Class A vs Class A vs Class AB

IndianEars

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Just for a perspective... a True 200 Watt Class A stereo amp will consumer 1600 Watts of power Continuously, even with no music playing.

Most of this (the equivalent of a Kitchen Electric Oven!) will be dissipated continuously as heat.

I suspect this will require 1 Ton of additional air-conditioning capacity in your room (its it not airconditioned, you will literally have a "Hot Seat" ;) )
 

rsjaurr

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Just for a perspective... a True 200 Watt Class A stereo amp will consumer 1600 Watts of power Continuously, even with no music playing.

Most of this (the equivalent of a Kitchen Electric Oven!) will be dissipated continuously as heat.

I suspect this will require 1 Ton of additional air-conditioning capacity in your room (its it not airconditioned, you will literally have a "Hot Seat" ;) )

Can you please explain how a 200w class A amp will consume 1600W power that too when no music is being played? I also would like to know how much power it will consume while playing the music.

My understanding is that Class A amps have roughly 50% efficiency. So it should consume almost double the power and not 8 times unless I'm too ignorant about how class A amp works. So kindly enlighten me.

And with due respect, I somewhat disagree with the second half of the statement unless you meant to be funny. :)
I have two pure class A monoblocks which output 300W at 4ohms which my speakers are rated at.
So even after 30-40 minutes of continuous play the heatsinks get warm and not hot as class A amps are blamed for.
BTW I have installed a 2.2 Ton AC in that room but the additional Ton is not for monoblocks, that's for the room size which is 14x30 feet.:)

1. Velvety detailed Vocals, Soft non irritating Treble and Bass Slam akin to adding a Sealed subwoofer to the setup. I'm re-exploring my library for listening to many tracks.

2. Doesn't break a sweat while driving my B&W floorstanders. Was quite curious to check out the surface temperature after running it for 4 hrs, well no heating at all. (pics attached temp recordings vs Freya).
Some experts can share their knowledge about different topologies of class A Amplifiers and their Relative Merits.
Congratulations on your purchase. You should keep it for a very long time.

I cant agree more with both of your comments regarding the sound signatures and the heat issue.
May be design also plays part in the heat issue.

And I doubt I will ever want to go back to class ab or d topology.
 

IndianEars

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Can you please explain how a 200w class A amp will consume 1600W power that too when no music is being played? I also would like to know how much power it will consume while playing the music.

My understanding is that Class A amps have roughly 50% efficiency. So it should consume almost double the power and not 8 times unless I'm too ignorant about how class A amp works. So kindly enlighten me.

And with due respect, I somewhat disagree with the second half of the statement unless you meant to be funny. :)
I have two pure class A monoblocks which output 300W at 4ohms which my speakers are rated at.
So even after 30-40 minutes of continuous play the heatsinks get warm and not hot as class A amps are blamed for.
BTW I have installed a 2.2 Ton AC in that room but the additional Ton is not for monoblocks, that's for the room size which is 14x30 feet.:)
Dear rsjaurr.

My post appears to have riled you.... My post and all its content was made in Full seriousness, I was Not being funny.... Just technically factual :)

Your questions demand putting much pen to paper, if they are to be fully clarified.

In this post, I will try to be brief. If a deeper technical interest is there, I will elaborate further, in subsequent posts.

There is often a incomplete understanding of what is Class A Bias (in the interest of Brevity, will restrict this discussion to the Output stage of an Audio Power Amplifier.

The Diagram below shows a very simplified "Push Pull" Output stage for an Audio Power amplifier. Depending on the biasing Adjustment, the SAME output stage circuit can be operated in Class A, Class B Or Class AB.


Output Stage.gif

TR1 & TR2 are transistors. Consider them to be like Taps in lines delivering water (actually power) to the Speaker.

The input signal consists of a Top (Positive) half and a lower (Negative) Half.

In a Class B Amplifier, TR-1 delivers (Pushes out) the Positive Half and then shuts off.

TR-2 then delivers (Pulls in ) the Negative Half and shuts off.

Note: There is ZERO Current flowing in TR-1 & TR-2 when there is no signal.

So NO HEAT is produced in a Class B amplifier when there is no signal / music

The 2 Halves will never mesh perfectly with each other in a Class B amplifier, since TR-1 & TR-2 Take a certain finite time to switch on and off. The Resultant is a signal shown below, delivered to the Speaker.

CrossOver Distortion.jpg


The result is distortion in the Crossover Region where TR-1 stops and TR-2 begins. This is called "Cross Over Distortion"


CLASS AB AMPLIFIER
In an effort to reduce / minimise (NOT ELIMINATE) the Cross Over Distortion, TR-1 & TR-2 are forced to carry a small current (approx 20 to 50 Milli amperes) all the time, so that hopefully, TR-1 & TR-2 never fully switch off and cause Crossover distortion. This is called a Class AB Amplifier.

A Class AB amplifier will produce some (a little) heat when there is no signal. As the signal increases, TR-1 & TR-2 need to feed more power to the Speaker, and they will heat up.

CLASS A AMPLIFIER
To completely Eliminate Cross over distortion, TR-1 & TR-2 are made to Carry a little more than the FULL POWER (eg 200 Watts) at ALL TIMES, even when there is no signal.

Hence TR-1 & TR-2 need to waste (dissipate) the Class A amplifier's Full power, when there is no signal. This is the time when a True Class A amplifier runs the Hottest! (ie when there is no signal).

As the signal arrives, part of this Full Power (say 200 Watts) is diverted to the Speaker, and the balance power continues to be wasted in TR-1 & TR-2. When the Class A amplifier is delivering 200 Watts into the speaker, it will waste / dissipate very little power in TR-1 & TR-2 since all 200 Watts are being delivered to the speaker. The Class A amp will Run the Coolest when it is delivering full power!


REAL WORLD EFFICIENCY OF AN AUDIO CLASS A AMPLIFIER

To ensure that TR-1 & TR-2 do not switch off when delivering 200 Watts power to the speakers, they are made to carry a Higher current that what is required by the speaker at 200 Watts. How much more ? That depends on the designer, and how keen he is to make his amplifier sound good with real world speakers. A speaker may be rated at 8 Ohms, but its actual impedance may typically vary from say 5 Ohms to 10 Ohms. To further complicate matters, the Speaker does NOT act like a simple resistor. It acts as a part capacitor at some frequencies and as a Part inductor at some frequencies! As a result, a speaker may demand 50% more current at some frequencies, than what a simple calculation will show, for an 8 Ohm resistor as a speaker. A Great Class A amplifier will be able to drive such demanding speakers. So good the Class A amplifier may be made to carry 50% more current than what a simple calculation will indicate. THAT lowers the Efficiency of the Class A amplifier to well below that of the theoretical 50% that you have quoted.

Now there is still the less than 100% efficiency of the Power Supply of the amp to be factored in. The Transformer and Rectifier could operate at 85% efficiency, even if a Regulated Power Supply is not used. If a regulated Power supply is used, efficiency will fall still further :eek:, ie even more heat will be generated!

To roughly summarise, let us assume typical efficiencies:

45% efficiency (instead of the theoritical 50%) for the Class A Output Stage (keep some extra current in the stage so that the transistors dont 'bottom out' when delivering full output.

30% extra current for demanding speaker loads ie 30% extra power or 70% efficiency

85% efficiency for the Transformer and power supply

The result is: 0.45 x 0.7 x .85 = 0.267 or 27.7% efficiency, ie the good, Genuine Class A amplifier will draw from the mains, 4 TIMES its Max output to the speakers... PER CHANNEL.

That is how a 200Watt Class A Audio Amplifier, Honestly and properly designed and specified will draw 1,600 Watts (approx) from the mains.


This is clearly exorbitant, and many Class A amps cut Deep corners, while trying to maintain their Class A Hype.

The short cuts are:
* Provide ONLY for enough Class A current into a resistive load.
* Provide ONLY enough Class A Current into an 8 Ohm Load (4 Ohm loads will make the amp run out of steam, demanding Double the current)
* Provide Only enough Class A Current for say 10% of the amplifier's rated power into 8 Ohms (eg enough current for 20 Watts Class A into 8 ohms resistive, in the case of an amp specified as "200 Watt Class A Amplifier per Channel".)

Each of the above shortcuts results in Non Class A operation for atleast some part of the amplifier's operation..... But Marketing Hype is maintained.

The above discussion is a bit technical.

If you want a not technical litmus test, if a Class A amplifier is Not running Very Hot.... Corners have been cut, and you do not have the best engineered Class A amplifier.

P.S: How hot is "Very Hot" ? Generally 65 deg C is "Too Hot To Touch for 10 seconds" This is also a temperature when Electronics begin to deteriorate rapidly, or causes the onset of 'thermal runaway" that will blow up the amplifier. An operating temperature of 45 to 50 deg C should not be exceeded for long term reliability.
 

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rsjaurr

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Dear rsjaurr.

My post appears to have riled you.... My post and all its content was made in Full seriousness, I was Not being funny.... Just technically factual :)



The result is: 0.45 x 0.7 x .85 = 0.267 or 27.7% efficiency, ie the good, Genuine Class A amplifier will draw from the mains, 4 TIMES its Max output to the speakers... PER CHANNEL.

That is how a 200Watt Class A Audio Amplifier, Honestly and properly designed and specified will draw 1,600 Watts (approx) from the mains.


This is clearly exorbitant, and many Class A amps cut Deep corners, while trying to maintain their Class A Hype.

The short cuts are:
* Provide ONLY for enough Class A current into a resistive load.
* Provide ONLY enough Class A Current into an 8 Ohm Load (4 Ohm loads will make the amp run out of steam, demanding Double the current)
* Provide Only enough Class A Current for say 10% of the amplifier's rated power into 8 Ohms (eg enough current for 20 Watts Class A into 8 ohms resistive, in the case of an amp specified as "200 Watt Class A Amplifier per Channel".)

Each of the above shortcuts results in Non Class A operation for atleast some part of the amplifier's operation..... But Marketing Hype is maintained.

The above discussion is a bit technical.

If you want a not technical litmus test, if a Class A amplifier is Not running Very Hot.... Corners have been cut, and you do not have the best engineered Class A amplifier.

P.S: How hot is "Very Hot" ? Generally 65 deg C is "Too Hot To Touch for 10 seconds" This is also a temperature when Electronics begin to deteriorate rapidly, or causes the onset of 'thermal runaway" that will blow up the amplifier. An operating temperature of 45 to 50 deg C should not be exceeded for long term reliability.

Absolutely not. Infact you missed the smily and my words "with due respect" used in the same sentence.
And thanks for explaining everything in detail.

BTW what is optical bias?
 

IndianEars

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Thanks for your revert, rsjaurr :)

Thanks also to so many forum members for liking my post.


Absolutely not. Infact you missed the smily and my words "with due respect" used in the same sentence.
And thanks for explaining everything in detail.

BTW what is optical bias?

In what context is this ? Are you referring to the Siltech Saga Power Amplifier that uses "Optical Bias" ? TAS Review Here

To explain Optical Bias, refer to the 1st Diagram in my Post # 26.

There is a potentiometer (Variable Resistor) VR-1 used as the "Biasing Adjustment".

VR-1 is part of the Voltage amplification stage of the Power Amplifier. This Voltage Amplification stage feeds the Output Stage (which is in effect the Current Amplification stage).

Instead of the simple Variable resistor, the (Mega buck) Siltech Amp uses a light source eg Bulb / LED that Illuminates a Light Dependent Resistor (LDR). The LDR's resistance varies with the amount of light falling on it. This Change in resistance adjusts / maintains the Bias current of the Output transistors ( which decides the level of Class AB or Class A operation).. The Optical bias makes it easy to also switch the mode of operation, eg from Class A to Class AB, by the user. Switching to Class AB enables the same amp to deliver much more power (in Class AB) but at the 'cost' of Class A sweetness..

The idea is to electrically isolate the Bias Adjustment. You can read about the SAGA amplification approach here at the Siltech Site.

The Siltech design splits the Power Amp into 2 sections each housed in a separate chasis !
It provides an option of using 1 or 2 Output stages, each housed separately!
The Biasing of the Output stage(s) is controlled optically, using a light source and an LDR.
I remember reading this in 2014, when it was launched with much coverage in the High End audio press.

If you are referring to Bias adjustment of another product, do mention. Most amps that use Optical Bias are exactly the same, but without the refinement / sophistication of the Siltech ... and without the option of adding an external output stage for even more power!
 
Last edited:

rsjaurr

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Thanks for explaining in detail. Every time I tried to read about optical bias this Siltech example always pops out.

The reason I am interested in optical bias is that Threshold SA1 amps which I own also use optical bias.
 

drkrack

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Dear rsjaurr.

My post appears to have riled you.... My post and all its content was made in Full seriousness, I was Not being funny.... Just technically factual :)

Your questions demand putting much pen to paper, if they are to be fully clarified.

In this post, I will try to be brief. If a deeper technical interest is there, I will elaborate further, in subsequent posts.

There is often a incomplete understanding of what is Class A Bias (in the interest of Brevity, will restrict this discussion to the Output stage of an Audio Power Amplifier.

The Diagram below shows a very simplified "Push Pull" Output stage for an Audio Power amplifier. Depending on the biasing Adjustment, the SAME output stage circuit can be operated in Class A, Class B Or Class AB.


View attachment 58116

TR1 & TR2 are transistors. Consider them to be like Taps in lines delivering water (actually power) to the Speaker.

The input signal consists of a Top (Positive) half and a lower (Negative) Half.

In a Class B Amplifier, TR-1 delivers (Pushes out) the Positive Half and then shuts off.

TR-2 then delivers (Pulls in ) the Negative Half and shuts off.

Note: There is ZERO Current flowing in TR-1 & TR-2 when there is no signal.

So NO HEAT is produced in a Class B amplifier when there is no signal / music

The 2 Halves will never mesh perfectly with each other in a Class B amplifier, since TR-1 & TR-2 Take a certain finite time to switch on and off. The Resultant is a signal shown below, delivered to the Speaker.

View attachment 58118


The result is distortion in the Crossover Region where TR-1 stops and TR-2 begins. This is called "Cross Over Distortion"


CLASS AB AMPLIFIER
In an effort to reduce / minimise (NOT ELIMINATE) the Cross Over Distortion, TR-1 & TR-2 are forced to carry a small current (approx 20 to 50 Milli amperes) all the time, so that hopefully, TR-1 & TR-2 never fully switch off and cause Crossover distortion. This is called a Class AB Amplifier.

A Class AB amplifier will produce some (a little) heat when there is no signal. As the signal increases, TR-1 & TR-2 need to feed more power to the Speaker, and they will heat up.

CLASS A AMPLIFIER
To completely Eliminate Cross over distortion, TR-1 & TR-2 are made to Carry a little more than the FULL POWER (eg 200 Watts) at ALL TIMES, even when there is no signal.

Hence TR-1 & TR-2 need to waste (dissipate) the Class A amplifier's Full power, when there is no signal. This is the time when a True Class A amplifier runs the Hottest! (ie when there is no signal).

As the signal arrives, part of this Full Power (say 200 Watts) is diverted to the Speaker, and the balance power continues to be wasted in TR-1 & TR-2. When the Class A amplifier is delivering 200 Watts into the speaker, it will waste / dissipate very little power in TR-1 & TR-2 since all 200 Watts are being delivered to the speaker. The Class A amp will Run the Coolest when it is delivering full power!


REAL WORLD EFFICIENCY OF AN AUDIO CLASS A AMPLIFIER

To ensure that TR-1 & TR-2 do not switch off when delivering 200 Watts power to the speakers, they are made to carry a Higher current that what is required by the speaker at 200 Watts. How much more ? That depends on the designer, and how keen he is to make his amplifier sound good with real world speakers. A speaker may be rated at 8 Ohms, but its actual impedance may typically vary from say 5 Ohms to 10 Ohms. To further complicate matters, the Speaker does NOT act like a simple resistor. It acts as a part capacitor at some frequencies and as a Part inductor at some frequencies! As a result, a speaker may demand 50% more current at some frequencies, than what a simple calculation will show, for an 8 Ohm resistor as a speaker. A Great Class A amplifier will be able to drive such demanding speakers. So good the Class A amplifier may be made to carry 50% more current than what a simple calculation will indicate. THAT lowers the Efficiency of the Class A amplifier to well below that of the theoretical 50% that you have quoted.

Now there is still the less than 100% efficiency of the Power Supply of the amp to be factored in. The Transformer and Rectifier could operate at 85% efficiency, even if a Regulated Power Supply is not used. If a regulated Power supply is used, efficiency will fall still further :eek:, ie even more heat will be generated!

To roughly summarise, let us assume typical efficiencies:

45% efficiency (instead of the theoritical 50%) for the Class A Output Stage (keep some extra current in the stage so that the transistors dont 'bottom out' when delivering full output.

30% extra current for demanding speaker loads ie 30% extra power or 70% efficiency

85% efficiency for the Transformer and power supply

The result is: 0.45 x 0.7 x .85 = 0.267 or 27.7% efficiency, ie the good, Genuine Class A amplifier will draw from the mains, 4 TIMES its Max output to the speakers... PER CHANNEL.

That is how a 200Watt Class A Audio Amplifier, Honestly and properly designed and specified will draw 1,600 Watts (approx) from the mains.


This is clearly exorbitant, and many Class A amps cut Deep corners, while trying to maintain their Class A Hype.

The short cuts are:
* Provide ONLY for enough Class A current into a resistive load.
* Provide ONLY enough Class A Current into an 8 Ohm Load (4 Ohm loads will make the amp run out of steam, demanding Double the current)
* Provide Only enough Class A Current for say 10% of the amplifier's rated power into 8 Ohms (eg enough current for 20 Watts Class A into 8 ohms resistive, in the case of an amp specified as "200 Watt Class A Amplifier per Channel".)

Each of the above shortcuts results in Non Class A operation for atleast some part of the amplifier's operation..... But Marketing Hype is maintained.

The above discussion is a bit technical.

If you want a not technical litmus test, if a Class A amplifier is Not running Very Hot.... Corners have been cut, and you do not have the best engineered Class A amplifier.

P.S: How hot is "Very Hot" ? Generally 65 deg C is "Too Hot To Touch for 10 seconds" This is also a temperature when Electronics begin to deteriorate rapidly, or causes the onset of 'thermal runaway" that will blow up the amplifier. An operating temperature of 45 to 50 deg C should not be exceeded for long term reliability.
Thanks a lot for a detailed explanation, clears lots of doubts. I was safe in assuming that threshold 4000 might be using a variant of Class A, which may not be Pure A and hence the smaller size and better heat management. Irrespective of technicalities its a Sweet sounding amp to own, but since it doesn't fit my future upgrade path of going for a highend integrated amplifier, sadly I will have to pass the Threshold Amplifier.

Hopefully we haven't derailed the thread and OP gets offers for what he's looking for.
 

IndianEars

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Yes, you are correct ! I should not derail this thread with a discussion on "Propha" Class A amps :p
Hi!
Looking for Mark Levinson 333 or Jeff Rowland amps or even Odyssey amps... in good condition... should not have been repaired or tampered...

Apologies to Malvai for4 hijacking his 'Wanted' post !
(
If there is an interest in this topic, maybe the Mods can spin it off in a separate thread?)

Just a Quick mention for a couple of points:

1. A Great example of a True Blue Blood Class A amplifier are the - now Discontinued - Vitus SM-011 Monoblocks

Some Interesting Specs:
* 40 Watts
in Class A or 200 Watts in ClassAB into 8 Ohms (Double into 4 Ohms)
* 1,400 Watts Power Transformer
* Power Consumption: 190 / 220 Watts Watts for Class A Mode (40 Watts) PER CHANNEL


Most manufacturers have now shifted to Pseudo Class A mode amps which utilise "Sliding Bias" (Threshold) or other variants with names conjured by their marketing departments.

In fact, various countries have introduced laws on power consumption by appliances that practically make True Class A amplifiers illegal !
 
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