A/V definitions..

[1tribeca]

If someone could clearly define the following 2 terms that'd be great...

1) Class A - as in Class A amplifier (as opposed to Class AB etc)

2) Discrete channel - as in 5.1, left and right discrete channels

I'm sure there's a bunch more I've never understood I'll come up with!!

Thanks!

 

[Seth=L]

If someone could clearly define the following 2 terms that'd be great...

1) Class A - as in Class A amplifier (as opposed to Class AB etc)

2) Discrete channel - as in 5.1, left and right discrete channels

I'm sure there's a bunch more I've never understood I'll come up with!!

Thanks!

Discrete is an advent that put Pro Logic to bed as the primary format for surroiund sound. With Pro Logic the surround and center where matrixed into a two channel it was decoded by using lots of algorithms to render it as 4 channels. Discrete does not mix into any channel. Each channel is completely seperate from the others.

 

[fmw]

If you want a technical answer to the question of Class A amplification, here it is.

I"n the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four classes of amplifier operations. They are class A, class AB, class B, and class C.

Class A Amplifier Operation

Class A amplifiers are biased so that variations in input signal polarities occur within the limits of CUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow.

Biasing an amplifier in this manner places the dc operating point between cutoff and saturation and allows collector current to flow during the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the input. Figure 2-12 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input.

The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples.

For a comparison of output signals for the different amplifier classes of operation, refer to figure 2-15 during the following discussion."

I like to think of it something like a water faucet. In a class A amp, the water is always running. You take a glass full and it just keeps running. In AB or B amps, the water is shut off during the times you aren't filling the glass. That's pretty simplistic but it might help picture what you have read above.

 

[1tribeca]

Given your water/tap explanation, why is a class A amp superior...or is it?

 

[fmw]

Given your water/tap explanation, why is a class A amp superior...or is it?

Superior in what way? If you mean does a class A amp sound better because it is class A, the answer is most likely not. If you compare an A and an AB amp in a blind test that have the same measured specifications except for the biasing, they will sound the same. There are some technical advantages to class A amps when you get to the limit of the power output of amps but that wouldn't be very meaningful in home audio except in the rarest of circumstances. There are some technical advantages for some radio frequency amplifers but that wouldn't apply to home audio.

AB amps run cooler and use less electrical power and sound the same for all practical purposes. Like tha faucet, there isn't much to lose by turning off the spigot during those times that you don't need water. That is why AB amps represent the vast, vast majority of amplifiers in home audio.

In general, I would say that, if you choose a big Class A amp do so because you want one, not because you think it will represent a sonic advantage. Amps with low distortion, flat frequency response curves and adequate power to drive your speakers in their environment are what you need. What we want in an amplifer is transparency and neutrality, not some "sonic signature." Don't spend any time worrying about how the output transistors are biased.

 

[dem beats]

holy crap

You guys are smart guys, and the glass analogy braught it into leymans terms. Me likey

Thanks!

I had the same question but never even bothered to ask.

As you get into c and d class how does that affect the was power is consumed?

 

[OttoMatic]

I'd suggest reading the Wikipedia entry. The explanation is pretty good, and there are pictures.

Class A is going to be the purest, and least efficient. In general, it uses a single transistor to amplify a signal (yeah, there will ultimately be more than one in the entire signal path, and amplification can occur in stages, but each transistor will amplify the signal itself). Yes, it's kind of "on all the time" in the sense that it's biased high enough to handle both positive and negative swings on the input signal. "Biasing" is on all the time simply means that there's a current flowing through the transistor or tube when there is zero input signal (but that current is not alternating at that time, and there is no sound). Properly designed Class A amplifiers will work only in the linear region of the transistor, which is a good thing. As transistors approach their non-linear regions, the output signal will be distorted with respect to the input signal. Class A amps are the least efficient, and a lot of the bias current, whether in use or not, is translated to heat.

Class B amps only use transistors in one half the range of the input signal. That is, one output transistor is used to amplify the positive part of the signal, and another transistor is used to amplify the negative part of the signal. As such, they can both be biased to be "off" when not in use; they will only be on if there's a signal coming in. The two halves of the signal are then recombined to create amplification of the entire signal. The problem here is that there can be crossover distortion where the signals are recombined (about the 0 point). Class B amps are not well suited for audio applications. They are more efficient than Class A or Class AB amps.

Class AB amps play some tricks to bias the transistors slightly on around the 0 point to eliminate the crossover distortion. They are more efficient than Class A, but less efficient than Class AB, and they are very common and well-suited for audio applications.

As to whether or not differences can be heard, you'll have to investigate for yourself. I'm not one to say that all amps sound the same, because there are simply too many variables to consider. There are definitely differences from an electrical engineering point of view; therefore, there's definitely potential for sound quality differences.

Good luck!

 

[Seth=L]

Superior in what way? If you mean does a class A amp sound better because it is class A, the answer is most likely not. If you compare an A and an AB amp in a blind test that have the same measured specifications except for the biasing, they will sound the same. There are some technical advantages to class A amps when you get to the limit of the power output of amps but that wouldn't be very meaningful in home audio except in the rarest of circumstances. There are some technical advantages for some radio frequency amplifers but that wouldn't apply to home audio.

AB amps run cooler and use less electrical power and sound the same for all practical purposes. Like tha faucet, there isn't much to lose by turning off the spigot during those times that you don't need water. That is why AB amps represent the vast, vast majority of amplifiers in home audio.

In general, I would say that, if you choose a big Class A amp do so because you want one, not because you think it will represent a sonic advantage. Amps with low distortion, flat frequency response curves and adequate power to drive your speakers in their environment are what you need. What we want in an amplifer is transparency and neutrality, not some "sonic signature." Don't spend any time worrying about how the output transistors are biased.

Class A amplifiers have a sonic difference (among transistor amplifiers) from Class A/B or Class B. The main reason why is linearity. A properly implemented Class A amplifier can drive virtually any load a speaker can impose on it. The SQ should be noticiably more dynamic at most levels of output with hard to drive speakers. The Klipsch RF series is a prime example of a speaker that needs very linear amplification to sound proper, especially at higher levels of output.

 

[fmw]

Class A amplifiers have a sonic difference (among transistor amplifiers) from Class A/B or Class B. The main reason why is linearity. A properly implemented Class A amplifier can drive virtually any load a speaker can impose on it. The SQ should be noticiably more dynamic at most levels of output with hard to drive speakers. The Klipsch RF series is a prime example of a speaker that needs very linear amplification to sound proper, especially at higher levels of output.

What is required to drive low impedance speakers is high current, not Class A operation. You can build an AB amp capable of delivering just as much current as an A amp. If your point is that most class A amps for home audio are designed to deliver more current than most AB amps, then I'll agree with you. That is a design choice. But I won't agree that there is a sonic difference because of biasing, OTHER THINGS BEING EQUAL.

 

[fmw]

I'd suggest reading the Wikipedia entry I'm not one to say that all amps sound the same, because there are simply too many variables to consider.

I'm not either but I will explain my position a little more completely. Amplifiers should be neutral. They shouldn't have a sound. If an audio frequency amp is designed to deliver enough power to the speakers for their intended application and then do so without audible distortion or audible swings in frequency response, then they will sound the same because they will be neutral. I'm not suggesting that all amps will meet those criteria but those that do will sound the same.

Output transistor biasing affects the efficiency of amps as we both mentioned but it doesn't necessarily affect the amp's ability to produce enough power with no audible distortion or swings in frequency response and those are the things that matter.

I spent 40 years as an audiophile. I finally quit it about 15 years ago after learning exactly what I could and couldn't hear in blind listening tests. So I'm very familiar with the audiophile world. Audiophiles crave big, powerful class A biased amps. That's fine. I don't have a problem with that at all. If I didn't have to pay for it, I would crave one myself. But I wouldn't be willing to assign a "sonic signature" to an amp just because it runs in class A. In my own experience, most of the high end amps are very transparent and accurate regardless of the class of operation.

I've done more blind tests personally than you would expect. I've tested A and AB amps together. I've tested tube amps. I've tested all kinds of amps. I've never heard a difference in sound that you could ascribe to the biasing of the output stage. Any differences in sound were due to differences in distortion or frequency response.

I once owned a very nice and expensive tube amp made by Audio Research. It was warm and cozy. It was big and heavy. It had a pair of handles on the front to help carry it around. It was beautiful to behold. It made me feel good. It sounded exactly like a well designed solid state amp. Why? because it was designed without audible distortion or wide swings from a flat frequency response. The fact that tubes were doing the amplification wasn't the issue. The design of the amp was the issue.

The audiophile world is riddled with beliefs, many of which don't stand up to blind testing. I gave up those beliefs a long time ago. Now I just stick to the real world in my audio. Audiophilia is a nice hobby. Unfortunately, it has less to do with sound than most people think.

Sorry if this came off as a rant.

 

[1tribeca]

All I know is, that a client of a friend who owns a shop, was blowing tweets in his B&W 800's (bit a of a volume hound) They did a bunch of elimination, and realized his Krells were too overpowering. we just swapped them for some McIntosh. Seems to be doing the trick.

As we were driving to/from the client's home we were talking shop...the two terms in my original post came up...never sure what they meant, now I do. Thanks to you folks!

The worst part about all of this, was I had to lift & carry 5 big Krells out of his basement into an SUV!!! How's your back? It's killing me, thanks.

 

[j_garcia]

My PM-7200 has a switch so it can be run in Class A mode. It does slightly improve the level of detail but it also reduces the amp's total output because so much more is lost to the considerably larger amount of heat generated. In A/B mode it gets barely warm. In Class A, it gets extremely hot, enough to actually alter the temp in my bedroom after a while.