Certainly it is easily possible to have a class "A" amplifier with some non-linear distortion. But a well designed class "A" amplifier will not have much distortion. Of course most untrained folks can not hear that 0.05% non-linear distortion on music peaks anyway.
And vacuum tubes are noted for their desirable distortion products, which is the only reason that tube amplifiers still sell today. It is the sound, which is pleasing but not really High Fidelity.

 

I finally found photos of the amp's circuit board online. Those here more experienced than I can probably tell at a glance if it is genuine class A. The specified output of the device is 1300mW. Here is the product info. https://xduoo.net/product/mt-602/

The power transistors are D882M (NPN) and B772M (PNP).

https://pdf1.alldatasheet.com/datasheet-pdf/view/967218/JIANGSU/D882M.html

https://pdf1.alldatasheet.com/datasheet-pdf/view/965941/JIANGSU/B772M.html

I eagerly await the verdict. Many thanks for the responses so far.

View attachment 266647

View attachment 266649

I can see two complementary pairs of transistors on the reverse side of the bord so I think you can safely assume that "class A" is simply salesmanship or bad translation. You can be pretty sure that each channel has a class AB push-pull output stage.

 

Can someone please confirm Keith's assessment of the circuit board in the photos (see previous page)?

I would like to be sure the amps are not class A before sending them back and having a refund refused.

Many of these amps have been sold and receive glowing reviews. If they are being falsely advertised, I thought someone should have blown the whistle by now. But I am perplexed by the lack of much heat sinking.

From the diagram below, it appears some class A's use a complementary pair.

 

I can see two complementary pairs of transistors on the reverse side of the bord so I think you can safely assume that "class A" is simply salesmanship or bad translation. You can be pretty sure that each channel has a class AB push-pull output stage.

The class is the biasing. There no reason complementary pairs of transistors couldn't be class A Push-Pull other than being a waste of power.


https://www.aikenamps.com/index.php/the-last-word-on-class-a

 

No reason at all that a complementary amplifier can't run class "A". Not efficient, but that is far different from "not possible."

 

Sir Kit:
EEK! The class-A headphones amplifier you posted has no negative feedback from its output to reduce its distortion.
All good audio amplifiers use negative feedback.
Circuitstoday.com is a website in India by students who copied that circuit or designed it wrong. Maybe the expensive Chinese headphones amplifier copied it.

 

I see what looks like two push-pull (class AB) on the bottom of the board.

Bob

 

This reviewer advises, "MT-602 is class A push and pull design amp. So there is some DC offset that can cause popping sounds when plugging in sensitive low impedance headphones." Seems credible enough.

https://www.audiosciencereview.com/...gging-in-headphones-dc-offset-tutorial.25470/

Like others here, I assumed a "class A" would NOT be push/pull, which is why I bought the amp. For a particular application, I wanted the current to flow only one way through the load, not differently for each transistor.

I suppose this answers my question, and thanks to the replies here, I have learned a lot as well.

 

This reviewer advises, "MT-602 is class A push and pull design amp. So there is some DC offset that can cause popping sounds when plugging in sensitive low impedance headphones." Seems credible enough.

https://www.audiosciencereview.com/...gging-in-headphones-dc-offset-tutorial.25470/

Like others here, I assumed a "class A" would NOT be push/pull, which is why I bought the amp. For a particular application, I wanted the current to flow only one way through the load, not differently for each transistor.

I suppose this answers my question, and thanks to the replies here, I have learned a lot as well.

The truth is you will never know the difference using human hearing to compare push/pull vs single ended typologies.

Class A in any power amp typology is defined as half the amp at twice the price.

 

Why are you so concerned about it being class A? Having a DC current flowing through your headphones will offset the diaphragms and cause non-linearity. I would have thought that low distortion would be the major concern.

 

The primary target of most amplifiers is minimum distortion, usually the second target is uniform frequency response. For most applications power efficiency is less important than either of those requirements. Minimum distortion and uniform frequency response are not limited to class "A" amplifiers. Design quality affects all classes of amplifiers. A class "A" amplifier can easily have serious non-linear distortion if it is not correctly designed.

 

Why are you so concerned about it being class A? Having a DC current flowing through your headphones will offset the diaphragms and cause non-linearity. I would have thought that low distortion would be the major concern.

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


It's a novelty amplifier that extols the virtues of tube sound distortion. Nothing wrong with that if it's what you want. Some distortions are better than others.

 

Certainly a cheap transistor amplifier running at it's max output and at least 10% harmonic distortion does not sound good with most music. Serious flat-top clipping and other non-linear effects.

 

Certainly a cheap transistor amplifier running at it's max output and at least 10% harmonic distortion does not sound good with most music. Serious flat-top clipping and other non-linear effects.

When you add dynamics compression, brickwall limiters and other processing tomfoolery to make it louder it's sometime hard to tell where the distortion is coming from.

 

The "sweet, soft, warm" sound of a tube amp is nothing more than a roll-off of the high frequency components in the sound.

 

Class-A usually produces an asymmetrical waveform that has "musical" even-order distortion harmonics that are cancelled in a push-pull output circuit. Some old people like the even-orders distortion and reduced high frequencies that very old class-A amplifiers produced.

I am old today but all my life I hated the severe distortion produced by acid rock and even I hate the lower distortion and muffled sound from an old tubes amplifier. I also hate the muffled sound from an AM radio. Good quality class-AB hifi amplifiers produce NO audible distortion.

 

https://xduoo.net/product/mt-602/
View attachment 266684

It's a novelty amplifier that extols the virtues of tube sound distortion. Nothing wrong with that if it's what you want. Some distortions are better than others.

I have always enjoyed music and hi-fidelity reproduction of it. In the 1960s I designed and built a number of stereo, hi-fi tube amplifiers and was able to achieve good clean crisp sound with excellent frequency response. The "sweet, soft, warm tube sound" is the result of limited design. The nostalgia for that sound comes from memories of AM radio sets from the 40s and 50s and from the cheap tube record players that every teen had in those days.

 

Can someone please confirm Keith's assessment of the circuit board in the photos (see previous page)?

I would like to be sure the amps are not class A before sending them back and having a refund refused.

Many of these amps have been sold and receive glowing reviews. If they are being falsely advertised, I thought someone should have blown the whistle by now. But I am perplexed by the lack of much heat sinking.

From the diagram below, it appears some class A's use a complementary pair.

View attachment 266666

Yes, that’s definitely class A. The load seen by the output transistor (Q3) is a constant current source consisting of D1, D2, Q2 and R5.
The current is 15mA, so it will stay in class A provided the load is greater than 390Ω.
True, it could do with some negative feedback, otherwise the distortion isn't going to be so great.

 

There's nothing magical about tubes.
It's all how they are designed into a circuit.
For example, I doubt that a McIntosh tube amp has a "tube sound".
I quite sure that a good McIntosh tube amp would be indistinguishable from a good solid-state amp in a blind A/B listening test (except for those with that proverbial "Golden Ear", of course).

 

I did not purchase the amp specifically for the tubes or headphone listening, but because of the relatively high wattage (1300mW) for a ready-made compact device.

Music today is engineered to make solid state CD and MP3 players sound better. I would not buy a tube hi-fi power amp unless I was into vintage LP's. IMO that would be the place to look for any differences between transistors and what the original recording was intended to convey.