Class A power vs Class A/B power

MMas · Dec 20, 2014

I have a question about Class A amplification vs A/B. Some amps I’ve owned were Class A up to a certain point, then would switch to A/B operating mode. Others were Class A entirely and even though the Class A amps were of lower wattage, they still seemed to have the drive of A/B,, but with more density and precision.

So, what might be a comparable “exchange rate” between Class A and A/B? Would a 20 watt Class A amp have the same driving power as say a 100 watt A/B on a 88dB/w loudspeaker?

Thanks in advance.

amirm · Dec 20, 2014

There would be no difference in the topology of the amps that would make one output more power than its measured one. A 20 watt class A would certainly not put out more power than the same design in class AB.

MMas · Dec 20, 2014

Thanks for the answers so far. Maybe I can ask the question a different way. Can a 20 watt Class A amp drive a 88dB/watt speaker to decent level as opposed to a 20 watt Class B?

I recall a Yamaha CA-800 integrated amp I had back in the 70s. It had a switch that could go between Class A and Class B. The output was 45w/ch in B and 10w/ch in A mode. I had DCM Time Windows at the time and, as I recall, didn’t hear a lower volume when I switched from B to A mode.

Does the math apply that the sound level will increase 3 dB every time you double the power? For example, start with 20 watts, lose one for x-o and wiring. So 88dB @2watts, 91 @ 4watts, 94 @8, 97 @ 16watts, ~100 @ max output.

Sorry for the seemingly obtuse question, but I’m trying to decide on a new integrated amp and would like a Class A for the sonics, but unsure whether I would have enough power.

MrAcoustat · Dec 30, 2014

amirm said:
There would be no difference in the topology of the amps that would make one output more power than its measured one. A 20 watt class A would certainly not put out more power than the same design in class AB.

PS: To me this is NO regular 45 watts per channel amplifier.

Classé Audio DR3-VHC the vhc stands for Very High Current this amp is only 45 watts per channel class A the weight is 102 pounds.

amirm · Dec 30, 2014

MrAcoustat said:
PS: To me this is NO regular 45 watts per channel amplifier.

Classé Audio DR3-VHC the vhc stands for Very High Current this amp is only 45 watts per channel class A the weight is 102 pounds.

If you put a couple of bricks on it, it will be even heavier.

Seriously, wattage is wattage. Watts=V^2/R. "Current" plays no role there.

Where current comes into play is when "R" keeps getting smaller and smaller. Higher current amps will not current limit in the face of small impedance of speaker. At higher impedance they put out no more power however than any other amp with the same wattage.

MrAcoustat · Dec 30, 2014

amirm said:
If you put a couple of bricks on it, it will be even heavier.

Seriously, wattage is wattage. Watts=V^2/R. "Current" plays no role there.

Where current comes into play is when "R" keeps getting smaller and smaller. Higher current amps will not current limit in the face of small impedance of speaker. At higher impedance they put out no more power however than any other amp with the same wattage.

Sorry but i don't know of many 45 watts per channel amplifiers that could drive 81db speakers for 13 years.

DaveC · Dec 30, 2014

amirm said:
If you put a couple of bricks on it, it will be even heavier.

Seriously, wattage is wattage. Watts=V^2/R. "Current" plays no role there.

Where current comes into play is when "R" keeps getting smaller and smaller. Higher current amps will not current limit in the face of small impedance of speaker. At higher impedance they put out no more power however than any other amp with the same wattage.

The big difference is the way the amps clip/overload. Tube amps do this without drawing much attention, you can overload a SET amp and get compression before it starts to sound ugly... most SS amps clip hard and sound really bad doing it. That's why people think tube amp power is greater than SS power, you can crank it up and get it into overload without even knowing it with a tube amp.

DonH50 · Dec 31, 2014

A class AB amp is in class A for low power then moves into class AB as the power level increases. The output is always pure class A as it crosses over from (+) to (-) devices, minimizing distortion in the critical low-level region. Class A is only ~27% efficient in single-ended mode, and a maximum of 50% using push-pull, meaning 1/2 to 3/4'ish of the input power is wasted (as heat). Class AB is up to ~67% so only ~1/3 of the input power is wasted.

W = I * V = I^2 * R = V^2 / R -- note R (ideal resistance) should be replaced by Z (impedance) for reactive (non-ideal) loads like speakers.

http://www.whatsbestforum.com/showthread.php?990-Units-Symbols-and-Terms-Oh-My!

An undistorted watt should sound the same no matter the type of amp. Differences in sound are largely due to the amplifier's output impedance (affecting interaction with the speaker) and saturation characteristics as mentioned previously.

http://www.whatsbestforum.com/showthread.php?8103-Example-Amplifier-Responses-into-Speaker-Loads

HTH - Don

microstrip · Dec 31, 2014

MrAcoustat said:
PS: To me this is NO regular 45 watts per channel amplifier.

Classé Audio DR3-VHC the vhc stands for Very High Current this amp is only 45 watts per channel class A the weight is 102 pounds.

View attachment 18342 View attachment 18343 View attachment 18344 View attachment 18345 View attachment 18346

The class A power specification is mainly established by the amplifier bias current . It is specified as the maximum power delivered by the amplifier to a 8 ohm resistor without switching off any of the active elements. The theoretical class A efficiency rating is calculated for an amplifier having a power supply with the minimum power supply voltage needed to deliver that power, ignoring any losses. Most of the time the amplifiers have much higher voltage in the power supply than this value, and can be operated at higher output power outside class A. I do not know the details of the Classé Audio DR3-VHC power supply, but it uses power capacitors with a 50V DC, 65V surge rating - I would expect such an amplifier to be able to deliver at less 200W before voltage clipping. But only measuring the power supply or looking at Classe data we can know exactly.

bonzo75 · Jan 9, 2015

What is the difference in class A and class AB from the same company when driving panels? For example, I think Pass Class A are luSh but don't have the grip of their AB models to drive Logans. I have heard a Lamm M2.2 on an AA, and it seemed to have good current and slam which I heard their class A (1.2 or 2.1, don't remember) doesn't have

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