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Battle of Battery Operated Portable Headphone Amps

amirm

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#1
This is a review, detailed measurements and comparison of two battery operated portable headphone amplifiers: then JDS Labs implementation of O2 and the Topping NX3s. The former is one that I own and it retails for USD which I purchased for about $140 including free Prime shipping. The Topping NX3s is on kind loan from a member and is a lot cheaper at just USD $59 on Amazon with free shipping. I believe it went on sale on Massdrop for $45???

Size wise, the Topping MX3s is much, much less chunky than the O2:

Topping NX3 and JDS Labs O2 Portable Headphone Amplifier Comparison Review.jpg

The NX3s is definitely pocketable and could I guess be strapped to the back of your mobile phone. No such prayer for the O2 amplifier. The NX3 feels quite slick while the O2 is like many DIY type products.

The NX3s uses any USB charge source which is very convenient. The O2 on the other hand uses a small but rather heavy AC mains transformer. AC can be converted to the positive and negative supplies easily for amplification and hence the choice of that by its designer, the aloof NWAVGUY.

Both units come with high and low gain and 3.5 mm headphone and input jacks as seen in the photo.

There is a bass boost on the NX3s which I did not test. There is no such feature on the O2.

Let's get into measurements and see how they compare.

Measurements
My first experience with the Topping NX3s was not good after I fired off the dashboard with the amp put in "unity gain" mode meaning it generates the same voltage output as what is fed input:
Topping NX3 Portable Headphone Amplifier USB Measurements.png


We see a rather large mains spike at 60 Hz and tons of smaller ones limiting ultimate rating of THD+N/SINAD. Specs online were a lot better so I disconnected the USB power from it and performance substantially improved:

Topping NX3 Portable Headphone Amplifier Measurements.png


This is a remarkable improvement of nearly 20 dB in channel 2! Performance was even worse when the battery was almost empty. I tested with an external USB power supply and it was even worse than what I got above which was from my computer's USB 3.0 capable port. Clearly the NX3s is made to charge and go rather than mostly desktop use while plugged in.

The JDS Labs O2 did not have much variation to speak of with or without its AC wall-wart:

JDS Labs O2 Headphone Amplifier Measurements.png


Its performance falls short of when the NX3s is running on batteries but better when the NX3 is USB powered.

Note that there is fair bit of variability in performance of O2. Every time I put it on the bench, it generates slightly different results. Even sitting there the SINAD numbers jump up and down. There is some kind of instability inside there (at micro/small level).

I measured signal to noise ratio but the data is junk because both devices severely clip at maximum level which is what is used for determining SNR:
Topping NX3 and JDS Labs O2 Portable Headphone Amplifier USB SNR Measurements.png


I need to devise better test parameters here.

The key test here is distortion versus power level when loaded down so let's look at that at 300 Ohms:
Topping NX3 and JDS Labs O2 Portable Headphone Amplifier THD at 300 ohm  Measurements.png


For reference I have shown the JDS Labs Atom desktop amplifier which is distortion-free until max power which exceeds both units. So if you need a desktop unit, don't look at either the O2 or the NX3s.

Both NX3s and O2 generate the same power prior to clipping. At that point, the distortion in NX3s is lower but the O2 keeps going after clipping so likely can get louder during momentary peaks than the NX3s.

In case you are interested in low gain performance, here is the data on that:

Topping NX3 and JDS Labs O2 Portable Headphone Amplifier THD at 300 ohm Low Gain Measurements.png


Switching to a much more stressful 33 ohm load we get this:

Topping NX3 and JDS Labs O2 Portable Headphone Amplifier THD at 33 ohm SNR Measurements.png


Both units put out far more power as is typical. The results are similar as 300 ohm with NX3s having lower distortion at clipping than O2 but not able to max out as well as O2 can. The desktop JDS Labs Atom leaves them both in dust though with double the power prior to clipping.

Both of these units naturally use analog potentiometers for level adjustment which can be subject to channel imbalance. Here is how the O2 does when I manually change the volume and measure the differential:

JDS Labs O2 Headphone Amplifier channel imbalance Measurements.png


Worst case imbalance is around 0.5 dB or so.

NX3s does worse at 0.75 dB of error:

Topping NX3 Portable Headphone Amplifier Channel Imbalance Measurements.png


Finally, here is how the output impedance compares:

Topping NX3 and JDS Labs O2 Portable Headphone Amplifier Output Impedance Measurements.png


The NX3s has an impedance of 5.5 ohm which is well above 1 ohm or so I like to see. The O2 does meet that criteria at 1.3 ohm. This means that the NX3s can modify the frequency response of headphones below 50 ohm which have variable frequency response (for good or bad).

Listening tests
I used my Topping DX3 Pro as the source DAC and set its digital level to max/0 dB. I then used a split RCA cable to feed both amplifiers at the same time. I then used an AB switcher to switch back and forth between the two amplifiers.

As usual I start with my Sennheiser HD-650 to see how the product performs with high impedance headphones. Here, the O2 stole the show right away. It could get louder than NX3s with better bass performance.

Switching to Hifiman HE-400i to represent low but flat impedance headphone, the O2 was louder but you could hear its distortion easier. Prior to getting to that point though, it did better than NX3s just as well.

Conclusions
From cost, looks and portability the Topping NX3s leaves the JDS Labs O2 in the dust. While I did not test it, it will likely have much longer battery life too.

Unfortunately in actual performance the JDS Labs O2 simply sounds better with the headphones I tested. It has much lower output impedance and has no variability between desktop and battery operation so you could use it as a desktop amplifier just as well.

I don't have a clear cut answer or recommendation here. Personally I would never lug an O2 around with me and for desktop use, I would get the JDS Labs Atom which is cheaper and higher performance. Maybe the right answer is to use the NX3s with a more efficient headphone. I don't really know.

So I say the battle is unfinished. Hopefully we will find more clear cut choices in portable, battery operated headphone amplifiers.

------------
As always, questions, comments, recommendations, etc. are welcome.

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Roen

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#2
I think I'm the prime audience for this and thank you, @amirm , for this comparison.

I still happily lug my D50 (Soon to be replaced by Tone Board) and O2 around and use it both at work and in transit, but the Grace M9XX / M900 will be the next battle that the O2 faces, which I will be keenly interested in!

One more useful piece of information would have been the O2 and NX3 both on mains power, and performance overlaid on the battery power graphs.
 
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#3
To my knowledge, the O2 is the only battery portable amp that doubles as a desktop amp. It is kind of unique in that regard. Amir measured power with transformer in the earlier post - around 120 me at 300, if I remember.
 
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#9
So it looks like if the O2 put in a digital volume control, it can becomes a state of art amp for people who start the hobby.
 
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#10
Holy mother of sasquatch! I want to thank you Amir, for the wonderful shootout. I have been greatly anticipating how the NX3s would measure, and at 300 ohms at least, measurement-wise, it is like the Son of Atom (or is it Adam, he he?). Such incredible value for money.

Being that both the O2 and NX3s measure better than adequate, and noise is at or below the threshold of hearing, I was surprised to learn that there was a definite preference soundwise for the O2. I was expecting at the very worst that you would hear no difference, as by objectivist standards, these amps are about as good as you can expect, by any measure, but especially in a mobile context. So now my brain is exploding with questions. I will start, or continue (similar) "what are we not measuring" thread, so that we might explore this further.
 

RayDunzl

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#11
So it looks like if the O2 put in a digital volume control, it can becomes a state of art amp for people who start the hobby.
It would appear to be an analog amplifier, so, no.
 

DDF

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#12
To each their own. I've owned both and you couldn't pay me to lug either around on a train or any trip. Those dimensions and weight are desktop only for me. I'll put up with some fairly sub par audio in the name of convenience.
Definitely. Its why I bought an HRT microstreamer, for those quiet times at the airport or hotel with a good set of iems. No sacrifice in sound but boy does it suck power!
 

amirm

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#13
Being that both the O2 and NX3s measure better than adequate, and noise is at or below the threshold of hearing, I was surprised to learn that there was a definite preference soundwise for the O2.
It is possible the battery was drained on the NX3s by the time I did my listening tests. I am charging both and will repeat the listening tests.
 

rebbiputzmaker

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#15
Unfortunately in actual performance the JDS Labs O2 simply sounds better with the headphones I tested.
I think you might want to adjust how you look at and test small or portable headphone amps/dacs. You are using big / inefficient phones that you actually would not be using on the go. Maybe iems or other small efficient phones would be more practical.
 
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#16
Actually, I think a lot of people might use the bigger cans with either amp. Maybe not to go, as in outdoor, but moving around the house.
https://www.shenzhenaudio.com/toppi...ne-amplifier-with-ad8610-and-buf634-chip.html
At the price of O2 I would say a comparison with this one is more appropriate. If the NX3 is decent then this one might actually be good.
The NX5 is much older. The NX3s actually measures better than the NX5 is rated. Of course, who knows?

Did you actually see the measurements of the NX3s? They are exceptional. I didn't exactly feel the love in the subjective evaluation, but Amir can be very tough in his listening tests. Also, I think he was concentrating on differences.
 
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#17
I had wanted a really thorough test of the O2, having seen bits and pieces. This review confirms what a competent performer the O2 really is. The low gain performance, especially.

And every time I look at the JDS Atom's distortion graph at 30 ohm, I just shake my head and say WOW. Most amps aren't this good at 300 ohm. No penalty for owning lower impedance headphones!
 
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#18
I think you might want to adjust how you look at and test small or portable headphone amps/dacs. You are using big / inefficient phones that you actually would not be using on the go. Maybe iems or other small efficient phones would be more practical.
Well the thing is most portable headphones are already designed to work with low power amplification saves for some special cases, which render these portable amplifier useless. If they don't provide enough power for power hungry cans I don't see much uses in them. For the one in the middle ground I think the HE400i is already a good target for testing.
The NX5 is much older. The NX3s actually measures better than the NX5 is rated. Of course, who knows?
Oh I didn't realise that. Seems like they have the S as newer one. Maybe they will release NX5S one day.
 
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#19
I think you might want to adjust how you look at and test small or portable headphone amps/dacs. You are using big / inefficient phones that you actually would not be using on the go. Maybe iems or other small efficient phones would be more practical.
I do agree that a portable amp should be able to power iem's. Kind of like part of its mission statement. The 5 ohm output of the NX3s is a bit of a bummer in that regard. I always thought that 16 ohm iems were designed to run on static electricity, or promises of future prosperity.
 
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