer of foam with a felt covering.

Lifting that out reveals the Sassy II underneath, seated securely in a similar felt-covered foam. Under the amplifier is a foam cutout which when pulled out reveals a space under the main foam body where you’ll find a USB charging cable. It’s a simple yet elegant unboxing experience but I would have liked to see some kind of 3.5 mm to 3.5 mm interconnect cable included so you had everything you need to get started right out of the box.


First impressions upon laying eyes on the Sassy II are of a premium product. It’s exquisitely crafted aluminum enclosure and machined panels give it a simple yet elegant look which is only exemplified when you hold it in your hand.

The Sassy II is designed with simplicity and ease of use in mind. On the front panel and surrounded by a protective roll cage (from left to right) are:

  • 6.35 mm headphone output
  • 3.5 mm line in
  • Power (green) and Low battery (red) LED indicators
  • Volume knob

The volume knob which also acts as the power switch has a really smooth action and is perfectly weighted with just the right amount of resistance that allows you to make precise volume adjustments. It’s the sort of knob that you would expect to find on a premium desktop or rack device, perfectly aligned and silky in its movement.

Thankfully the LEDs have just the right amount of brightness so they’re not distracting and don’t clash with the glow from the tubes. Thank you Phatlab for making the power light green because I’m so tired of blue LEDs!

On the back panel are (from left to right):

  • Low/High Gain switch (High: 0dB / Low: -10dB)
  • Charging LED (red)
  • micro-USB port (for charging)

Brushed aluminium adorns the topside of the chassis. It’s smooth and resistant to fingerprints to boot and surrounds the rear-printed glass viewing window that sports the phatlab logo and emits the warm orange glow of the tubes. On the underside are four rubber feet that keep the Sassy II firmly in place on your desk, table, counter or whatever you decide to place it on.

If there was one thing I’d change about the design of the Sassy II it would be the addition of a 3.5 mm headphone jack. All of my IEM’s and the majority of my headphones have 3.5 mm plugs and it would simplify the experience having the ability to plug directly into the amp without using a 6.35 mm adapter.


Frequency Response10 Hz – 100 kHz
Output power180 mW + 180 mW (300 ohm)


1000 mW + 1000 mW (30 ohm)

Headphone impedance30 ohm – 600 ohm
Gain SwitchHigh: 0dB / Low: -10dB
Power supply5V/1A via micro USB port
Operation time (on battery)Approx. 10 hours (depends on headphone impedance)
Charging time (>1A supply)Approx. 3 hours
Dimensions119 mm x 93 mm x 23 mm
Net weight275 gram

Let’s have a look at the internals. With a full-discrete solid-state output stage, the Sassy II is designed to work with any headphones or earphones regardless of impedance or sensitivity. The hand-selected military-grade JAN6418 tubes are responsible for the major amplification of left and right channels, each one taking care of the left or right side respectively.

When it comes to testing each individual tube there are two criteria to meet. The first one is the low microphonic effect. Using very low consumption and small size pencil tubes they are more vulnerable to microphonics than bigger size tubes, so they need to see the spectrum of every tube for low microphonics. The second criteria is the Gm ( transconductance) value or so-called voltage to current gain. For optimal performance, they need the tubes with the right Gm to fit into the right operating point at the circuits.

The Sassy II has a very low noise floor (generally below -110dB) and I’m told the reason is rooted in the unique circuit topology of Sassy (and all Phatlab products). Sassy has only two gain stages, one for voltage gain based on tubes and another for current gain based on high power transistors, all with discrete devices (meaning no integrated circuit) and without global feedback. This is why the selection of the perfect tubes is so important (only 30% pass the criteria for testing). The less gain stage, the lower the noise.

Phatlab’s amplifiers have extreme driving capability beyond the chassis size. Unlike traditional Op-Amp circuit topology, they do not count on feedback to make everything right. After screening tubes and making every transistor work at the right operating point, they supply them with higher voltage. It is the high voltage (>20 volts) that provides the extra driving capability on high impedance (>300 ohms) headphones.

Unlike many tube amplifiers, the Sassy’s direct heated triode (DHT) is ready to work immediately on powering up without the need to wait until the tubes warm up. That leads me to something I find really unusual (in a good way) about the Sassy II. The unit stays completely cool even after hours of operation. The aluminium enclose doesn’t get even slightly warm during use.

You might on occasion hear ringing from the amp after you’ve switched headphones or just turned the unit on. Don’t panic! This is normal and is just microphonics from the tubes within. You’re more likely to notice it with low impedance headphones or IEMs. After a few seconds, the sound will disappear.

Sassy II Measurement Report

Frequency Response (L: yellow and R: red channels)

The above graph shows both left (yellow) and right (red) channel frequency response at moderate output level from 20 to 20k Hz.  Counting on the minimum deviation (phase shift) from flat response, the real -3dB bandwidth extends over 10 times both below and above the frequency limits, which is equivalent to 2 to 200k Hz (-3dB) bandwidth meeting Hi-Res requirements.

Noise Floor ( L and R channels)

Just turning on the power of Sassy II and turning the volume to minimum (meaning no input signal), the graph shows the residue noise spectrum generated by Sassy II itself. The inherent noise (low gain) is below -120dB. That is why there is hardly a hiss even working with most sensitive IEMs.

Signal to Noise Ratio (SNR)

The graph shows left SNR (70.3dB). With 1 KHz input signal, Sassy II output 1KHz base frequency and its 2nd (2 KHz) and 3rd (3 KHz) harmonics.

The graph shows right SNR (72.4dB). It has only obvious 2nd harmonics and barely 3rd harmonics, because of superior characteristics of right channel tube compared with left channel.

Total harmonic distortion (THD)

The input signal is intentionally shifted to 3 KHz and output is lowered to simulate the worst-case scenario. It shows about 0.1% THD for the left channel.

It shows 0.125% THD. Again the performance difference between L and R channel is due to variations of tube electrical performance.

Battery life

Average battery life is rated at around 10 hours but I found that when using IEMs or lower impedance headphones at lower volume (think evening in a quiet room) then it can be significantly longer. Obviously, this will depend on headphones and output levels used and will vary for everyone. There’s no need to worry about the battery though if you have a power source nearby. The unit can still be used when powered or charging via the USB port.


Sources used for testing
Music used for testing
  • Solar Fields – Blue Moon Station
  • The Gloaming – The Gloaming
  • Radiohead – Hail to the Thief
  • Trevor Morris – Vikings OST
  • Roque Banos – In the Heart of the Sea (Original Motion Picture Soundtrack)
  • Thundercat – Drunk

Some readers might find this section a little on the “lite” side after seeing other reviews that wax lyrical about how their amplifier’s sound with various headphones and IEMs but in my personal opinion that essentially breaks down into a series of mini-reviews of those headphones and IEMs… An amplifier will have a basic signature (hopefully close to flat) that carries across to whatever you connect to it so while I will cover some pairings below, this section will be a very brief description of how I hear the Sassy II.

As to output impedance of Sassy II, since there is no feedback scheme to reduce its value, it is a very straightforward series resistance at the amplifier output specified at 2.4 ohms. With the added gain select switch the Sassy II is more suitable than the original model for low sensitivity IEMs but has more than enough power to drive high impedance or demanding full-sized headphones, including planars.

Even on sensitive IEMs at very low volume, I haven’t been able to detect any channel imbalance whatsoever which is quite remarkable. This is largely due to integrating gain adjustment when driving higher sensitivity IEMs or headphones and gain setting is low, naturally, the user will turn the volume higher to potentiometer’s linear region.

There’s little colouration added to the music, just some of that tubey goodness that comes across as a bit of extra warmth, the effect of 2nd harmonics taking off the clinical sterility of digital recordings, added airiness to the highs and a slightly increased soundstage. In regards to the extra warmth, it doesn’t take anything away from the clarity, far from it.

The effect is barely noticeable but you can tell it’s there like a little voodoo (the good kind) has been sprinkled over your tunes. You could liken it to a polished vs brushed aluminium: The surface is noticeably changed but it’s still smooth, however now it looks and feels even better than before. Okay, I’m digressing there and probably a good amount of people will be left scratching their heads at my analogy. Sometimes I feel like I’m bordering on autistic and things only make sense in my private little world…

Overall the Sassy II is a low-distortion tube amp so avoids any drastic splashiness or smearing that you might find in a cheaper or intentionally high-distortion tubefied amp.

In use

In this section, I’ll be discussing very briefly some pairings that I’ve tested with the Sassy II. As I have already covered the basic sound signature above this is simply meant to provide the reader with a little insight to the driving capabilities of the amp.


The first time I heard the Sassy II was at CanJam Singapore 2017 with the Abyss planar headphone. I hadn’t heard the Abyss prior to this but had seen people mention its fairly demanding power-wise. Well, I can state conclusively that the Sassy II took on the challenge like a champ. The sound was full across the spectrum and I was listening at under 50% volume (high gain) so still had plenty of power in reserve.

With my Beyerdynamic DT990 Pro (250 ohms) I generally sit around 30-40% volume (low gain). This is a nice pairing even though the 990 is pretty weighted in the bass already but there’s some added liveliness in the treble that lifts the presentation beautifully.

The Ultrasone Performance 860 (32 ohms) is a linear headphone with a reference signature. As a result, it benefits greatly from the Sassy’s touch of warmth and is a more exciting listen with more energy and gusto than with a pure digital amp. In “Opening Set” by The Gloaming the Ultrasone’s resolving qualities are perfectly matched resulting in a full-bodied bass, warm inviting midrange. I found myself listening at about 15-25% volume (low gain).

Sennheiser’s HD600 was another great match for the Sassy II at a level of around 40% in low gain, or 25-30% when set to high gain. The HD600’s forward vocals sound natural and really engaging along with the Sassy’s ability to produce light, shimmering treble.


In all the IEMs I tested there was no noticeable hiss or background noise. I’m usually not overly sensitive to hissing but in low gain setting, I didn’t detect any at all.  I tested a few of the IEMs in my current rotation, including LZ A4, Thinksound ms02 and Trinity Phantom Master 4. Although none of those is super sensitive the noise floor was black to my ears. The Phantom Master 4 seemed to benefit the most from the earphones I tested gaining a little more body and fun to their somewhat clinical default signature.


I’ve had the Phatlab Sassy II portable headphone amplifier for almost a month now and although I rarely use portable amps on the go I have definitely become accustomed to using this one at home. In fact, because the Sassy makes it so easy to use full-sized headphones in any room or around the house I’ve been using them almost exclusively instead of my IEMs.

The battery can get me through a full week on a single charge if I listen to 2-3 hours a day (keeping in mind none of my headphones are especially hard to drive). The Sassy II does everything right for me. Even the green power LED is a refreshing change since nowadays everything seems to have blue ones!

The Phatlab Sassy II portable headphone amplifier has impeccable build quality, great looks, fantastic battery life and is lightweight yet has power output similar to what many desktop units produce. What’s more, is it gives you a hint of tubey sound whilst retaining a fairly neutral signature and with selectable gain output is versatile enough to drive anything from sensitive IEMs to demanding planar headphones.

Try as I might, I simply can’t find anything negative to say about it. Granted it’s not cheap but neither is what it offers in material or sound quality. If you’re looking for a serious portable amp to compliment your kit the Phatlab Sassy II is simply stunning.

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