Is Rotel considered a Class "A" Amp.?????

[chinets]

Is Rotel considered a Class "A" Amp. ???? I've heard that Rotel are NOT considered Class "A" amps. Is that true??? I also know ,as I own the Rotel 1065 that they heat up too much that the fan starts blowing. How come they heat up so much? Would upgrading to Rotel 1095 Amp. solve that problem?? They are driving N803 front and N804 rear plus HTM1 center.

[DrJRapp]

This is all very technical in nature, and very difficult to explain in laymans terms.You are using the terminology as if it were grading the quality of the amp, which it is not, although pure Class A amps are considered superior due to the lack of switching distortion. The Class of an amp is the design criteria of its output circutry switching and bias. Most Rotel amps operate Class A/AB, which is common amongst high power, quality Hi Fi amps. The amps operate Class A up to a certain output and then shift to class AB. The theory is that by the time spl has risen to a certain point, the switching (crossover) distortions introduced in Class AB are awash in the spound and therefore unoticable.

The heat issue is even more complex. While Class A amps tend to run hotter that others (due to constant current flow on input) there are many criteria that determine how "hot" a unit will get in terms of temperature. Chasis design, power supply design and heat sink design are key players. The 1065 get as "hot" as it does since there is a lot going on in a relatively small chasis.

[mitch57]

Actually, I believe the Rotels are Class AB and not Class A/AB. I could be wrong though. Where did you get the information that they are Class A/AB?

[RebelMan]

According to Rotel they are Class AB. At least that is what they said when I asked them about two months ago.

[mitch57]

Audioholics did a review on the Rotel RMB-1095 and listed it as a "a push-pull class AB amplifier design". They discussed this under the "Build Quality" heading on the first page.

http://www.audioholics.com/productreviews/avhardware/RotelRMB-1095amplifier1.php

[Kens1]

Should not have read that - now I want one.

[mitch57]

Should not have read that - now I want one.

I'm selling mine. It's less then a year old. Do you want to buy it?

[Kens1]

I would like to, however new speakers are the first priority. How come you are getting rid of it?

[DrJRapp]

Actually, I believe the Rotels are Class AB and not Class A/AB. I could be wrong though. Where did you get the information that they are Class A/AB?

That came from a test report I read on the 1080 or perhaps the 1090 about two years ago. I'll have to dig it up. I could be that the stereo amps are different than the multichannels. The 1075 and 1095 I know are AB.

[mitch57]

I would like to, however new speakers are the first priority. How come you are getting rid of it?

It's a long story but in a nutshell I upgraded to the Parasound Halo A51.

[greggz]

Excerpted from here... http://www.aikenamps.com/ClassA.htm

The fundamental problem is in how class AB is defined, and how people interpret it. The people who say a class AB amp is "class A at lower volumes" are technically wrong, but for the right reasons. If you were to define class A as being only conduction for a full 360 degree phase angle, you would be correct. However, there is more to the definition of amplifier classes than that.
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Here is where the problem comes in: because a class AB amplifier is biased so that the plate current flows for the entire cycle at lower output levels (which is done to reduce crossover distortion), many people claim it is a "class A amplifier at lower volumes". This is simply not true. It is operating in conditions *similar* to class A, but is not a class A amplifier by any means. It is still a class AB amplifier, no matter what you choose to call it.

[mitch57]

Excerpted from here... http://www.aikenamps.com/ClassA.htm

So based on this information there is no such thing as a "Class A/AB amplifier? He doesn't really discuss that in his article.

[greggz]

I'm not sure what the real answer is. I did some Googling on "Class A/AB" amps. I see manufacturers claim their amps are Class A/AB but all the sites I can find on amp building don't reference Class A/AB.

This was the best explanation I could find of "Class A/AB"

The MOSFET output stages in Perreaux amplifiers run what is known as extended Class AB or A/AB. By balancing all the trade-offs; physical size, heat dissipation, power usage, and sonic quality, we have achieved what we consider to be the most beneficial result. By applying even more quiescent current, more than Class AB, but less than the requirement for pure Class A, we have biased the power MOSFETs to be ON at all times and are held just above their non-linear area, even during their OFF times.

Source: http://www.perreaux.com/class_ab.php

[Taito]

What some manufacturers call Class A/AB is really just class AB that has both 'halves' biassed 'on' more strongly than is strictly necessary for Class AB operation. (although it could also be their way of saying that the earlier stages in the amp are class A, and the output is class AB -but almost everyone does this)

Not sure if this'll help or not (trying not to get too detailed); The output stage of Class B amps consist of two halves, one covers the positive half of the signal, the other covers the negative half. However, transistors 'lose' approximately 0.7V. So if you think of a signal that swings 10V each way of '0', the first 0.7V of that 10V in each direction is lost. This is called crossover distortion. Class AB amps are class B amps that are biassed so that the transistors on each half are always fed 0.7V (or a little more) so that when the signal crosses over '0', it adds to the 0.7V and the problem of crossover distortion is reduced. What people are calling Class A/AB is when the transistors are more strongly biassed 'on' than with just Class AB. This can help reduce crossover distortion further, but any amp. which is based around the class B topology will exhibit some crossover distortion. It's a trade-off (as are all things engineering) between distortion and efficiency. If you can dramatically reduce power consumption and heat generation at the cost of a little extra distortion....

Might be helpful, might not.
-Ben

[mitch57]

What some manufacturers call Class A/AB is really just class AB that has both 'halves' biassed 'on' more strongly than is strictly necessary for Class AB operation. (although it could also be their way of saying that the earlier stages in the amp are class A, and the output is class AB -but almost everyone does this)

Not sure if this'll help or not (trying not to get too detailed); The output stage of Class B amps consist of two halves, one covers the positive half of the signal, the other covers the negative half. However, transistors 'lose' approximately 0.7V. So if you think of a signal that swings 10V each way of '0', the first 0.7V of that 10V in each direction is lost. This is called crossover distortion. Class AB amps are class B amps that are biassed so that the transistors on each half are always fed 0.7V (or a little more) so that when the signal crosses over '0', it adds to the 0.7V and the problem of crossover distortion is reduced. What people are calling Class A/AB is when the transistors are more strongly biassed 'on' than with just Class AB. This can help reduce crossover distortion further, but any amp. which is based around the class B topology will exhibit some crossover distortion. It's a trade-off (as are all things engineering) between distortion and efficiency. If you can dramatically reduce power consumption and heat generation at the cost of a little extra distortion....

Might be helpful, might not.
-Ben

Parasound Halo Amps claim to be Class A/AB amplifiers. Their literature claims that they operate in high bias class A at 10 watts (A51 amplifer) on both the input and output stages of the amplifier. Their JC1 monoblocks operate at 25 watts on both the input and output stages. The JC1s also have a toggle lever to switch between high bias and low bias.

I don't even come close to being able to understand all of this techno babble! But it would be nice to know which manufacture's claims are indeed factual and which are not.

[Taito]

I might be mistaken, but the way I read it, the input of the the A51 is run in class A (biassed to (on average) use 10W). The output is class AB, with the transistors biassed 'on' with 10W. This will reduce crossover distortion, but does not mean that there is no crossover distortion. Similar story with the JC1, only with a greater reduction on crossover distortion. (not sure about digital, but) As far as analogue amps are concerned, only pure Class A amps will have no crossover distortion.

However, a well implemented amp in any topology will outperform a poorly implemented amp in another topology.

-Ben

[mitch57]

It sounds like you understand it better then I do.

[JonMarsh]

I might be mistaken, but the way I read it, the input of the the A51 is run in class A (biassed to (on average) use 10W). The output is class AB, with the transistors biassed 'on' with 10W. This will reduce crossover distortion, but does not mean that there is no crossover distortion. Similar story with the JC1, only with a greater reduction on crossover distortion. (not sure about digital, but) As far as analogue amps are concerned, only pure Class A amps will have no crossover distortion.

However, a well implemented amp in any topology will outperform a poorly implemented amp in another topology.

-Ben

I like the last statement you made a lot. There's an awful lot of other stuff in this thread which is "mostly" right, and partially/sometimes wrong. I haven't designed audio amps as a primary living for almost 25 years, but I still dabble in consulting now and then.

The thing I like about the last statement Taito made is that it is the clue for why many other axioms or generalizatios in this thread are at times wrong. I'd probably refer folks to sites like Hugh's at AKSA or Douglas Self's site on audio topics for the lengthy dissertations explaining the nitty gritty- I'm too tired tonight to summon the effort, and need to pack for a biz trip to Singapore.

Just a few points I'll throw out for consideration or discussion...

1. Crossover distortion is dependent on other factors besides bias level, and is heavily influenced by output stage topology and the specific transistor technology - extended beta bipolars (as produced by On Semi and Toshiba) having the lowest I've measured outside of true class A (bias so high that it exceeds the maximum required output current)

2. Many manfacturers do bias their amplifers into a relatively high state of AB- that is, so that the first several watts up to the first tens of watts may operate without any of the output devices going into cutoff. Sometimes I think this is done more as an audio mythology thing than for any practical reasons. Why? See number 3.

3. A well executed "blameless" Class A/B amplifier biased at the critical minimum bias for it's lowest designed low impedance, done with Extended beta transistors and the right bias circuit, will exhibit essentially unmeasurable crossover distortion running open loop (i.e., before any feedback). This is usually at a relatively low, cool running level of bias. Self claims this, I've verified it for myself. (though Self uses conventional feedback amplifier configurations, and I don't). Raising the bias from this point INCREASES the distortion because of higher beta in the crossover region and the generated discontinuity in the output stage transfer function. This can be verified by measurement of open loop stages as well as by simulation.

4. Much of the notch distortion in many solid state output stages is due to charge storage in the base/collector junction in relatively slow bipolar transistors- MOSFETs or 30 MHz extended beta types don't seem to exhibit this. Increasing the bias doesn't reduce this distortion much, unless you go up to class A, and then there are still some residual problems. I.E., these older slower transistors don't measure well and don't sound good. period.

5. MOSFET output stages most often have difficulty because the transistor Gfs is low, unless you parallel lots of them or use compound amplifier structures which are more difficult (but not impossible) to compensate for stable operation. The inherent open loop output impedance is quite a bit higher, and this means either using more feedback, suffering more level related non-linearities, or using more transistors. Early MOSFET amps like the proposed Hitachi designs or the Haflers had HUGE amounts of opend loop gain and bandwith- only the MOSFET speed allowed them to run that much bandwidth. Not very linear, though.

Moderate cost commercial amps that I've heard and think sound pretty good for the money are the JC designs for Parasound - they use extended beta output transistors, too. Aragons aren't bad with the right preamp- ditto on extended beta transistors. My commercial faves, though, are Ayre and Theta Digital. No loop feedback, extended beta output devices, and lots of them. Output FETs in some models. Again, lots of them. Of course, there are other good midrange and top end amps, but I certainly haven't heard all of them. Oh, I do like Conrad Johnson and Sonic Frontiers tube monoblocks quite a bit. Not necessarily the most accurate at the frequency extremes, but very fun and musical.

Just my 0.02.

[Taito]

And what a two cents! Thanks for the input, Jon.

[lvhung]

I listen some tube amp with BW
I said very smoth but don't like the sound of class A amp