Nobsound NS-02E Review (Tube Headphone Amp)

[amirm]

This is a review and detailed measurements of Douk Audio Nobsound NS-02E vacuum tube headphone amplifier. It is on kind loan from a member and costs US $60 on Amazon including Prime shipping.

NS-02E is a cute little thing:

Volume knob is a bit hard to turn due to detents in it but is livable. There is not much else to the unit than 3.5 mmm input and output jacks and a switching power supply:

OK, there is a stupid blue LED that lights up the tube but that is common these days. I can't read the faded writing on the tube so can't tell who has made it. Maybe the owner can advise later.

Nobsound NS-02E Measurements
As usual we start with our dashboard of feeding the unit 2 volts and adjusting the volume control to get 2 volts out:

One channel has much more distortion than the other, bringing the average down even more than one wants to see:

Noise floor is actually better than distortion:

Frequency response rolls off in the bass indicating capacitor coupling:

Tube stages tend to do well with high impedance loads as far as power and this is no exception:

My threshold is 100 milliwatts for this test and the NS-02E sails past that if you allow plenty of distortion before clipping.

Conversely, they don't like low impedance loads:

I was pleasantly surprised by the low output impedance though.

Channel matching is not great:

Nobsound NS-02E Listening Test
I started with my usual 25 ohm Ether CX and found what I expected: the sound gets distorted badly from medium level on up. With 300 ohm Sennheiser HD650 though, the NS-02E woke up and delivered early rattling levels with seemingly low audible distortion. Listening was enjoyable across wide range of content.

Conclusions
Objectively the results are horrible other than available power into high impedance load. With plenty of power before getting to severe distortion, the NS-02E delivers a good subjective experience into high impedance headphones. If you have an itch for "tube sound" and have a high impedance headphone, the Nobsound NS-02E is low cost way to experience that. Otherwise it is a strong pass.

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[LTig]

Seems even @amirm can handle quite a load of distortion...

EDIT: nice price though!

 

[SIY]

Any pix of the innards? The output impedance suggests there's a solid state buffer on the output.

 

[LTig]

I wonder how it's possible to get high voltage required for tubes from a 6V DC power supply, especially for this very low price. Agreed the distortion is typical tube, but could it be that the tube is just for display (hence the blue LED)? Maybe it still works if one removes the tube ...

 

[LTig]

Any pix of the innards? The output impedance suggests there's a solid state buffer on the output.

One more hint to a fake tube ...

 

[amirm]

I wonder how it's possible to get high voltage required for tubes from a 6V DC power supply, especially for this very low price.

I assume there is a step up power supply in it. Let me see if I can open it easily....

 

[SnoopKatt]

Thanks for the review amirn!

I'm the original owner of this headphone amp (or the owner before sending it in for review - actually bought it used!), and I want to say the tube that's in there is the one that the headphone amp normally ships with brand new; it's not a nice GE tube or anything like that. I'm not 100% certain, but I didn't pay very much for the headphone amp off eBay, so I would be very surprised if it was shipped with a non-OEM tube.

 

[LTig]

I assume there is a step up power supply in it. Let me see if I can open it easily....

I'm an EE and know how tubes work but I'm far from being an expert. Standard tubes need quite high voltages (up to several 100 V) but I remember having played as a child with a DIY tube radio which ran from a few batteries, so there are tubes out there with low voltage requirements. Does anybody know what's the lowest supply voltage a tube can work with?

 

[amirm]

OK, took the top off:

This thing is cuter inside than out!

As we see there are two independent setup switching power supplies. Capacitor voltages are 16 so likely the step up value is below this. I am assuming the opamps are the front-end and the tube is the output stage.

 

[SnoopKatt]

I'm an EE and know how tubes work but I'm far from being an expert. Standard tubes need quite high voltages (up to several 100 V) but I remember having played as a child with a DIY tube radio which ran from a few batteries, so there are tubes out there with low voltage requirements. Does anybody know what's the lowest supply voltage a tube can work with?

I remember watching this when the video was first published - it's a teardown of a tube headphone amp. The tubes in these types of devices tend to operate more in a buffer state rather than as amplifiers (I believe this Nobsound amp has discrete opamps inside for amplification).
Edit: amirn beat me to it by seconds!

 

[LTig]

OK, took the top off:

View attachment 110263

This thing is cuter inside than out!

As we see there are two independent setup switching power supplies. Capacitor voltages are 16 so likely the step up value is below this. I am assuming the opamps are the front-end and the tube is the output stage.

It's hard to believe that a tube output stage has 0.8 Ohm output impedance. However two NE5532 opamps in parallel can drive a 300 Ohm load, so I rather assume the tube is at the input.

 

[amirm]

It's hard to believe that a tube output stage has 0.8 Ohm output impedance. However two NE5532 opamps in parallel can drive a 300 Ohm load, so I rather assume the tube is at the input.

Could be. I got my doubt after I post that.

 

[Doodski]

Hmmmz 36 minutes that this review has been up and Google has it as the fifth search result when searching for, "Nobsound NS-02E."

 

[tomchr]

Interesting with the 10.5 dB difference in THD+N between channels. Any idea what causes that?

Tom

 

[Francis Vaughan]

The key phrase here is starved anode.
It is quite possible to operate a simple triode with say 12 volts on the anode. There is zip current, but you get a quite real voltage gain.
The near definitive guide is from Merlin Blencowe. PDF Here.

 

[tomchr]

Yeah. It wouldn't surprise me if it runs on ±15 V. All the electrolytic capacitors are 16 V types (possibly except one where I can't read the markings). The resistors are 0805 or 0603 types (so 0.125 W or 0.0625 W types). Not the several watt types you'd need for an anode load in a high voltage circuit.

Tom

 

[Ralph_Cramden]

I’m old enough to remember tube car radios. They used mechanical vibrators to create a rough AC that could be stepped up by a transformer to the 300V or so needed by the tubes, from the car’s 6-12V battery.

 

[SIY]

It's hard to believe that a tube output stage has 0.8 Ohm output impedance. However two NE5532 opamps in parallel can drive a 300 Ohm load, so I rather assume the tube is at the input.

This. And likely the tube is configured as an incompetent cathode follower, less likely that it's a gain stage.

edit: Just watched the video, skipping to the circuit. Dave nails it.

 

[tomchr]

I’m old enough to remember tube car radios. They used mechanical vibrators to create a rough AC that could be stepped up by a transformer to the 300V or so needed by the tubes, from the car’s 6-12V battery.

Heh. Yeah. SMPS the old-fashioned way.

Tom

 

[pavuol]

Hmmmz 36 minutes that this review has been up and Google has it as the fifth search result when searching for, "Nobsound NS-02E."

Some 2 hours that this "non headless tubeamp review" has been up and Wallmart is out of stock

Amir "the stock influencer"