Author Topic: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier  (Read 47842 times)

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Offline EEVblog

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EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« on: January 07, 2016, 07:46:11 am »
Dave reverse engineers the XuanZu X2-U808 Hi-Fi Headphone Amplifier to see if the valves actually do anything.
What is the circuit topology?
E180F (6J9) pentode valve datasheet: http://frank.pocnet.net/sheets/009/e/E180F.pdf
Doug Ford's Microphone design series: https://www.youtube.com/playlist?list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw

 

Offline Radio Tech

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #1 on: January 07, 2016, 10:18:49 am »
I was really surprised to see that the tubes actually did something in the circuit. As you said it could have worked without them.
Nice reversal on the circuit Dave.

Offline c4757p

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #2 on: January 07, 2016, 10:27:24 am »
For the little bit that it's worth, the tubes are doing something if you want "tube distortion", even a follower configuration will distort (though perhaps with a slightly different profile than a voltage amplifier).

Might be interesting to look at an FFT of the output. (Disclaimer, I haven't had a chance to watch all the way through yet, I got about halfway...)
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Offline thomastheo

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #3 on: January 07, 2016, 10:42:20 am »
I was really hoping for a little high voltage boost circuit, and a proper tube based amplifier. Not because it would have better audio output than a set of op-amps, because i don't buy all that nonsense, but because it would be a great little piece of nostalgia. I would totally buy one if it was, novelty or not, but alas.
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #4 on: January 07, 2016, 10:49:54 am »
Drawing an  analogy to a J-FET is simply wrong.
A more apt analogy would be a MOS-FET since there is an insulator between the elements like in a vacuum tube.


Sorry Dave Ya got it wrong.
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Offline Ampere

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #5 on: January 07, 2016, 10:55:06 am »
I'm a bit disappointed that they're just being used as buffers (and noise generators), but at least I learned something about tubes.
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #6 on: January 07, 2016, 11:09:23 am »
Not the kind of thing I would waste my money on.
Yah it's Cute, but so much of the electronic trash that comes out of China also has a cuteness factor.
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Offline Richard Crowley

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #7 on: January 07, 2016, 11:09:41 am »
I have found it to be a pretty reliable indicator that if they put an LED under the fire-bottle to "light it up", then they are going for sizzle and there's no steak there.  Move on, nothing to see here.   :=\  (second time I've used that emoticon in 10 minutes after never using it before.)
 

Offline gadget73

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #8 on: January 07, 2016, 11:17:56 am »
yay for Chinese numbering.  Same "number", but completely different part thats not even remotely close to the original.  Nothing like swapping a triple triode for a sharp cutoff pentode and sticking the same number on it for confusion reasons.

Honestly the reason that solid state makes no sense to me is because I get electron flow.  "conventional" flow is backwards and makes no sense in my head. 
« Last Edit: January 07, 2016, 11:24:14 am by gadget73 »
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #9 on: January 07, 2016, 11:43:47 am »
yay for Chinese numbering.  Same "number", but completely different part thats not even remotely close to the original.  Nothing like swapping a triple triode for a sharp cutoff pentode and sticking the same number on it for confusion reasons.

Honestly the reason that solid state makes no sense to me is because I get electron flow.  "conventional" flow is backwards and makes no sense in my head.

I discovered a long time ago it makes no difference which way the little packets of energy go, it all works out just fine. :)
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Offline jolshefsky

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #10 on: January 07, 2016, 11:54:00 am »
If my really quick look is correct, it looks like you can remove the valves and jumper pin 2 (control grid) to pin 3 (cathode) and get a much lower power, halfway decent headphone amp.
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Offline V42bis

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #11 on: January 07, 2016, 12:14:12 pm »
If it's not torn down yet, try putting a couple of square wave through it, say 2KHz to check the upper frequency response, should not have significant under or over shoot. Then a 20hz square wave and it should not have signify tilt.

 

Offline N2IXK

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #12 on: January 07, 2016, 12:17:50 pm »
yay for Chinese numbering.  Same "number", but completely different part thats not even remotely close to the original.  Nothing like swapping a triple triode for a sharp cutoff pentode and sticking the same number on it for confusion reasons.

The tubes are actually Russian tubes, which can be translated from Cyrillic as 6Sh9P or 6J9P.  Equivalent to the European E180F or the US type 6688.

Original USSR datasheet:

http://www.magictubes.ru/sprav/pdf/6j9p.pdf
« Last Edit: January 07, 2016, 12:21:27 pm by N2IXK »
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Offline max666

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #13 on: January 07, 2016, 12:21:07 pm »
Drawing an  analogy to a J-FET is simply wrong.
A more apt analogy would be a MOS-FET since there is an insulator between the elements like in a vacuum tube.


Sorry Dave Ya got it wrong.

No I think you got it wrong, if the control grid voltage would become positive a grid current would flow, just like in a J-FET.
 

Offline rrinker

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #14 on: January 07, 2016, 12:22:33 pm »
 My favorite is that one Gigabyte PC motherboard that had a single tube audio amp on it. Talk about silly...


 

Offline gadget73

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #15 on: January 07, 2016, 12:39:41 pm »

I discovered a long time ago it makes no difference which way the little packets of energy go, it all works out just fine. :)

Also just too many parts to keep track of :) 

24v K-A and a cathode follower arrangement?  Whats the point?  It would give lower output impedance, but feeding into an op-amp its not like that would be required. Not enough voltage to make any useful gain anyway.  Wonder how far out of the linear operating range it is. 
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #16 on: January 07, 2016, 12:51:46 pm »

I discovered a long time ago it makes no difference which way the little packets of energy go, it all works out just fine. :)

Also just too many parts to keep track of :) 

24v K-A and a cathode follower arrangement?  Whats the point?  It would give lower output impedance, but feeding into an op-amp its not like that would be required. Not enough voltage to make any useful gain anyway.  Wonder how far out of the linear operating range it is.
In my opinion there is no point, That is not a good tube for a cathode follower, the designer is not taking advantage of the high input impedance a vacuum tube offers, and as you pointed out the plate voltage (B+) is too low.

The designer isn't even trying to prove something.

Why not?
build a 30W headphone amplifier using all tubes, no output transformer, using an output differential amplifier with cathode output. (to keep the voltage and impedance low)? Run it on say............125V regulated(+/- 65.5V).

That would be a concept amplifier worth building, if for no other reason than to prove the idea. :)
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Offline retrolefty

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #17 on: January 07, 2016, 12:52:18 pm »
My favorite is that one Gigabyte PC motherboard that had a single tube audio amp on it. Talk about silly...

 I remember seeing a picture of that, couldn't believe it when I first saw it, but then I remembered the golden ear crowd. Suspect it wasn't a big seller. You know that could be somewhat of a collectors item.
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #18 on: January 07, 2016, 12:53:00 pm »
My favorite is that one Gigabyte PC motherboard that had a single tube audio amp on it. Talk about silly...

I wish I had three hands....
This is a triple face palm.  :palm: :palm: :palm:
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Offline retrolefty

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #19 on: January 07, 2016, 12:56:14 pm »

I discovered a long time ago it makes no difference which way the little packets of energy go, it all works out just fine. :)

Also just too many parts to keep track of :) 

24v K-A and a cathode follower arrangement?  Whats the point?  It would give lower output impedance, but feeding into an op-amp its not like that would be required. Not enough voltage to make any useful gain anyway.  Wonder how far out of the linear operating range it is.
In my opinion there is no point, That is not a good tube for a cathode follower, the designer is not taking advantage of the high input impedance a vacuum tube offers, and as you pointed out the plate voltage (B+) is too low.

The designer isn't even trying to prove something.

Why not?
build a 30W headphone amplifier using all tubes, no output transformer, using an output differential amplifier with cathode output. (to keep the voltage and impedance low)? Run it on say............125V regulated(+/- 65.5V).

That would be a concept amplifier worth building, if for no other reason than to prove the idea. :)

 Why 30 watts, not many headphones can accept even peaks at that level? About the only thing that might differentiate a 'headphone amp' form other audio amps is that it might be designed to only work at the higher output impedance of most headsets.
 

Offline AF6LJ

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #20 on: January 07, 2016, 01:13:19 pm »

I discovered a long time ago it makes no difference which way the little packets of energy go, it all works out just fine. :)

Also just too many parts to keep track of :) 

24v K-A and a cathode follower arrangement?  Whats the point?  It would give lower output impedance, but feeding into an op-amp its not like that would be required. Not enough voltage to make any useful gain anyway.  Wonder how far out of the linear operating range it is.
In my opinion there is no point, That is not a good tube for a cathode follower, the designer is not taking advantage of the high input impedance a vacuum tube offers, and as you pointed out the plate voltage (B+) is too low.

The designer isn't even trying to prove something.

Why not?
build a 30W headphone amplifier using all tubes, no output transformer, using an output differential amplifier with cathode output. (to keep the voltage and impedance low)? Run it on say............125V regulated(+/- 65.5V).

That would be a concept amplifier worth building, if for no other reason than to prove the idea. :)

 Why 30 watts, not many headphones can accept even peaks at that level? About the only thing that might differentiate a 'headphone amp' form other audio amps is that it might be designed to only work at the higher output impedance of most headsets.

In a word "Headroom" but even 10W would be enough.
Sue AF6LJ
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Offline jitter

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #21 on: January 07, 2016, 01:39:45 pm »
Dave reverse engineers the XuanZu X2-U808 Hi-Fi Headphone Amplifier to see if the valves actually do anything.
What is the circuit topology?
E180F (6J9) pentode valve datasheet: http://frank.pocnet.net/sheets/009/e/E180F.pdf
Doug Ford's Microphone design series: https://www.youtube.com/playlist?list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw

A lot of wank indeed. Made for the true audiophile that sees the tubes/valves and just "knows" that these must sound good.
Tell the same people that it has SMPSs in there, and it'll fall off it's pedestal in the blink of an eye.

Other than guitar amps, which are all about distortion, and for good looks (tubes just look cool without the blue LEDs), I see very little use for hollow state amplifiers.
 

Offline retrolefty

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #22 on: January 07, 2016, 01:44:36 pm »
Dave reverse engineers the XuanZu X2-U808 Hi-Fi Headphone Amplifier to see if the valves actually do anything.
What is the circuit topology?
E180F (6J9) pentode valve datasheet: http://frank.pocnet.net/sheets/009/e/E180F.pdf
Doug Ford's Microphone design series: https://www.youtube.com/playlist?list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw


A lot of wank indeed. Made for the true audiophile that sees the tubes/valves and just "knows" that these must sound good.
Tell the same people that it has SMPSs in there, and it'll fall off it's pedestal in the blink of an eye.

Other than guitar amps, which are all about distortion, and for good looks (tubes just look cool without the blue LEDs), I see very little use for hollow state amplifiers.

 Still popular and useful for HF Kilowatt RF amplifiers used on the ham radio bands.
 

Offline BobC

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #23 on: January 07, 2016, 01:58:34 pm »
So, in our world of programmable analog circuit arrays and perfect rail-to-rail opamps, when would you *need* to use a tube?  When does using silicon not make sense?

I've been fortunate to have worked in a wide variety of fields, including radiation monitoring.  Some of the sensors provide signals in the microvolt/nanoamp  range, and local gain is needed before forwarding the signal elsewhere.  But when the sensor is near the center of an operating nuclear reactor, gain can be tough to find.  How to get local gain?

Specialty tubes to the rescue!  A vacuum is naturally radiation-resistant. But careful selection of materials is needed to make a tube that will have a useful lifetime and still work at high temperatures in the presence of massive levels of ionizing and neutron radiation.  It turned out that minor process tweaks to existing tubes did the trick, and a dual-stage high-gain amp provided all the gain needed.  (Our work was also used on satellites operating in the van Allen belts.)

What finally killed tubes in this application (decades after tubes disappeared everywhere else) was their accelerated aging.  Inside a nuclear reactor, a lifespan of a 7 years often isn't good enough.

So we had to find ways to get the raw sensor signal away from the reactor to external amplifiers.  The cost was moved from the tubes to ultra-low-loss cabling, and the signal was fed to what was basically a logarithmic nano-ammeter.

Fun all the way, no matter how you slice it.  The conversion to a nearly all-digital system (reducing the circuitry between the ADC and the sensor) was boring in comparison.
 

Offline Brumby

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Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #24 on: January 07, 2016, 02:00:47 pm »
Just one word .... context.
 


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