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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
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
audiobrian OP
96 posts
 
I truly appreciate the ongoing discussion. Another point of confusion to me is comparing let's say a Belles MB-01 class A and Karan 450 class A both using approx. 1000 VA transformers/channel, similar capacitor banks, and 20 transistors/channel. However the Belles is said to deliver 75-80 watts pure class A into 8, and the Karan, 450 watts pure class A into 8 ohms.......this huge difference in pure class A output power is not easily understood, even though Mr. Belles and Mr. Karan obviously use different toplogies, transistors, etc.

For what it's worth, in the context of my system only, my personal favorite solid state amplifier, which I do own, and alternate with the VAC, is the class AB Gamut M250 monoblocks. As many know, these monoblocks use one large mosfet pair/channel. I'm not sure if using a single mosfet pair/channel is theoretically better than paralleled devices, as Gamut and, I believe, Vitus believe....another interesting topic, perhaps for a different thread :)

My system comprises a VAC Ren mkII pre and either the Phi 300/Gamut 250 into Kharma Midi Grand Ceramiques :)

Thanks
jmcgrogan2's avatar
jmcgrogan2
6,346 posts
 
Well some will call their amps Class A, but obviously it's just a marketing tool for some. As you say, "buzz" word.
All 'Class A amps are not equal, just as all Class A/B amps are not equal, and all tube amps are not equal.

FWIW, I have not heard the Karan KA-S 450, but just looking at photos I find it VERY hard to believe that it could be pure Class A. 450 wpc in pure Class A would require monoblocks with much larger heatsinks.

As a comparison, Pass Labs XA series, which runs in pure Class A, goes to monoblocks with their 60 wpc XA-60.5
Their top of the line XA-200.5 puts out 200 wpc, and each monoblock is larger than the stereo Karan 450. So I would be inclined to believe that the Karan slides into Class A/B at around 50 wpc. This is no ding on Karan, as I have not heard this amp. I'm simply saying that physics makes it impossible for me to believe that this amp delivers 450 wpc of Class A power.

It's just physics, Class A generates a LOT of heat. If it truely is Class A, the power rating will be about 20-25% of a similar size amp which slides into A/B. Many amps will give you 5-10 wpc of Class A before sliding into Class A/B, and some may be bold enough to claim that they are Class A amps, as it is a selling feature, and marketing folks know what buzz words attract customers.

I also agree with Ngjockey, that the best way to compare A to A/B is in the same amp. AFAIK, Plinius and Clayton are the only two manufacturers who offer an A>A/B switch that you can flick on the fly. I've owned both, and experimented. I would say the Class A sounded better, more relaxed and articulate especially at low to moderate listening levels. At higher listening levels the differences decreased, IMHO.

Cheers,
John
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