I recently purchased a Chinese-made headphone amp because it is advertised online as "class A".

https://xduoo.net/product/mt-602/

Wanting to be certain, I asked the company for a circuit diagram which I have attached below. From prior discussion, I understand a complimentary pair can be biased to operate in class A mode.

Does anyone see that happening here? If so, I would please like to understand exactly how it works.

 

I understand a complimentary pair can be biased to operate in class A mode.

Why?
You do understand that class A operation has more distortion than class AB.
Or is that what you are looking for?

You should know that this is an engineering forum and it may be hard to find anyone here who is interested in the obsolete technology and generally inferior audio performance of tube amps.
You may need to find an audiophile forum for that.

 

Yes, I wanted a class A amp, as do others for their own reasons.

My question involves the design of the circuit, not the relative pro's and con's from an audiophile perspective. With due respect, there is no point having a debate. That's been done before and silly people like me keep buying enough tube amps to keep the industry alive.

If you are not able to address the point I raised in my OP, maybe someone else here would be kind enough to do so. I hope they have not been dissuaded by your comments.

 

The tubes add some even harmonics distortion that some people say sounds "musical". But audio should have no distortion.
The distortion from the tubes will increase as they wear out. Are replacement tubes and high voltage capacitors available anymore?
The output transistors are missing negative feedback that is used to reduce the distortion in the output transistors in all other audio amplifiers.
The fairly low distortion rating does not say the output level. It is probably when it is turned down.

The complex parts of the circuit steps-up the 12V supply to the high voltages used by the old tubes.

 

I recently purchased a Chinese-made headphone amp because it is advertised online as "class A".

https://xduoo.net/product/mt-602/

Wanting to be certain, I asked the company for a circuit diagram which I have attached below. From prior discussion, I understand a complimentary pair can be biased to operate in class A mode.

Does anyone see that happening here? If so, I would please like to understand exactly how it works.

From the circuit diagram, I see that there is a preset potentiometer to adjust the bias to the output pair on each channel. It will most certainly be set to give minimum distortion, which, in that configuration, will be class A-B. If it was set to bias both of the complementary transistors on (as close to class A as the circuit would go) the transistors would dissipate a lot of heat and the distortion would be awful.
The vacuum tube preamplifiers are working in class-A. I guess the marketing guys took the liberty of using that to appeal to the non technical audiophials.

 

KeithWalker confirmed my suspicion. It may be technically capable of biasing into class A, but is it delivered that way?

Now that I am stuck with the amp (looks pretty and sounds nice), I am reluctant to readjust the trimmer and possibly cause damage. Is there any way I can tell if it is true class A by checking the output on a scope?

I have emailed the manufacturer for an explanation. The promo clearly sates the output stage is class A.

Thanks for all the replies so far.

 

The preset does not adjust the bias. It adjusts the DC offset.
The output stage is a circuit known as a "diamond buffer" and is self-biassing with impressively low distortion for a simple circuit (better than -90dB with matched transistors). Note that the preset does not adjust the voltage between the driver bases therefore does not adjust the bias.
Broksie has a very good explanation of the diamond buffer https://www.tubecad.com/2012/09/blog0244.htm

The circuit is simply a pentode valve connected as a triode which gives some amplification and, as @Audioguru again says, introduces some second-harmonic distortion.
The voltage amplification stage is indeed class-A but the output stage isn't, as @KeithWalker says.
@Audioguru again is wrong about the output stage. It is class-AB, but nevertheless a very good circuit. It has 100% feedback, as it is simply two sets of cascaded emitter followers.
The only way you can use a scope to determine the class of the output stage is to insert a current shunt into the collector of the output transistor. If the current never reaches zero, it is class-A - but there's no point, because it is class-AB.

If you want to know more about valve design have a look at:
https://www.lulu.com/shop/merlin-bl...cover/product-1v4g7dd8.html?page=1&pageSize=4
A properly designed valve amplifier can produce impressive distortion performance, better than a lot of amplifiers that get recommended on this forum, and better than most class-D designs (but don't mention the efficiency)

 

Thank you for the detailed explanation. If the manufacturer replies in a timely manner I will post same here for further comment.

 

Blencowe suggests that the distortion of a triode-strapped pentode at 1V rms input is about 0.2% (varies linearly with amplitude), and Broskie suggests that the distortion of the diamond buffer is about 0.003%, so I think most of what you hear will be the contribution from the valve.
There are a few provisos:
1)The distortion from the diamond buffer will be crossover type (odd harmonics, and reducing with amplitude)
2) It probably won't be as good as 0.003% because that was measured with matched transistors. So you might just hear a contribution from the output stage at low levels.

 

If the tube is not biased class A, you will know it from the harmonic distortion. You can rest easy.

 

The simplest way to tell if an amplifier is working in class-A is to measure the DC current it takes. If it stays the same when the volume is increased, it is class-A.

 

The simplest way to tell if an amplifier is working in class-A is to measure the DC current it takes. If it stays the same when the volume is increased, it is class-A.

The converse is not true. If the current remains the same it definitely is class A. But it can still be class A if even if the current varies, provided it doesn't reach zero.

 

The output transistors do not operate in class-A:

 

I think perhaps audiophiles want tube amps to operate in the linear (active) region since they believe that means the amplification is linear.
Of course that's not true since tubes are non-linear devices (as are BJTs and MOSFETs), and generate significant harmonic distortion.

 

I remember the "warm" sounds produced by my grandmother's tubes AM radio. It sounded different than the muffled sounds from the solid state AM radio given to me. The tubes class-A distortion filled in the AM radio missing high frequencies.
They both sounded BAD.

The amplifier kit I made had tubes and it sounded OK. Until I took it to a McIntosh amplifier clinic where they compared its distortion and limited bandwidth with an expensive solid state McIntosh amplifier.

 

There is no such thing as accurate in a studio recording, in that it is never what you would have heard in the room.