CLASS A AMPLIFIERS

I could be very wrong about this, but, I thought Aragon's trick was that they offered their rated class A opperation only so long as they were driven into an 8 Ohm load, below which they reverted into class AB. From what I understand the best ss class A/AB amps half their class A output as their class AB output doubles into progresively halved impdedances.

The total output is given as 400 watts into 8 ohms and 1000 watts into 2 ohm, I guess that why the full name is Aragon Palladium 1K. They can't really be running that high in class A they just aren't that big and never run really hot just warm. Look at Avalon's (an agon dealer) listing for the claim. They do have enormous power though, huge headroom, never clip to my ears. I use 936 JM/ Focal Electras with them. They have double 7 or 8 inch woofers and they probably drop their resistance to 4-6 ohm at times. I am suspicious, believe me, but I like them anyway.

Even 25 watts of class A will run quite warm. If you want to check if your amp is class A, rig up an ammeter onto the power. While playing music at increasing volumes, when the dial/readout starts to oscillate, that's when it kicks into A/B. For that 25 watts, that would be 0.25 amps and that's loud.

they supposedly run up to 125 watts class A

Wow! They must be HUGE (even with a switching PS)... We're talking about a +500 VA PS per channel for class A... Now that's a nice monster!

BTW, if they claim 125/channel class A, they must be able to back it up; so, you probably DO have 125W one way or another (but it does sound like overkill as you note...)

I am still at a loss for comprehending the general sentiment expressed in this thread that class A amps are only beneficial due to lower distortion. Heat dissipation and the efficiency of their use of power are well known, but they are also given credit for having a typically fuller sound and are known for their ability to control bass well.
I own very high biased (reportedly) class A amps but can't run them as pure class A. If and when they would switch to A/B is mysterious to me. I don't hear any noticible change in the sound. It is entirely possible they never reach that point, in fact they shouldn't because they supposedly run up to 125 watts class A which would blow my head off except for perhaps dynamic bursts.
Does anyone know if the claim that Aragon Palladiums are really class A in the first place. They don't distort much if at all, but they don't run really hot, temperature wise, either. Any one who knows something about these amps and can tell me something other than the canned corporate stuff would be appreciated. Sorry if that is a thread jack.
Also why all the advertising about class A capabilities if it doesn't matter?

Those are all excellent answers but if you want to know how they sound different, visit a Plinius dealer as some of them are switchable between class A and A/B.

Thank you very much to all of you for those very clear and competent explanations. I feel much more smarter now!

There are very high powered pure class A amps. Needless to say they are typicaly very expenisve, very big, very heavy, run very hot and are costly to operate.

Charlot

This web site goes into a bit more detail concerning the differenct classes of amplifier circuit designs http://www.knowledgeisfun.com/E/El/Electronic-amplifier.php

In general class A is much less effecient than either class B or A/B(most common design) but procduces less distortion (e.g.cross over distortion in class AB) than either class B or A/B. With the inefficency comes heat,caused by bias current, which must be dealt with by utilizing large heat sinks.This generally makes pure clas A unsuitable for high powered amplifiers. An example that comes to mind is Nelson Pass his X seriers is class A/B and ranges in power from 150 (X150) watts to 1000 (X1000)watts per channel. The XA amps are pure class A and range in power from 150 (XA150) watts to 200 (XA200) watts per channel.

Chuck

Chuck

Generally, they are not more powerful. Class A designs are extemely inefficient because it requires that a lot of heat be dissipated to maintain stability, robbing power to drive speakers. For technical as well as practical reasons, this limits their power output stages to around 100 watts and their ideal design power rating is usually 80 wpc or less.

Class A amplifiers nearly always exhibit more authority, but this has nothing to do with power.

I should point out that a Class A amplifier can be either push-pull, single-ended, tube or transistor. What is important is that the amplifying device or devices never goes into what is known as 'cutoff'. IOW the device or devices each amplify the entire waveform all the way up to full output regardless if single-ended or push pull.

Distortion is generated when an output device goes into cutoff. In a push-pull circuit, when the device is cutting off while at the same time passing off the signal to its complement that is just barely turning on, the resulting distortion obscures low-level detail (as in the case of Class AB2 and Class B push-pull amplifiers). This can be complicated by the presence of an output transformer in the circuit as magnetics tend to react badly to this sort of thing!

So ultimately, Class A is the lowest distortion form of amplification, regardless of the circuit topology or amplifying device.

The price paid is Class A amplifiers are less efficient as they are 'on' all the time and therefore they make a lot more heat. They also require fairly beefier power supplies for the same reason. Such is the price of increased performance!

Much of the newer A/B designs will run up to nearly 25% of the output in pure class A and switch to AB for cooler operation and lower power draw.. and many of the newer A/B designs today rival and beat many of the older Straight Class A in my opinion, newer technology capacitors, transformers, transistors can make this possible. Older Thresholds, CODA, and Levinson's I owned are good, but my newer odysseys and many mcintosh amps sound better sometimes and do not run hot at all... so your taste after audition is going to determine if they are better for you or not, I hated the rediculous Heat and draw of electrical with low efficiency in comparison, so even if not sonically better I find some reliablity, ease of use, and cost benifits to the newer Class A-A/B designs.

Class A operation is the transistors are fully biased (turned on) meaning there is no crossover distortion (notch??) when the signal swings from + to - and vice versa.

A watt is a watt, so Class A watts are not more powerful than say Class AB watts.

Of course, the drawback of Class A is heat and energy consumption. The amp must dissipate all of it's power whenever it is turned on whether playing music or not. Either the energy goes out the heatsinks as heat, or to the speakers as electrical energy, or a combination of both in most cases.

No. The sonic benefit is that the entire signal is swung by a transistor (or several in parallel) instead of individual halves of the signal being split up by two different transistors. Since there is no "handing off" of the signal to another transistor, there is potentially less distortion. But then feedback comes into play but that is another story...

Some will say that Class A always sounds better than Class A/B but in fact, it largely depends on the circuit design.

The details are below; the bottom line is that in a class A amp, the output stage is on and passing current for both halves of the signal (+ and -). Consequently, during the time there is no signal, the devices dissipate power as heat. Class AB amps are designed so that there are devices dedicated to each half of the waveform so that less power is dissipated as heat and they're more efficient. The down side is that during the time the signal "crosses over" from + to - there may be a small delay in the device turn on that handles it and this cause a type of distortion known as cross over distortion. Many modern designs today work in class A up to a certain power level then switch to AB. Sorry for the verbose answer, but this isn't a simple topic.

Class A Amplifiers
Class A amplifiers operate over a relatively small portion of a tube’s plate-current or a transistor’s collector-current range and have continuous plate- or collector-current flow throughout each RF cycle. Their efficiency in converting DC-source-power to RF-output-power is poor. DC source power that is not converted to radio frequency output power is dissipated as heat. However, in compensation, Class A amplifiers have greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. They are most commonly used in small-signal applications where linearity is more important than power efficiency, but also are sometimes used in large-signal applications where the need for extraordinarily high linearity outweighs cost and heat disadvantages associated with poor power efficiency.

Class AB Amplifiers
As the designation suggests, Class AB amplifiers are compromises between Class A and Class B operation. They are biased so plate- or collector-current flows less than 360 degrees, but more than 180 degrees, of each RF cycle. Any bias-point between those limits can be used, which provides a continuous selection-range extending from low-distortion, low-efficiency on one end to higher-distortion, higher-efficiency on the other.

Class AB amplifiers are widely used in SSB linear amplifier applications where low-distortion and high power-efficiency tend to both be very important. Push-pull Class AB amplifiers are especially attractive in SSB linear amplifier applications, because the greater linearity resulting from having one amplifier or the other always conducting makes it possible to bias push-pull Class AB amplifiers closer to the Class B end of the AB scale where the power-efficiency is higher. Alternatively, push-pull Class AB amplifiers can be biased far enough toward the highly-linear Class A end of the scale to make broadband operation without resonant tank circuits possible in applications where broadband operation or freedom from tuning is more important than power-efficiency.