I am confused about how to equate and compare class A to class AB solid state amps. Is there a conversion factor, or is it not that simple? I have heard people say that "this 30 wpc class A amp is easily equivalent to 150 wpc class AB" etc.
The two top Luxman integrated amps cost about the same. One is the 30 wpc pure class A L-590Aii, and the other is the 120 wpc class AB L-509u. How do these amps equate or compare to each other? Which is more powerful (or are they about the same?)? I am confused!
An AB amp that is class A to X watts is not equivalent to a class A amp, bear in mind - the distortion spectra are different even within the class A range (and that is largely what defines the sound of an amp). Nelson Pass has an excellent article explaining this on his FirstWatt site.
Furthermore, only a full class A amp can be single-ended as well (as the Pass Alephs are) - that change in topology seems to make an even bigger difference to the sound.
There sure are some great-sounding class AB (SE and PP) amps these days, tho!
In general, I love tube amps. I know this is a generalisation, but the solid state amps I have found listenable to, were nearly all Class A, the Aleph 3 being the best. I would love to hear the new integrated based on the XA30, I expect that will be quite special. Otherwise, I found an edge, lack of musicality, harshness, call it what you will, to class A/B, SS amps. The only exception was the Lavardin IT, which was very nice indeed.
One other thought here is this... listening levels.
Listening levels are achieved only by the input of power to the speakers... ergo, watts. Class A watts, Class D watts or Class A/B watts... it'll take a certain amount of them to produce a certain SPL.
AS often as I've come over this issue, I continually forget the where abouts of the conversion chart online.
The amount of watts needed to produce the level you desire at times going into your speakers is quite important! An insufficient amount can put them in harms way either immediately or over some period of time if they are in fact being under driven, or starved for power, when trying to achieve the levels you wish now and then.
80% of the music is made in the first 20 watts.
But if the Verity's aren't very eff, or have some severe impedance drops with some freqs, more power is surely going to be needed than those first 20 watts.
I'm no paint peeling listener myself usually. 85-90% of the time I'm under 90db SPL. But there are those times one may wish to twirl the fun knob up.
Don't risk downtime or addded expense by runnnig with insufficient power.
Mark - When you listen to 200W peaks your class AB amp really delivers to speakers on average only few percent of peak power. It is because music is not always at the peak and half as loud means 1/10 of power. In addition there are gaps (unless you listen to sinewaves). Nobody sane would design power supply in 2x200W amp for 800W required (50% efficiency at best). Typical amp can deliver full power at limited period of time (like 1 minute) before thermal protection kicks in. There is no need to design large heatsinks for the same reason - otherwise they would have to be the same size as in class A amps.
Class A is extremely wasteful but has one big advantage - is more linear before negative feedback is applied. At the end (with feedback) both class A and class AB might have the same linearity but class AB required 20x more gain (before feedback) to achieve this. This huge gain (in order of 4000) in connection with limited response of the amp (limited slew rate) is causing late feedback correction. In time domain it shows as small overshoots of pulses and in frequency domain it shows as enhanced higher order odd harmonics (bright sound). This is called TIM (Transient Intermodulation Distortion) and was unknown until 70's. There are great class AB amps where designer minimized the problem and also some bad class A amplifiers. You have to listen. My general intuition is that better specifications mean worst sound (achieved by deeper feedback). Same with power - what is on the paper is useless. The only usefull parameter you can find there is weight (some correlation with power).
Mark I think the deciding factor should be your ears;if you can bring the 2 Luxman units home with the speakers to see how they interact in your listening room;let them warm up for a set period of time then sit down and enjoy the decision making process.Hopefully you can do a home audition of the pieces in question;good luck and enjoy the listening.
Many thanks for all the helpful replies! Now I am less confused.
The speakers that I am considering are the Verity Audio Parsifal Ovations, driven by either a Luxman 590Aii or a 509u. Both Luxman integrateds are heavy beasts with large transformers/power supplies, and they exactly double their output into 4 ohms. One is class A (590Aii, 30 wpc) and the other class AB (509u, 120 wpc).
The 509u probably operates in class A at low volumes, and may be the way to go. I know that the 590Aii is highly regarded, but I am concerned about a hot and power hungry pure class amp in the long-term.
Adding a Wadia 381 or an Esoteric X-05 cdp to the mix may be the beginning of a great system.
I owned an Aleph 30 (Class A) amp rated at 30 glorious watts per channel. Based on the different amps I have owned previously and a direct comparison to them, it had no more power than just a 30 watt per channel AB solid state amp.
An amplifier is a tool to drive your speakers, select them first and the select an appropriate one that you like the sound of on THAT speaker. There are many factors involved, ghostrider has pointed several of them out. Generalizations about the qualities of Class A, Class B, Tubes etc. are less important than the qualities of a particular amp and how well it matches your speaker.
A watt is a watt is a watt. It costs a lot more to build a pure class A amp than a class AB amp of the same power. Class A amps are very inefficient, need large transformers (power and output, if applicable), and dissipate a lot of heat.
For all their disadvantages, Class A amps often have a kind of liquidity that is hard to find in Class AB designs.
Class A amps go into clipping more gracefully than class AB amps. This sometimes leads users to push the class A amp hard into a load because of the gradual move into distortion (and largely 2nd harmonic distortion at first, which is pleasing to the ear).
So the L-509u is capable of much more power than the L-590Aii. There is one more factor to consider, though, and that is how the two amps behave as the load impedance changes. Speaker impedance often varies across the frequency spectrum, and it's not clear how well the two amps deliver current to lower impedances. Class A amps often but not always have an edge here due to their large power supplies and basic topology.
If you are using efficient speakers consider the L-590Aii. It will emphasize finesse over brute force.
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