EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier

Dave reverse engineers the XuanZu X2-U808 Hi-Fi Headphone Amplifier to see if the valves actually do anything.
What is the circuit topology?

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[MrCarlsonsLab]

Hi Dave. The reason they are using that simplified 6688 tube, is due to cathode area or (size.) Larger cathode area in a small vacuum tube means they will operate at lower Voltages ( providing they don't have to produce much drive power) They would have been better off to use a "space charge" tube and arrangement. A space charge tube is designed to work with 12 Volts on the plate.
Enjoy your holidays!

[transistortester]

Not trying to start a war, but I think putting LEDs under tubes ruins the look of them. The dim glow of a tube looks great on it's own, so I see no reason to overpower it with gimmicky, incredibly cheap looking lighting. On that topic, I personally think LEDs under nixie tubes should be considered a crime.

[rbduffer]

You brought a tear to my eye, I didn't know anyone still uses vacuum tubes.
Back in 1959, when bits were flat, I went to a very good Engineering school we were taught vacuum tube circuit design. . Seeing the plate characteristic graph brought it all back.
Immediately after graduation I went directly into computer programming.

[NatureAndTech]

23:40 "so the tubes actually do something..."
Yes. They add 1.6% distortion.

[RandyLott]

I'm a musician and an electronics engineer. Tubes are not better for purely listening to music. I want to hear the music as close to the original as possible. Give me a low noise solid state amplifier for that. No filtering or "coloring".

However, for guitars and basses, the tube sound is real! Tubes sound fantastic for their dynamic range compression and harmonics they add. Not ideal for playing recorded music at all!

[LazerLord]

What I want my audio to be is what is stored on the file on my computer. No coloration, no distortions, no filtering. I want to listen to sound the way it's intended to be heard, and what's on the file is what I want to hear.

Note how I said *MY* sound, and not *everyone's* sound. Some people (actually, probably a lot) enjoy the coloration of a tube amp, or a specific EQ setting, or some tasteful deviations from what the sound file actually contains. That's fine, it's what they enjoy. For me personally, I would want an amp (really, a DAC-amp combo) because the headphones I have can't be properly driven by what I already have.






Although I think we can all agree that _"audiophile"_ grade SD cards and Ethernet cables do absolutely nothing. I really wish that the term audiophile didn't have such a negative connotation to it. When I (and maybe other people) see 'audiophile' on something, it immediately triggers our snake oil detectors.

Just thoughts :)

[MattExzy]

These things remind me of that one friend I had that re-ripped his entire MP3 collection into FLAC (from the MP3 files, not original source), telling me it was for 'better sound quality'. Biggest thing though, as with 'oxygen free' cables - don't engage with them in argument or present logic. *backs away slowly*

[larkenfield]

I actually own one, and even using only two tubes, this fine little amp has the delightful, noticeable *soundstage* of a tube amp. I find that quality of sound unmistakable without dismantling it technically. The tubes also make it easier to notice where the musicians are located in space. So, it's a hybrid amp with a genuine tube ambience... Regardless of whatever its distortion may be rated at - and that's probably going to vary according to the tubes being used - I have personally noticed much less listener's fatigue... I have enjoyed this amp hours on end playing music that I usually avoid because it's not a warm or full, especially at the bottom bass end of the musical spectrum. The tubes add warmth and fullness and spaciousness of sound. But my initial impression is that I was amazing at the improvement of the soundstage that even my Grado GS1000 headphones could not produce. As a musician, this is the most fun I've had improving the sound quality and my entire library. So, I'm disappointed that the technician in this video apparently didn't actually describe the amp's sound before technically analysis it. The only analysis that truly interests me are by those who have the ability to hear the different qualities in the equipment they are analyzing, and I found that lacking here... This means saying right off the bat whether this or any other amp sounds pleasing to the EARS... or why bother going to the trouble of analysis it? In the meantime, I've ordered some NOS GE 6688 tubes that should make this little amp sound even better. It's the EARS that take over after the analysis ends. So of course the tubes are having their effect.

[hobo]

Keep it up Dave, you're informative and fun to listen to. I'm old enough to remember when a TV or radio went on the fritz, pulling out all the tubes and riding to the drug store on my bike to run them all through the tester, then hoping that they had a replacement in the cabinet underneath. Also, TV repairmen who made house calls and always had a red and black RCA tube caddy. The good old days!

[Harlequin]

"They're filled with vacuums." :D 8:39

[BrandtRedd]

Since the tube output is nearly the same level as the input, I would love to see what happens if you pull the tube and simply short from pin-to-pin - letting the op amps do all the work. If I'm reading the schematics right that should give a fairly good quality result.

Probably too late since the video is from a year ago.

[YAUUN]

I have to say you are really good at explaining circuits. I was expecting to be totally lost with my high school knowledge of circuits, but I followed the gist of most of what you had to say.

[dogastus]

I wish Dave had spent 5 seconds longer and switched the input to a square wave.

[NoFearXJs]

What would be SUPER AWESOME is for Dave to point us in the right direction with a link or 2 of some choices of TRUE tube pre amps that are headphone sized and or a little more powerful... Thank you for the great detail Dave top quality as usual, hope the masses can help you find some proper choices for us, Cheers !

[unclebarry]

Dave, I loved this bit. I've always been fascinated with vintage technology. I'd love to see more pre-transistor teardowns and explanations of how they work.

[parentteachernight]

I know it's not necessarily your area of interest, but I really enjoy your audio equipment teardowns. I would love to see more!

[neilhuband]

So it's not a skookum choocher then Dave, is that what you're telling us?

[runardamnes]

Fun fact:
The European ECH83 mixer/oscillator tube will work with a plate (or anode) voltage as low as 6 Volts. It was mainly used for car radios.

[SeanBZA]

Biggest improvement is remove the 2 tubes and install 2 links from grid to cathode, and remove the cathode resistors. You save 2W of power and improve reliability by at least an order of magnitude and reduce the

Of course if you want the valve sound leave the resistors in and use a 1N4001 for the link so it is forward biased and able to generate the lovely distortion with less power and noise.

[keyen]

+EEVblog, Dave, a video on audio distortion theory (for beginners) and measurement might be pretty popular.