2019 January 4     Audio      Reviews

Introduction


If you listen to headphones at your computer, then you might benefit from a headphone amp.  A volume-control box you can place next to a keyboard is very convenient.

I was looking for a low-cost headphone amp that doesn't generate a lot of switching noise.  Vacuum tubes a plus.

So there's this little hybrid tube-amp called the NS-08E.  Let's see if it's any good.


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In This Article


The NS-08E

Some Specs

LED's

Noise

Power Output

Turntable Compatibility

Sound Quality

Vacuum Tubes

Volume Control

Conclusion



The NS-08E


It's a very small amp, only about 3 5/8" by 2 3/4".  The main housing is metal that looks like anodized aluminum. It has a gold-tone brushed aluminum faceplate.

The power switch and power input jack are on the back of the unit;  the volume knob and audio jacks are on the front (faceplate).

And of course, there are two vacuum tubes protruding from the top of the unit.



Some Specs


Headphone Impedance:  18-600 ohms
Made In:  China
Power Supply Voltage:  6V DC in


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LED's


Like many of these hybrid mini amps, there's a blue LED down in the center of each vacuum tube socket. 

At first I thought this was going to be gimmicky and kind of a nuisance.  It may be gimmicky, but the LED output is nowhere near as strident as I expected.  I'm not saying I'd want this in every type of equipment, but it's tolerable here. 



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Noise?


There's a slight hiss, like tape noise, but only when you turn it up farther than you'll probably be using it.  It has a higher noise floor than a pure solid-state amp, but you'd expect this.  One reason for using vacuum tubes here was to impart some coloration to the sound.

When your fingernail hits the volume knob, you can hear that shimmery, microphonic sound that vacuum tube gear often makes.  Some people might not like that, but I think it's OK. 

The headphone cable can pick up noise from nearby appliances, including your computer.  Try moving the headphone cable-- and the pre-amp-- and the noise may go away completely.


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Power Output

This amp is loud.  With 47-ohm headphones, it's plenty loud at one-eighth volume. 

In fact, I often listen to it at just four clicks:  ten percent volume.

The factory specs say that it can output 1100 milliwatts when connected to 32-ohm headphones.  That's 1.1 watts.



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Turntable Compatibility


If your turntable has its own phono pre-amp, then it will be compatible with the NS-08E.  The Audio Technica AT-LP120 works great with this type of amp.  Line Out from the turntable goes to Line In of this headphone amp;  then you can listen to vinyl LP's on the headphones.

The NS-08E requires Line Level input;  anything that outputs Line Level will be compatible with it.  CD player, tape deck, MP3 player, some phonographs, etc.  You can use this mini headphone-amp to listen to any of these devices through your headphones, even if they don't have a headphone jack of their own.


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Speaker Compatibility


Powered speakers will not work with this as their amp.  At least, not the ones I tried.  They pass no sound when their internal amp has no power.  (Powered speakers are powered because they have their own built-in amp, and usually they have their own volume control.)  So it would be redundant to try using this headphone amp with powered speakers anyway.

It would be cool, though, if you could plug a small non-powered set of stereo speakers into this, rather than headphones, temporarily.  For example, to play those crazy YouTube videos where some guy built a 1,400 horsepower vehicle that bounces over massive boulders.  People ask me about connecting up speakers and stuff like this all the time (not monster 1,400 horsepower rock bouncer mobiles), but I haven't tried this particular way to connect an amp. 

Most of your generic computer speakers are 8 ohms, I think;  this headphone amp was designed for headphones that are 18-600 ohms.  However, because this is a "current amplifier" design, it might actually produce some sound output.


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Sound Quality

Compared to a pure solid-state headphone amp, the NS-08E has a deeper, more bass-rich sound.  The higher frequencies are somewhat attenuated, so there's not as much detail there. 

This may be the result of using the pentode tubes in triode configuration.  See Vacuum Tubes.


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Vacuum Tubes

Two Chinese 6J9's that jut out from the top of the unit (be careful;  they are pointy.)   These are pentodes with 9 pins.  They are not the same as US-made 6J9's;  the Chinese 6J9's are equivalent to US 6688's.

They have the Compactron-style pins, like the old TV tubes. 

At first I thought the tubes might not be doing anything but acting as a decoration.  But they're doing more than that.  Dave from EEVBlog took apart a similar (or identical) amp and showed that they are wired as triodes, which apparently is a well-known technique in vacuum tube technology.

The input voltage goes to the grid (the anode), with the output voltage being tapped from the cathode (negative terminal).  This is not the normal way to hook up tubes, but it is a normal way to hook up a cathode follower tube.  The cathode voltage follows the anode voltage, meaning they're almost identical.

The voltage stays about the same, but the tube acts as a high-impedance input with a low-impedance output. 

In other words, it is a current amplifier. 

What good is that?  Well, it lets the amp supply enough current for low-impedance headphones.  It would be easy enough to do this with semiconductors, but the tubes are supposed to give some coloration to the signal.  And there is also something about vacuum tubes that just suggests they're better for handling current.  (Whether it's a real advantage, who knows, but it looks cool.) 

The tube configuration might explain why this little amp has so much "juice".  I ran this thing next to a pure solid-state amp, which normally has to be run near full volume to power the headphones.  The NS-08E could run the same 47-ohm headphones much louder with the knob turned up only a fraction. 

Converting a high-impedance source to a low-impedance source is a real thing, not a gimmick.  So the vacuum tubes in this device have a legitimate function. 


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Volume Control

The adjustment is not continuous, but stepped.  This is one of the only things I don't like about this unit, but it's not that bad.  You can always adjust your computer's sound levels to get just the right volume at whatever setting you like to use with your amp.

The volume control is in the path of the line-level signal, which means it is attenuating that voltage before it gets to the vacuum tubes.  Then the tubes take that voltage and pass it to the op-amps, albeit with a higher current-output capacity.

The other thing I don't like is that the cables sort of get in the way of adjusting the volume knob.  They plug into the front of the unit.  That said, you may not be adjusting the volume that often, so again it's tolerable.


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Conclusion


For a long time I thought these tiny hybrid amps were sort of a novelty.  Then I tried one.

This little headphone amp is surprisingly compact, surprisingly loud, and it's actually one of the lowest-priced headphone amps you can get.  It's difficult to find any headphone amp for this price, let alone an amp that has vacuum tubes.

Overall it's very much worth getting;  I like this amp and use it all the time.



         

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