Do You Prefer Class A or AB?

J

JLagoon

New member
Hi everybody,

I am curious of people's preference in their amps. Do you prefer playing through a class A amp/mode or class AB?

I have a VHT 2502 power amp, which as class A/AB switch. I have not switched the amp to class A for a long time. When I ran it in class A earlier today, I noticed the big difference. It's much smoother, responsive, and the sound of low strings are rounder.
 
Re: Do You Prefer Class A or AB?

I prefer what sounds good.

Vox AC30s and 15s are A/B and I love the sounds. Actually most amps I love are ABs but I never played a proper class A amp but my Ibanez Valbee which isn't THAT great to begin with...
 
Re: Do You Prefer Class A or AB?

The Vox AC15 was a class A If i'm not mistaken. I like class A, driving the tubes hard, all the time.
 
Re: Do You Prefer Class A or AB?

Nope. Both are class A/B I'm pretty sure.

As a matter of fact I believe that 99% of amps that have more than one power tube are class A/B. Any push pull would make an amp class A/B.
Though I am by no means an expert.

Also class A refers to CLEAN sound. So the overdriven sound of class A is like talking about how well a plane handles on the ground.
 
Re: Do You Prefer Class A or AB?

AB. I always think of Santana's syruppy sound when I think of class A and Hendrix's roar when I think of AB.
 
Re: Do You Prefer Class A or AB?

Can somebody please once and for all explain me the difference between the two ..... am I always thingking that a class A is a non-master volume amp where a lot of gain comes from the power tubes and the way to get clean is to turn down the volume on the guitar .... Class B is everything else ..... am I right ???
 
Re: Do You Prefer Class A or AB?

I had an Electrical Engineer explain it to me quite a while ago. I will try and explain, but I don't REALLY understand.

Class A runs the power tube(s) in series, no need for matched sets. This type of circuit wastes a lot of energy (not sure why) and is therefore not as loud as class AB.

Class AB-->Power tubes run parallel, requiring matched sets. This is a much more efficient configuration.

He had some information on specific frequency responses and damping effects, but that information went right over my head.

This what I remember from the conversation, anyway. I hope it helps.
 
Re: Do You Prefer Class A or AB?

Woah!!! There's a lot on misinformation flyin' around here. Let me try to clarify a few things...

It is confusing, because Class A was originally defined with a single device in mind, and it had to do with where the operating point, or idle bias, was set for that tube or transistor. But it was a terribly simplified definition that led, and continues to cause, much confusion today.

For output stages, it's a lot easier to include the load as part of the definition. If fact, it's absolutely necessary. I'll explain why with a much more practical and real world example. As you may know, the load , or speaker, completes the amplifier circuit. Suppose you have a 10 Watt amplifier connected to an 8 ohm speaker. The output current will be 1.1 Amps. If the amplifier's output bias is set such that the plate current (or collector current for a transistor) is 1.1 Amp or higher, it is running in Class A. Put another way, if the output current never exceeds the bias current, it's Class A. Now, here's why the speaker is so important. Suppose you connect a four ohm load. If the amp has the juice to produce it, it will now put out 20 Watts, and the output current will increase to double, to 2.2 Amps. You've only got 1.1A at 10 Watts in Class A. The next 10 Watts will be produced in Class A/B - the signal itself pushes the bias up in Class A/B to deliver the extra current and power.

The class of operation has nothing to do with how the devices are connected (series/ parallel). By setting the bias as explained above, any amp, single ended, or push/pull, can be run in Class A. It's just extremely inefficient, because when the power isn't being used, it's converted to heat.

Most push/pull A/B amps have their bias set high enough to eliminate crossover distortion, the point at which the positive and negative devices are switching. It's usually pretty low, because even a few watts of Class A gets things cooking, and it's a lot harder on the power supply transformer, caps, and output devices as well.

Class A is known for it's smooth, rich sound, and sounds more powerful than its rating suggests.

Hope this all makes some sense.

Noth
 
Re: Do You Prefer Class A or AB?

Class A power amps are inefficient in comparison, because they are "always on". Class A power amps are designed to amplify the entire signal waveform above zero (idle)...just like airplanes can only climb and descend above ground! Everyone knows what happens to airplanes when they stop using energy!

You get high fidelity, with low efficiency.

Class A/B power amps split the work. 1/2 of the signal is amplified by one tube(s) on the (+) positive side of the waveform , and the another tube(s) is used to amplify the (-) negative half of the waveform. Since each power tube in the A/B amp works only half the time, the potential output is greater. They can simply create a larger waveform. Class A/B amps are often called push-pull amps, and
I liken the operation to two lumberjacks using one of the old two handled saws on a large tree.

You get high efficiency, but the fidelity suffers, as there is a point where the the halves switch on and off alternately.

There are other classes, like class D. They are even more efficient, use little power...but are better of when used for voice intercoms.

BTW, gimme typical class A/B. I needs my crunch-nosity, and do not have the talent to exploit Class A design.
 
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Re: Do You Prefer Class A or AB?

Go Noth! We were typing at the same time! There really is no way to describe some of this stuff, without taking the time to understand the basics...and understanding of the 360 degrees of an AC waveform.

I doubt that any of the greats understood this stuff either, so it may be like Frank Zappa said "Shut up, and play yer guitar"! <<I'm not telling this to anyone...it's just a reminder to myself!
 
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Re: Do You Prefer Class A or AB?

Sludgenutz said:
Class A/B power amps split the work. 1/2 of the signal is amplified by one tube(s) on the (+) positive side of the waveform , and the another tube(s) is used to amplify the (-) negative half of the waveform. Since each power tube in the A/B amp works only half the time, the potential output is greater. They can simply create a larger waveform. Class A/B amps are often called push-pull amps, and
I liken the operation to two lumberjacks using one of the old two handled saws on a large tree.
Click to expand...

So, I guess I shouldn't try to pull my on EL34 out to "bring down" my 50 watt laney to 25 watts? :scratchch
 
Re: Do You Prefer Class A or AB?

So if I pull a tube out of my 15 watt traynor which is class A, it will become a 7.5 watt amp with no negative side effects?
 
Re: Do You Prefer Class A or AB?

Sludgenutz said:
Class A/B power amps split the work. 1/2 of the signal is amplified by one tube(s) on the (+) positive side of the waveform , and the another tube(s) is used to amplify the (-) negative half of the waveform. Since each power tube in the A/B amp works only half the time, the potential output is greater. They can simply create a larger waveform. Class A/B amps are often called push-pull amps, and
I liken the operation to two lumberjacks using one of the old two handled saws on a large tree.
Click to expand...
Not exactly. You are describing Class B, where one tube amplifies the + side and the other tube amplifies the - side. That results in crossover distortion, A/B biases the tubes so they are both active in a small range near the 0 axis. (The - tube is active for a small region of the + side of the waveform)

I agree with everything else Noth and Sludgenutz said.
 
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