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Buzz about pure class A solid state amplifiers ?

Fellow Audiogoners:

I would greatly appreciate all your input/insight.

Lately both in and out of Audiogon, I have been aware of a "buzz" and numerous praises about the benefits of pure class A solid state amplifiers, as an alternative to the more common high bias AB designs, and an alternative to the transparency and coherence of tube designs.

I have recently auditioned a current production 60 watt Accuphase class A solid state design, a fine amplifier indeed. However, up against my similarly priced VAC Phi 300 class AB stereo tube amplifier (used in ultralinear), it sounded mildly anemic, less transparent, and less coherent/detailed. This is with due respect to Accuphase products, which I have owned and enjoyed greatly.

So....please help me to understand all the purported benefits of the lack of crossover notch distortion, etc with class A push pull soid state designs, when excellent AB tube amps, with crossover notches, are IMHO more cohesive, transparent and resolving in comparison. Have designers, both tube and solid state, basically succeeded in rejoining positive and negative waveforms seemlessly, without perceptable crossover notch distortion?

Yes I did, admittedly, have an interest in simplifying my system, and going back to solid state after 14 years (with tube preamp); However my tubes are gonna stay :)

To all my solid state friends, please know that I have owned many solid state amps which I have truly enjoyed and have the utmost respect, dating back to the Reference Levinson 32's...and I know there are great SS amps out there, and primarily class AB designs.

Just wanted to share my experience, and learn alittle more about class A solid state, specifically.

With thanks in advance for your thoughts

Brian
audiobrian
dpac996's avatar
dpac996
433 posts
 
the "buzz" about class A amps usually results in popular discussions about crossover distortion...it's easier to imagine this voltage based boundary condition than, say thermal distortion mechanisms manifesting non-linearites which may actually do more sonic harm than crossover distortions. The class A approach minimizes the thermal distortions by maintaining a more or less constant device temperature, and therby reducing the distortions caused by heating and cooling (consider a class B amp: junction temperatures and the gain non-linearities rise and fall much faster than the massive heat sinks they are strapped to, which are more an RMS type thing).
Other positives about class A involve less modulation based noise on the power rails (since the output devices are constantly "on" the rails are subject to a more fluid load), and of course the often mentioned cross-over distortion. As Ngjockey states and as I can attest to, the primary differences when running the sa102 in class AB/ vs A are:
When thermally stablized (after about 45 minutes-- and btw, more indication that the benefits of class A are not due entirely to absent crossover distortion) in class A
1)image depth increases noticably
2)vocalists take on a rounder more coherent tone
3)fine treble shadings becoms more apparent
4)overall 'feel' seems more natural and grainless

no big changes in bass power or dynamic "slam"
just more refinement overall...its like you have two amps in one...and its fun to listen for a while in one mode then another and hear this over and over...

hope this helps!


shadorne's avatar
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shadorne
7,333 posts
 
The class A approach minimizes the thermal distortions by maintaining a more or less constant device temperature, and therby reducing the distortions caused by heating and cooling

Hang on - a pure Class A device will cool when driven hard - it will run hottest at idle. So you still have thermal issues - like with any device.
unsound
6,115 posts
 
Perhaps your preference for the class AB tube amp, might invite comparison with a pure class A tube amp?
jallen
502 posts
 
Class A is the least efficent, generate the most heat, and cost the most to repair all those fried transistors. Check the price to repair a pair of Krell MDA300's...about 1500-2500.00 Not cheap, I find it easier to just change tubes. However, can't drive MBL's with tubes very loud so SS it is. Stick with the companies who know how to make Class A, AB, and D sound great. ARC, Pass Labs, Rowland, Krell, Copland, Ayre, and the others. The most important thing is not whether it is Class A or not, it is how the damping factor of the amp is matched to the needs of the speaker. If this is off, all A-D will sound like bad hifi.
liguy
228 posts
 
The advantage of a Class "A" amplifier is not:

"The class A approach minimizes the thermal distortions by maintaining a more or less constant device temperature, and therby reducing the distortions caused by heating and cooling (consider a class B amp: junction temperatures and the gain non-linearities rise and fall much faster than the massive heat sinks they are strapped to, which are more an RMS type thing).

Other positives about class A involve less modulation based noise on the power rails (since the output devices are constantly "on" the rails are subject to a more fluid load)"

as Dpac886 suggests. It is that the amplifying devices are constantly operated in their linear region, above cutoff and below saturation. By using Class "A" topology we can set the bias of the device to it's most linear region. This negates large amounts of negative feedback needed to correct for the extremely nonlinear characteristics of the transistors at the collector cut-off region and the turn-on/turn-off times of the devices in class A/B topology. There is a good explaination at Passdiy.com, Nelson Pass's contribution to the DIY Community.

http://www.passdiy.com/pdf/classa_amp.pdf
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