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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

Showing 3 responses by shadorne

shadorne's avatar
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shadorne
7,467 posts
 
At elevated levels it is likely that both sound nearly indistinguishable.

At low levels Class A is likely to have an advantage.

Crossover distortion is not that "level" dependent. Therefore when the music is very soft then the crossover distortion may become noticeable. As you increase the music in relation to the distortion then it becomes less audible - much in the same way "hiss" when playing loud becomes a non issue but can be a nuisance in very quiet passages in classical.

Of course an extremely precise Class A/B design that has negligible crossover distortion may be just as good as a Class A.

Of course components drift with age - so a Class A amp that is five years old may outperform a Class A/B of the same age ( unless you has the Class A/B serviced). On the flip side the higher operating temperature of a Class A may also mean that it ages faster.

There exist power amps that run Class A to significant power levels before switching to Class A/B. This may be the best compromise (power but also years of reliable quality signal at low levels) depending on your viewpoint. Another user could argue that it is simpler just to service the Class A/B amp regularly to keep crossover distortion low.

Note that designers can deliberately choose parts so that they partially compensate eachother in the component aging process...this is probably what distinguishes high quality designs from cheaper low quality ones.

There is no absolute answer - except that build quality (design, component selection) may ultimately be more important than Class.
shadorne's avatar
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shadorne
7,467 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.
shadorne's avatar
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shadorne
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perhaps in some designs, but you would have to be running the amp at 100% rms power levels for that to occur, (and the heatsinks would not cool appreciably) most speakers and ears would have been damaged. This argument holds no water. [/url]

Good reply - I do see your point now. I agree that at more modest levels the Class A is going to be thermally much more stable (run hotter but stay that way).

[quote] BTW what amp drives your ATCs?

These are active speakers so each speaker has three separate Class AB amps that run Class A to two-thirds power. The amp circuits are completely discrete (all individual components - no packages) and are actually very simple. They run rather hot when idle of course. The design uses a sliding bias so that the operation stays Class A until 2/3 power. The power output stages of the amp look like any typical simple amp design with two complementary sets of push pull high power MOSFETS - but remember each of the three separate amps are only driving one transducer (no complex crossover) over a very limited frequency range (so the work load is shared and kept as simple as possible - this is perhaps the biggest benefit of the overall design rather than the Class A operation). There is actually more complexity in the line level buffering/pre-filtering for each power amp stage which includes some phase adjustment to maintain accurate phase over the crossover. The three power amps to each driver are totally identical - so power output must be controlled/limited by the load resistance (woofer taking more power than a tweeter).
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