CLASS A POWER RATINGS
In well designed SS class A power amps I was told that although power could increase to double in class AB mode from speaker demands of 8 ohms to 4 ohms, class A suffered to be cut in half.
For example a class A denominated amp rated at 50 watts class A into 8 ohms would double to 100 watts class AB into a 4 ohm load but only have 25 watts of class A power before switching to class AB.
But I've noticed of late claims of manufacturers stating their amps double their class A power as impedance demands are halved.
So is the explanation that technology has advanced ?
@rost The confusing bit here is that any amplifier that goes to 'AB mode' is a class AB amplifier and is not class A. By definition. If you are conflating a class AB amplifier with a real class A amplifier, I can see why the question. The technology hasn't advanced in this regard. When a class A amp can double its power into half the load impedance, its simply because its built to be able to do that. The technology to do that has been around for decades. |
I like what @atmasphere said. i'll add that the heat generated by the "AB amps that operate in class A until X wpc" is never enough to convince me that they are truly operating in "Class A mode". My tube amps, class A, all the way. Jerry |
Real Class A or not, this is the Class A amp I wanted to get (never say never). I had 2 lower powered amps in the same lineup, and they were great. Krell KSA-i400 40th-Anniversary Reference Stereo Power Amplifier (hifiheaven.net) Read the specs on it and how it operates in 'Class A'. |
While I do not understand their magic, this guy does all class A only, I believe his theory is, "if the first watt sucks, why continue". His go up and down from 8ohm to 4ohm, in either direction. All I know is, the stuff sounds pretty awesome. While I don't go to magic shows, their stuff is pretty magical. |
The only amps that you can be sure are always class A are single ended amps because their topology makes them always class A; they cannot go into class B since the output devices must be on all the time. Push/pull amps are different and what you read is true. Most 'class A' push/pull amps go into class B at some point, especially into lower impedance loads. Look at the Pass class A push/pull amps. They are rated at 30 watts and they are class A at 30 watts but all the tests show they actually develop about 70 watts but by then they are class B They are rich AB amps and Pass is honest since he only calls them 30 watts where they are class A. It's hard, expensive and difficult to make a push/pull amp of any power that stays class A essentially all the time. The only one I can think of was the Mark Levinson ML2 amp of decades ago It was huge and yet only rated at 25 watts 8 ohms. But it had huge bias that kept it class A all the time even doubling into low impedance loads. It was 50 watts 4 ohms and even 100 watts 2 ohms and I believe it still stayed class A. But stop and think even an 85 dB speakers only needs 1 watt to play that loud and 10 watts for 95 dB. So the Pass 30 watt class A amp is still class A when it's so loud you shhould not be in the room. It's only split second peaks like banging a piano where the amp goes out of class A and then for such short times the amp is in class A probably over 98% of the time and you don't notice the spit seconds it's class B. Most of us were super happy when we got test scores in school over 95. |
+1 This makes absolute sense. Either it’s a legitimate true class A designed and built circuit or it isn’t. Amplifiers that claim generous class A power and are barely warm to the touch are suspect to me. Perhaps some degree of class A biasing but not genuine full “A” circuit. Charles |
An excellent example of a pure class A push-pull amplifier. It was/is a beast. |
Why not just buy a Sugden amp and end it. You will no longer have a yearning for anything else....read up on Sugden, they were the first with SS Class A in 1967. No BS class A that transitions to class A/B....I had not even heard of Sugden, but a few years ago. My curiosity peaked, and I broke down and bought one. It set me back a decent penny ..but man I’m glad I did it.....A21SE Signature...once that was in place, I again broke down and purchased my Tannoy Speakers, now I’m in the poor house lol. I’ll tell you what, it’s one heck of a combo. You would not believe how good only 30 watts per channel can sound (40 watts into 4 ohm). I’m pretty much set for life, as I’m old. |
I have a pair of mark levinson 33h mono blocks each one weighs over two hundred pounds they double every time the impedance is halved and they are stable to 1/2 an ohm. They stay class a all the time and when they were tested the output power was limited by the input power. Each one the minimum power requirements are two dedicated 20 amp circuits. They do not switch to an ab amplifier and there power doubles not halves as the impedance is halved. |
@rost Wrote:
My understanding is a class A /AB amplifier will have a switch to differentiate between the two classes. The switch changes the amplifier's quiescent current. My amplifiers have a switch on the front panel in class A for 30 watts output, @ 8 Ohms the quiescent current is 250 watts and in class AB for 200 watts output, @ 8 Ohms the quiescent current is 80 watts. In my audio system my amps are always in class A mode. I hope that answered the OP question. 😎 Mike |
I've got a Pass 250.8 amp that runs 25 watts in pure class A and then transitions into class AB and can generate 250 RMS into 8 ohms. It idles in Class A and sucks 400 watts of juice from the wall when not reproducing music- just turned on. It's about 110 pounds and when on is hot- not warm- hot- and will heat my dedicated room to 73 degrees after being on for a couple of hours. The power meter on the front which shows when it starts to move from Class A into AB mode never budges and I play it loud into a pair of Wilson Sasha 2. It's an incredible amp, super musical and dynamic and great bass. I've never been able to get it to come close to moving into AB, the first 25 Class A watts will drive the speakers louder than you can stand! |
I read the specs. The amp runs in Class B and has a circuit that senses when there is a peak in the programme. It then puts the amp into Class A before the moment of the peak, so it can be fully reproduced. OK read that again please. That is of course impossible. Without a time machine. Krell called it an 'anticipator' circuit. But of course it cannot anticipate. If you want Class A, like I do, the Krells to buy are the pure Class A KRS200s like mine. Class A, the whole Class A and nothing but Class A. |
@sngreen No. The Dynaco SCA35 is an EL84-based push-pull amplifier that is class A to full output. This is so because both output tubes are fully class A, only operating out of phase with each other. In this way the amplifier generates lower distortion than would ever be possible with a single-ended amplifier, and easily has wider bandwidth (and more power) too. No downside, except for the heat. We've been building class A push-pull OTL tube amplifiers that are class A for decades on now (nearly 50 years). Our largest model makes over 500 Watts in class A. FWIW though, class D has seriously eroded any reason to go class A. The reason a class A amplifier can sound good is because of its distortion spectra, which tends to be more benign than an AB amplifier. However, the way the feedback is handled has a tremendous effect on the distortion spectra as well, which is why merely being class A isn't always enough. Feedback can be poorly applied (and often is), resulting in a harsh sounding amp with measurably low distortion, on account of non-linearities in the feedback loop introducing higher ordered harmonics. A benign distortion product will be mostly the 2nd and 3rd harmonics, with enough amplitude that they can mask the presence of any higher ordered harmonics (the latter of which, if unmasked, are responsible for an amplifier sounding dry, bright and/or harsh). Turns out that class D, if designed properly, has a similar distortion spectra to a really good class A tube amplifier, and sounds similar, since the distortion signature of any amplifier is also its 'sonic signature'. |
Another 'Class A' amp that uses the iBias is the Westminster Labs. Similar to the KRELL XD that I said is 'Class A'. I have a CODA #16 amp that is Class AB and runs the first 100 watts in Class A. The 'pure Class A' KRELL XD is very similar sonically. A bit smoother sounding. I hope to hear the uber new KRELL KSA i400 which on spec is incredible. |
@charles1dad .. but class B operation is push/pull, not? Does not push/pull imply switching in between? |
No. Class “B” and push-pull aren’t synonymous. @atmasphere has given a clear (At least to me) example/explanation. His OTL amplifiers and the Mark Levinson mentioned earlier are examples of pure class A push-pull topologies. There’re others as well. Charles |
@sngreen Class B only means that the amplifier is conducting for half of the waveform. You can build a single-ended amplifier that is class B (not that you would want to listen to it). IOW Its not an amplifier, its a class of operation. You can make single-ended class D amplifiers too. |
@clearthinker the krell ibias amps do not run in class B, they are class A the circuit used increases the class A bias as more class A power is needed. Krell's old sustained plateau bias was also class A, in fact if you drive those old amps hard they will generate enough heat that the top two bias positions will drop down to class B until the temperature of the amp comes down. |
@invalid I spoke to someone who has the KRELL KSA i400 at their shop. A bit amazed at what I heard today about it. |
@reg19 Possibly. More will depend on other aspects of the circuit, such as how feedback is implemented, the construction of the power supply and so on. |
@yyzsantabarbara - Would you care to please share what you heard about the Krell KSA i400 that you found so amazing? TIA |
I used to own the KRELL Duo 175XD and the K-300i so I know how the new KRELL sounds. I loved it. I sold the 175XD with the plans of buying the KSA -i400 or a CODA #16. I got a deal on a used #16 so I went that way. However, I do miss that new KRELL sound. I expected the i400 to be better than the 175XD and 300XD. That was confirmed to me by this dealer. However, what was surprising is that the dealer sells other amps that are in the same price range, and he was saying that the i400 is much better than the main line he sells. I think he said it kills the other amps he sells. He was trying to get me to buy some of the other gear in the past that he loved, but now his opinion is a bit different. In terms of priority the i400 is the last item on my list that he thinks I should buy since I already have the CODA #16. He is not really trying to sugarcoat his opinion on the i400 to me. The amazing part on the amp is the specs. It doubles down to 1 Ohm and can drive anything. It also does not get Class A hot and the sonics for me were the most relaxing amps I had owned (the 2 lower end XD ones). The only negative for me is the depth and 160lbs weight. A huge reason why I went with the smaller and lighter CODA #16. Though the C0DA #16 sounds so good in my office system. The plan was to use in in the Livingroom system.
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