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What exactly is Class A operation?


flatgrass

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In the simplest terms....

Class A operation means that the amp's amplification circuts are always switched on. Results in smooth sound...cost is high power consumption and lots of heat generated.

Class A/B, lets skip that for now....

Class B, means that the amps amplification circuts power up when music is present and is off in between passages. Results are acceptable...power consumption is lower than Class A and is relative to the dynamics of the music...heat is less than class A.

Now, class A/B, the amp operates in class A mode for the first few watts. Most listening is at lower power levels anyway. Then switches to class B mode if music demands increases. Combines the advantages of B with the benifets of A.

My personal take is that class A amps operate the closest to how a tube amp sounds. The switching of class B is what makes music on these amps sound fatiguing.

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"My personal take is that class A amps operate the closest to how a tube amp sounds. The switching of class B is what makes music on these amps sound fatiguing."

Most tube amps are class AB.

"Class B, means that the amps amplification circuts power up when music is present and is off in between passages. Results are acceptable...power consumption is lower than Class A and is relative to the dynamics of the music...heat is less than class A."

Class B means that one half of the output stage handles the positive half of the waveform, and the other half of the output stage handles the negative half of the waveform.

"Class A operation means that the amp's amplification circuts are always switched on. Results in smooth sound...cost is high power consumption and lots of heat generated."

Class A is divided (generally) between single-ended and push-pull. A single-ended design is biased at the full current required to reproduce the signal peaks. A push-pull design is biased at half of the peak current requirement. Under normal opperation the devices do not turn off during any portion of the waveform.

"Now, class A/B, the amp operates in class A mode for the first few watts. Most listening is at lower power levels anyway. Then switches to class B mode if music demands increases. Combines the advantages of B with the benifets of A."

Sort of correct, other than thinking you can get a few watts in class A.

A typical 100W class AB amplifier will have 40W of heat to get rid of at full power. Most of these amps cannot run more than about 5 minutes at full power without overheating and shutting down. This works because of the high peak-to-average ratio in music. A typical 100W class AB amplifier will run on a raw DC voltage of ±57V. Let's make our amp with heatsinks twice as big as normal 100W class AB amplifiers so it can run at full power all the time. With our huge heatsinks we can run 350mA of bias and only have 40W of heat to get rid of. 350mA in a push-pull design allows 700mA peak signal before one half of the output stage shuts off (goes into class B). 700mA is 495mA RMS. 495mA RMS into 5 ohms (minimum impedance of most Klipsch speakers) is 1.22W !

A real 100W class AB amplifier will be biased at about half of the previous example (as it has half the heatsink our example did).Power goes with the square of the current, so that means a typical class AB amplifier will move into class B above about 300mW !

Class A is not a cure all.

Yamaha made some really high quality amps in the late 70s that had class A switches on the front panel. The switch lowered the power supply voltage so they could increase the bias without generating too much heat. In Class AB the big model put out 125W, and 30W in class A. Listening tests showed that the amplifier sounded no different in class A than it did in class AB (as long as you did not drive it beyond 30W and clip it).

Big class A amplifiers designed as such do sound different than class AB amplifiers, but for reasons other than the bias levels.

Last but not least, it should be pointed out that most McIntosh amplifiers are optimally biased class B designs (or very low bias class AB), and run quite cool.

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Gentlemen,

Thanks. I am beginning to get the picture. Although, must admit, still learning about exactly how bias acts and affects.

My paraphrasing then, more power efficient, quicker response?. Not necessarily cleaner signal?

As a matter of fact, one of the items I was looking at was one of the Yamahas mentioned.

kevin

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Gentlemen,

Thanks. I am beginning to get the picture. Although, must admit, still learning about exactly how bias acts and affects.

My paraphrasing then, more power efficient, quicker response?. Not necessarily cleaner signal?

As a matter of fact, one of the items I was looking at was one of the Yamahas mentioned.

kevin

Kevin,

The historical perspective is that when the amp switched (remember: "Class B means that one half of the output stage handles the positive half of the waveform, and the other half of the output stage handles the negative half of the waveform.) there was a byproduct called "crossover distortion". This was a harmonic distortion that was most audible at lower levels (hence the motivation to run as it as Class A for the first fraction of a watt). However, amplifier designers were quite effective at conquering this form of distortion and now days folks do not worry as much about crossover distortion, since the engineering and design has successfully tackled this issue (folks can still argue however ... and not all the fixes were successful, especially in the earlier days).

Regarding the Yamaha amps (M series, or at least some of them). These were good sounding amps (whether or not the Class A could be distinguished). They do not have a large following, so they can be picked up at reasonable prices.

Good Luck,

-Tom

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Everyone,

Again, thanks for the edification. The responses, along with the articles, have really helped. I actually know much more than 24 hours ago. Isn't this forum great?

I am now thinking seriously about trying out a Yamaha CA series integrated. It is supposed to have 10W when switched to class A mode, roughly 50W when switched to B.

Should give me plenty of power to work with. I plan to use this mainly for some ceiling and outdoor speakers (most likely in B). But, may give it a spin on the main Belles (in A) just to satisfy curiosity.

kevin

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Tom gave some great info. Let me get a little more philosophical.

Our experience in air has a natural bias. We are sitting in one atmosphere of pressure. Sound is little variations of pressure plus and minus from 14.9 pounds per square inch on our ear drums.

Despite this, once we translate this into electrical storage or transmission, the typical system is to make "no music" or quiet, into zero volts. Then we have to have to start representing the rarification as negative numbers. The plus and minus sine wave is the typical model.

The invention of the vacuum tube and transistors was wonderful. However, they have a flaw or basic charicteristic. They are all controlled rectifiers which can only pass current in one direction. Like a water valve, you can twist the handle to increase or decrease flow, but not make the flow go backwards.

In Class A operation of a single tube or transistor, we make it flow of current to mimic the plus and minus with a bias. The negatives of inputs are close to zero flow, the zeros of input are half turned on, and the pluses are all the way turned on. This is a gross generalization.

Importantly, it assumes we only have a single "battery" of power supply with positive voltage. Keep that in mind. Maybe we can make a separate amp and put in another battery backwards, and have a supply of negative voltage. A single valve doesn't know, in the proper circuit. The valve is still doing its thing, forward biased. It just doesn't realize that it's hooked up backwards relative to its cousin.

Single "valve" circuits can be operated in Class B, but it means that only half the waveform is amplified. This can be done in radio frequency amps where we're just making one frequency and don't care about fidelity and can filter out the gross distortion. Class B does have it's place elsewhere.

As Tom points out, the problem with Class A with a single valve device is that when there is no music, or the level is low, we have to keep the transistor or tube turned half on. This is okay in circuits where we are just making analogies to the music in mixers and low power levels. In fact, just about all audio cirucits before we try to drive a motor, i.e. speaker, are working in Class A. The stuff in the pre-amp and the like are typically working in Class A.

Part of the problem of terminology is that a single valve is said, correctly, to not conduct in Class B, and half of the plus and minus waveform is lost. But, if you have have two valves and the new, second one, is connected to the backwards battery, then the entire waveform is preserved. Two valves,each in Class B, combined, do work well. This is a push-pull circuit. As pointed out by others, the very clever circuit designers have found ways of allowing even the push pull to only push (Class A) at low levels,

I've used the term of a backwards battery. That is just a restatement of a negative and positive supply rail from a power supply circuit. .

Gil

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"....

If you like Yamaha gear and it's in your budget, you could look for an Internet dealer who's carrying the MX-D1 amp. 500Wpc of clean class D power, and it's happy to run at a fraction of a watt.

...."

You want to be careful and not lump all the Yamaha gear together. The stuff I was referring to was made before the mid 1980s (or so). At that time they had some very well designed equipment. Don't assume that is still true today.

Good Luck,

-Tom

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Tom,

The MX-D1 is no ordinary Yamaha amp. Some reckon it to be one of the best amps ever made. It has nothing to do with any of the other Yamaha product lines past or present.

It is also true that it is probably not the amp you would automatically choose for Horn loaded speakers. Not many people need 500 wpc with a 100 (+) dB/w/m sensitive speaker.

Does drive a B&W 801 with consumate ease though....

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Max isn't kidding about the MX-D1's build quality. According to the review in Stereophile,

"The MX-D1's overall construction and parts quality appear to be extremely high; the heavy, copper-plated chassis has extensive magnetic shielding and vibration control."...

...Inside is a true dual-mono design, including separate power supplies, that Yamaha calls Twin Monaural Construction.

Although the list price is US$5000, John Atkinson, Stereophile's lab tester, stated: "Summing up these measurements, the beautifully finished Yamaha MX-D1 offers both very high power and very high dynamic range at a very attractive price."

In the review in the UK magazine, Hi-Fi News, the reviewer wrote: "As you might expect from a product at this price, the standard of its construction, inside and out, is meticulously detailed. Mono channels are the order of the day with independent power supplies and even independent, screened chambers for the input amplifier, the output filtering and switching supply. No expense has been spared - even the output filters are exclusive, low-impedance toroids with double cores."

It's definitely not a mass-market item like most Yamaha gear. A Yamaha USA tech told me that only about 200 were built. Happily, all that effort resulted in a very pleasant-sounding unit that provides high levels of presence and clarity when listening to music.

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