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Old 24th January 2007, 06:00 AM   #1
batee is offline batee  United States
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Default Class A and Class AB - How Does This Work?

I just read an eBay ad for an X350 that said that it was 50W Pure A, and 350W total. I'm wondering how this is accomplished?

Is this a 400W Class AB amp with the input level shifted / biased to allow 50W of swing to occur on one set of transistors (ie the set that handles the "+" portion of the Class AB wave)? Then when the signal goes above 50W (or whatever voltage this corresponds to), the "-" set of transistors starts to conduct during portions of the cycle? If so, could this be biased even higher to get a higher power 175-200W Class A wave at the expense of a lower or nonexistent higher power class AB output and an astronomical electric bill?

The search tool returned 8712 results. None of the entries on the first few pages seemed to cover this topic. Sorry for my English - I graduated from a public school somewhere in the Midwest

Thanks everyone,

Bryan A. Thompson
bryan@batee.com
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Old 24th January 2007, 07:42 AM   #2
pgbhat is offline pgbhat  India
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50W is the audio output power & 350W is the total power consumption of the system.
check this link for more about amplifier efficiency
Amplifier efficiency
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Old 24th January 2007, 08:41 AM   #3
SunRa is offline SunRa  Romania
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Quote:
50W is the audio output power & 350W is the total power consumption of the system.

Well, I think that's wrong, look at the specs here: X350
The power consumption is not 350 W but is somwhere around 600W (idle)-1800W at full load.

The amp is biased in class A operation up to 50W or so, and then switches to AB operation up to 350W.
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Old 24th January 2007, 09:54 AM   #4
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Hi Bryan,

it probably could be biased a lot higher to get more class-a power.
The problem is that the case / power supply / heatsinks are dimensioned for 600W idle.

So if you up the bias X% the power dissipation at idle would also go up X% and the temperature would become higher.

To change the output power (class A ) from 50 watts to 175 watts you would need to up the bias by a factor of 2. This would also change the temperature from around 50°C at ambient 20° to 75-80° at ambient 20°.

I suppose the 50 watts are chosen because most of the time you will need a lot less than that.

William
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Old 24th January 2007, 10:15 AM   #5
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How many REAL 200 watt class A amplifiers have you heard of ?
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Old 24th January 2007, 01:58 PM   #6
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I understand that raising the bias would produce more heat that has to be dealt with.

I asked the question because I'm curious to know what technology is used to allow the switch from Class A to Class AB at higher power levels, not because I want to hotrod one of Nelson's amps...

I assume if it's Class AB with bias, then it still has crossover distortion at the point where one set stops conducting and another set starts. Does the output wave reflect the crossover distortion at a lower/higher portion of the wave rather than at the center? Or does the bias change as the load (or output power) changes, to keep the crossover at the center of the wave?

Thanks!
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Old 24th January 2007, 07:55 PM   #7
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Quote:
Originally posted by jacco vermeulen
How many REAL 200 watt class A amplifiers have you heard of ?
1

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Old 24th January 2007, 08:19 PM   #8
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Bryan,

the big guy is modest as usual, that 1 and only is his own XA200.

Download some of the Passlabs power amp owner manuals.
NP is a great teach. ( even looks like one )
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Old 24th January 2007, 08:55 PM   #9
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Bryan Hi,

To further knock on the nail...

Pure Class A of 200 watts see the monobloc XA200:
http://www.passlabs.com/amplifiers/xa200.htm

This is THE pure Class A monobloc of 200 Watts in 8 Ohms load...
Idle at 650 watts for each monobloc. {it's also the max power consumption}
2 * 175 lb weight. {2 case of X1000}
Nelson did it !!

- - - - - - - - - - - - - - -

And for Class AB look at the X250

http://www.passlabs.com/amplifiers/x250_5.htm
Idle at 350 watts ; max 1000 watts for this stereo amplifier
110/150 lb weight total.
But you cannot bias it higher ; you are limited by the heatsink dissipation !!
And No, there is no "crossover distortion" when going from Class A to AB
in these design. You only ask the Power Supply for more current!
{If I remember, Nelson or Grey did answer about this point some time ago}

Take a look at the manual of each model...

I had a chance to listen to a pair of XA160 a few years ago at Festival du Son
in Montreal ; and WOW, that's nothing to compare with a class AB,
Pure delight for your ears!!! "fasten you seat belts" ; it's pure Magic !!

Not saying that the X350 is not a great amplifier, but the XA 160/200 are
on the "upper class" without any doubt!!!

Regards.

PS: On that day, back home I was still delighted by my little A75 {highly biased!}
pushing the Martin Logan Aerius' i {but missing the XA 160 WOW!!!}

Alain.
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Old 24th January 2007, 09:36 PM   #10
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Wasn't the Aleph 1.2 a 200W amp? That would make two, although the 1.2 is not in production at this time.
As far as the A vs. AB discussion...it looks distressingly like the cook throwing spaghetti at the wall to see if it sticks. Indeed, some did, but an awful lot fell to the floor and it's in danger of turning into a terrible mess.
The X amplifiers don't use any 'technology' to 'switch' the bias from class A to class AB. There's no dynamic biasing, no sliding bias, no funky sales-spawned bafflegab. The output stage is a conventional complementary follower. Nothing weird. The "X-ness" is all in the front end. Nothing in the output. It just happens to be biased somewhat hotter than some of the other class AB designs out there.
The idea of running class A up to some arbitrary power level before shifting to class B is a nifty one. Almost like having your cake and eating it too. The question is where to set the bias, which in turn determines where the A-to-B transition will occur. Some people set it as low as possible; their intention being to keep heat dissipation to a minimum. As a corollary--something you never see pointed out in the sales literature--they get to use cheaper transformers and heatsinks. Hmmm. There's no way that could factor into their decision, is there? Nah...not a chance.
On the other hand, you've got people who use higher biases in order to keep the amplifier functioning in class A for as long as possible. For an average--or even above average--listener, even when they decide to kick some backside, 50W of class A will pretty much cover all the bases and then some. Remember that musical peaks are greatly higher than the average listening level. By the time you've used up the 50W of class A, odds are pretty good that you're clipping peaks anyway.
And clipping is a whole lot worse than any namby-pamby crossover distortion. If you've used up your alloted 50W class A, and the 300W of class B behind that, you've got far worse problems than any of the usual bench-test distortions.

Grey

P.S.: The most inefficient commercial speaker I can remember was the Stax F-81--an electrostat. Incredible midrange. Highs good to very good. Lows were weak due to the relatively small size of the panels. I believe it was rated at something like 80 or 82dB. It took 200W to get it moving. Nonetheless, I lusted for a pair of those things. That midrange got hold of me and would not let go.
These days, assuming I was in the mood to buy electrostats, I'd probably go for something else, but back then I wanted a pair of F-81s in the worst kind of way.
(There was a stacked version--the F-83 [I think I got that right]--but I never had a chance to hear them.)
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