DaveVoorhis
Addicted Member
Some feel Class A sounds better than an equivalent Class AB amplifier.
Class A Watts ?
- Thread starter luxmaster
- Start date
Yes. It's usually about twice as powerful as an equivalently rated class AB amp. Class A amps are rated according to their bias point and the class A envelope is usually about twice the bias. For instance, my F5 is biased about 30 watts, but it's class A envelope is 60 watts, and then it transitions to class AB operation and can reaches up to about 110 watts. I'd take a 30 watt class A amp over a 100 watt class AB amp any day of the week.
DaveVoorhis
Addicted Member
No. 100 watts = 100 watts whether the amplifier is Class A, AB, C, D, T, H, TDAA, you-name-it.
I'm gonna have to disagree with you. I can't think of anybody that describes the power of a class A amp in terms of anything but what it's biased at. Again, take my F5. Is it a 30 watt amp? A 60 watt amp? A 110 watt amp? It'll do 100+ watts all day, but it's not a 110 watt amp. Is it a 60 watt amp? No, it can't be, because the class A envelope slides all over the place depending on the impedance presented to the amp. It's only 60 when you assume an 8 ohm load. The one thing it is all of the time is biased to about 30 watts. That's what anybody who builds them calls them. That's how Nelson Pass rates the thing. That's the rating convention for pretty much all class A amps.
DaveVoorhis
Addicted Member
Whilst there has been some deviation from the FTC rating standard (mainly on HT, Class D and "party" amps), wattage rating of an amplifier is normally determined as the power it can continuously deliver into speakers at a rated distortion level. See, for example, http://www.iqaudiocorp.com/pages/FTC-amplifier-power-spec.html
But regardless how it is determined, the underlying principle applies: 100 watts is 100 watts -- amps times volts -- no matter whether it's generated by an amplifier, a battery, or a line cord plugged into a wall socket in series with a resistor.
I think you're talking about how much wattage an amplifier class may be capable of delivering, all other things being equal. Indeed, Class A is less efficient than Class A/B, such that only about 25% of the wall current power in Class A makes it to the speakers. The remaining 75% is simply emitted as heat, which explains the large heat sinks in typical Class A amps.
By comparison, Class A/B is about 70% efficient -- which means only 30% of the wall power is emitted as heat -- and Class D can be over 90% efficient. That's why Class D has the smallest heat sinks -- only 10% of the wall current is emitted as heat instead of going to the speakers.
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When anyone buys an amp, they want to know how much power it puts out, and that's how all amps are specified.
All amplifier power outputs are specified into an assumed load or a quoted load. Yes, load impedance is a factor in the power output of any amplifier.
Btw, what's a "class A envelope"?
I'm talking about a guy walking into a hifi shop and they have a line of amps sitting there. Let's say a Pass XA25, a Parasound A23, and a 120 watt class D amp. (Can't name one because I don't care much about them.) Pass specifies the rated power at 25 watts, which is common convention for a class A amp, which is what the tag on the shelf will say. Next to it the Parasound will say 120 watts because that's what Parasound rates it at.
I'm assuming the guy here asking the question is asking about rated power. Should he assume the XA25 is less than a quarter as powerful as the Parasound? That's not even close to the case. It's actually more like 2/3rds the power. You simply don't see class A amps rated by anything other than their bias. When somebody tells me what a class A amp is rated I assume that's it's bias. That's the selling point of the amp more than what it can peak out at.
nj pheonix
Lunatic Member
Side note
All class A amps aren't equal
All class AB amps aren't equal
Many class AB amps operate in class A for a watt or so.
I agree. A, on all the time , no crossover distortion.
Whether that presents itself as better sound is debatable.
My point is , the best of AB sound better than the worst of A.
A by itself doesn't make an amp sound good.
Pending something I don't understand watts is watts.
If your combination of available power to your efficiency of speaker makes it loud enough and clear enough to you (in this context , you're not turning volume to 11), the specific power output is somewhat meaningless.
I see the merit in getting the more valuable one.
For sake of the hobby. Pick the one which sounds best to you.
All class A amps aren't equal
All class AB amps aren't equal
Many class AB amps operate in class A for a watt or so.
I agree. A, on all the time , no crossover distortion.
Whether that presents itself as better sound is debatable.
My point is , the best of AB sound better than the worst of A.
A by itself doesn't make an amp sound good.
Pending something I don't understand watts is watts.
If your combination of available power to your efficiency of speaker makes it loud enough and clear enough to you (in this context , you're not turning volume to 11), the specific power output is somewhat meaningless.
I see the merit in getting the more valuable one.
For sake of the hobby. Pick the one which sounds best to you.
The class A envelope is the power envelope within which the transistor remain conducting. Forgive me for busting out the example of my F5, but it's a simple topology that a lot of push-pull circuits can be derived from. Mine is biased at 1.4 amp which means each MOSFET is dissipating about 44 watts. 1.4 amp by about 32 volts. In practice were going to throw out the top few volts and calculate the useful bias at something more like 38 watts because as you run out of voltage the FETs get unhappy. That's it's rating. That's what it would be called if I put it on a shelf to sell it. That's how Pass Labs rates their class A amps.
The class A envelope is about double the bias current into a prescribed load; 8, 4, 2 ohm. Given the bias point above, that means my F5 is good for 62 watts into an 8 ohm load and 31 into 4 ohm. But, it will transition into class AB and continue pushing power up past 100 watts
At idle each output device is drawing equal current so that there's no power on the output post. As a signal is applied, one device begin to conduct less while the complimentary part conducts more and a signal arrives on the output post. This process will continue until you get to the point of one device conducting nothing while the complimentary device conducts the full 2.8 amp and put it on the output, in the case of my amp. That is the class A envelope within which the signal is fully carried inside the bias. Usually that's a little less than double the actual bias because of mismatches in parts and degeneration.
Hope that makes sense.
DaveVoorhis
Addicted Member
I'm not sure what you mean by "rated by anything other than their bias."
An amplifier is normally rated at the maximum continuous power it can deliver into a load at a specified distortion rating, independent of any bias. A Class A amplifier that delivers 100 watts into the load (i.e., a speaker) is exactly the same power (by definition) as a Class AB amplifier that delivers 100 watts into the load.
If both amplifiers are rated for 100 watts at (say) 0.05% THD, that generally means any attempt to eke out more power will either result -- in both amplifiers -- in more distortion or the amplifier shutting down (or burning up.)
The difference is that the Class A amplifier will generally be physically larger than the Class AB amplifier because it will only turn 25% of the wall current into speaker motion and emit the rest as waste heat. The Class AB amplifier will only waste 25% of the wall current as heat; 75% of the wall current will turn into speaker motion.
I'm not trying to be a weenie here, but by your description of how a class A amp is rated, there are no actual push-pull class A amps. Virtually all of them transition to class AB way before they run out of voltage. One could argue they're all just high bias class AB, and I've seen that argument. Likewise, single ended class A could just be called high bias class B. Nobody seriously call them AB or B amps, respectively.
A good class A amp is biased generously, has a bit more voltage than it really needs, and are universally rated by their builders by their bias. I think it's much more descriptive of the amp and that's what buys and builders of them are designing and paying for.
infullview
Super Member
As George Carlin might say "watt's watt and that's that" (slightly changed from original quote)
DaveVoorhis
Addicted Member
I made no reference to whether an amplifier is push-pull topology or not, only to its power capacity in watts.
I've not noticed that. In my experience, informed buyers look for a wattage rating that will deliver a given sound pressure level in a given room at a given distance from a set of speakers of a given sensitivity, out of a preferred amplifier class. Thus, I have seen buyers choose Class A over Class AB because they believe Class A will sound better; i.e., the buyer's preferred amplifier class is Class A. I don't recall any buyers mentioning bias, aside from whatever is implied, bias-wise, by Class A vs Class AB (but not Class D, as there's no relevant bias to speak of.) But that's generally a builder's consideration, not a buyer's.
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I have never heard that a class A amp is twice as powerful as an equivalently rated AB amp. Sure, they are biased higher, but that has to do with optimizing the operating point of the output devices and reducing crossover distortion. A watt is a watt, even if there are some perceived or measurable sonic differences.
Blue Shadow
Waiting for Vintage Gear from this century
Easy for a Yamaha guy to say.The one with the highest resale value.
Me, I'd go class A Luxman as it is a sweet amp and I'm one that needs power because of speaker choices. The sound quality will be there with the class A. And I've learned to enjoy 100 watts of high quality Luxman power when using the 5M21 as the main amp in the sound room instead of the Crown DC-300a, HK Citation 16 or Crown SA-2 all with a receiver or two more watts per channel.
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whoaru99
Epic Member
I do not have an F5 but have considered on many occasions to build one of the PassDIY ones. As such, I'm a little familiar with the specs.
The standard design is rated 25W @ 8 ohms and calls for 24V power supply.
Considering there will be some drop through the outputs, I opine there is around 20-21 V to actually work with for output considerations.
If you consider that is the peak voltage the amp could deliver we need to figure the RMS voltage. Let's split the 20-21V so at .707 the amp could produce 14.5Vrms output.
14.5Vrms into 8 ohms is ~26W, which aligns almost perfectly with the 25W @ 8 rated power.
EDIT: I recognize later you mention 32V rails, which more or less means it must be an F5 Turbo variant. Applying the same methodology as previously stated, the calculated output comes at 49W, which, again, is in almost perfect alignment with the 50W @ 8 ohms rated output.
How far either of those stay fully Class A I don't know, but it's clear they are 25W and 50W (@ 8 ohms) amps respectively both from a technical perspective and from advertised rating.
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Blue Shadow
Waiting for Vintage Gear from this century
Or the amp is huge. Look at some of the high end amps the size of coffee tables rocking class A and no fans, bristling with heat sinks. Well beyond the scope of AK's vintage (which they can be) and cheap (which with the cost of a rebuild, maybe they ought to be).
I had an M-45, 125 wpc switchable to class A with a dozen watts give or take (some sources show it as 10 others as 20 IIRC) and it did not sound good in Class A/B and sounded just fine in Class A. Big difference in the operating temp of the amp. The amp was not recapped other than corrosive glue abatement. I do not expect the same sonic difference just because of a class change.
If you have super efficient speakers, you don't need 200wpc. You can easily get away with a 50wpc Class A and have no idea it isn't more powerful.
I just went from a 400wpc into 4ohm to a 150wpc into 4ohm and the SQ and control is 3 fold better or more. Both AB but the new one is a Luxman
