Class A Solid State Sound


Would someone kindly describe the differences in class A sound of Pass XA.8 series, Accuphase A-70/75 series, and Gryphon class A amplifiers. Does much or any of the differences relate to mosfet (Pass and Accuphase) or bipolar (Gryphon)  output devices?  Thank you!

audiobrian
The classic comparison is between Pass And Boulder Warm VS Dry, Tubish vs Transistorish. I am not totally sure about this but there is something about Class A bipolar amps which causes the designers to want to use fans and fancy bias tracking schemes. Do they generate more heat or require higher bias voltages. Pass seems to always avoid this but he does not run his power higher than 300 watts whereas Boulder will go much higher. JC simply switches over to AB which you can hear in the JC 1. Push it into steady AB and it loses its effortlessness. It still gets as hot on low impedance loads as a Pass amp. 
@erik_squires 

I cant find anything to disagree with in your post up there. It's totally silliness to make any unqualified statement about a particular devices currently delivery ability since so many other factors contribute to it. 

I agree with your assessment of feedback too. My F5 is a class A amp that utilizes "generous" amounts of negative current feedback and I've never heard anybody say that amp sounds bad for it. Feedback is something that needs to be applied with careful consideration and shouldn't be as universally disparaged as it is. 

Myjostyn,

The reason you see BJT amps with fans and thermal tracking schemes has to do with the characteristics of BJTs. They're highly prone to thermal runaway where as the device heats up, it conducts more current, which in turn raises their temperature, which causes them to conduct even more current. This characteristic does produce distortion which also requires compensation. MOSFETs, when strapped to a big enough heat sink, need no thermal tracking because their conductivity vs temperature will reach an equilibrium at some point so long as the bias is within the operating range of the device. Some MOSFETs, like laterals, have virtually no thermal drift and you can set bias on stone cold devices to find the bias only creep up slightly as the thing hits 75°C. 

It all depends on what the designer is trying to achieve as for what gain devices he chooses. Pass certainly uses plenty of BJTs in his older designs, but he uses MOSFETs these days for the exact reasons you described. Simpler circuits and a more organic, tube-like sound. 
It all comes down to the way the amp sounds into a specific speaker and the only way to know that is to hook them up and have a listen. You can make assumptions all day long and many of them will prove right. But, assumptions are the mother of all f--k ups. A less expensive amp may sound just as good as a more expensive amp into a specific speaker and we are all price sensitive to one degree or another.  
I have heard great examples of both types. It is interesting that Atma-sphere amps have been forgotten in this discussion. They are class A OTLs Tube amps and many say better than any type of class A SS amp.
They all get hot so why not have a little glow with it? 
Ralph designs his amps to drive fairly benign, efficient speakers. If you like speakers like Focal or Wilson or KEF, forget Atma-sphere amps.