Verify Genuine Class A Operation with Scope

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
I bought a hybrid headphone amp from overseas, tube preamp with transistor power stage. The specs claim the output is "class A". No circuit is available. How can I verify this by looking at the outpt on my scope?
 

KeithWalker

Joined Jul 10, 2017
2,619
How many transistors are there in each channel? If there is only one, and there is no audible distortion at low and medium volume setting then it is class A.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
I believe there are class A amps with more than one output transistor.

As per my OP, how can I check with scope juyst to be sure?
 

nsaspook

Joined Aug 27, 2009
10,705
I believe there are class A amps with more than one output transistor.

As per my OP, how can I check with scope juyst to be sure?
Your scope would need to be able to see cross-over distortion (or the lack of it). Signal generator on a line input. Channel A on the amp input, Channel B on the amp output. Use scope math to diff A/B, adjust gains for a scope signal null.
Look for a crossover distortion signal.

Amp-Crossover-Distortion-GIF2_large.gif
Random internet GIF of Distortion.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
Thank you for your suggestion. But maybe a good amp will have minimal distortion?

Can it also be done by observing the DC level of the amp's output, or using a meter to determine if the output current is mono or bipolar?
 

nsaspook

Joined Aug 27, 2009
10,705
Thank you for your suggestion. But maybe a good amp will have minimal distortion?

Can it also be done by observing the DC level of the amp's output, or using a meter to determine if the output current is mono or bipolar?
Unless it's horrible amp it will have minimal distortion. The blackbox way to tell is to look at efficiency under no and full signal load. A true class A power AMP it will be a hot, power hog all the time. A practical 50W amp might have a idle power of 100W and use 200W at max power. A low power class A headphone amp is unlikely to have this obvious amplifier class profile.
 
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Audioguru again

Joined Oct 21, 2019
5,419
I agree that a class-A amplifier gets hot all the time including when it has no output signal.
But a headphones amplifier has a very low output power then its heating will also be low.
Why would it use antique vacuum tubes in its preamp??
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
As mentioned, this is a headphone amp. It puts out about 1W. Not going to get noticeably hot.

Here is another idea.

What if I feed in a monopolar signal from a function generator, rather than a bipolar sound card? If the amp is class A should not the entire signal be in the positive region as viewed on a scope?

With no input signal, it would sit at half rail. Whereas A/B would be "0" volts.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
Now Audioguru is insulting my amp ;-) When the tubes glow, they attract friendly spirits that work hard to make music sound like it used to.
 

crutschow

Joined Mar 14, 2008
31,128
it would sit at half rail. Whereas A/B would be "0" volts.
Not at the output, since it has a DC block capacitor.

One possible way is to observe the amp power supply current.
A Class A will have little change in the current with a change in the output signal volume to the headphones.
A Class AB or B should have a noticeable change.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
155
Oh yes, the blocking cap. I like to sometimes short them out.

I will certainly try crutshow's suggestion. If I understand correctly, the "little change" in class A current is because it already sits at 50% rail with no signal.

At full volume, the current would therefore increase by about 25% for A as opposed to 50% for AB. Is that correct?
 

Audioguru again

Joined Oct 21, 2019
5,419
I never had a class-A audio amplifier/heater but I think its power supply current does not change much with output level.
Most audio amplifiers are class-AB so that the class-A occurs with a low current only at idle and low levels. The difference in power supply current from idle to full output can be huge.
 

crutschow

Joined Mar 14, 2008
31,128
At full volume, the current would therefore increase by about 25% for A as opposed to 50% for AB. Is that correct?
No.
The average current draw of a Class A amp is largely independent of signal level.
A Class AB can have much more than a 50% change.

Where did you get those 25% and 50% values?
 

BobTPH

Joined Jun 5, 2013
6,080
A class A amp outputting 1W will probably dissipate about 10W, so yes, there would be heating. Just measuring the quiescent current should be enough. If it is under 100 mA it is likely not class A.

Bob
 

crutschow

Joined Mar 14, 2008
31,128
Below is the LTspice simulation of a simple Class A amp:
Note that the average supply current (right box) is essentially the same as the quiescent current (left box, horizontal cursor).

1651939087323.png
 
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Thread Starter

Sir Kit

Joined Feb 29, 2012
155
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.

xduoo_mt-602_pcb_top.jpg

xduoo_mt-602_pcb_bottom.jpg
 
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