Class A naman.. sir, i see a lot of amps with emblems on the faceplate claiming Class A or even Class AA (Technics does this alot). I also saw in the norh site that their Le Amp II is class A in the gain stage and then class AB on the bipolar stage. What is the gain stage & bipolar stage? Also, how does a pure class A amp differ in sound from a semi-class A amp? Thanks!
it is very common to see the low level stages in an amp operate class A, for a typical solid state power amp, there are three stages:
1. input stage, almost always a long-tail pair, differential amplifier, this is normaly Class A, the stage amplifies the difference between the input signal and the feedback signals from the output stage.
2. the VAS or voltage amplifying stage, or simply voltage gain stage, it is this stage that provides to full voltage swing to the speakers..this is also operated class A. this stage has the highest voltage gain..
3. the output stage, which can be bjt's or mosfets, the function of the output stage is to couple the low impedance speakers to the VAS, since the VAS can not drive the speakers directly. this stage has a voltage gain of unity (1) and a current gain defined by the product of beta's of the output and driver transistors...
>for a true Class A amp, output transistors are biased to the desired operating point, regardless of wether there is an input signal or not,or wether current is flowing thru the speaker load, it is constant and always flowing thru the output devices. a class A amp avoids the problems associated with transistors cutting-off and cutting-on, this according to some is the best sounding of all the three configurations, but the costs involved may not be worth it eventually....
>for a class B, there is no quiscient current at all, the output devices alternately swing from conduction to cut-off, that is why this in this type of output cross-over distortion is indeed very noticable,
>for class AB, the output devices a slighly biased, to about 50ma or so per device, increasing this bias can lead to a regenerative phenomenon known as thermal run-away, this is for bjt's., for mosfets, bias can be set higher to say 100ma or more since the thermal-run-away phenomenon is not present with this type of devices, in fact increasing bias gives a better sounding amp. but this can not be done arbitrarily, there are other factors to consider.bjt' devices also have this secondary breakdown characteristics, wherein a high voltage and current can not occur simultaneously, mechanism, creates a pinhole size hotpots on the die surface that can destroy the transistors in a sec. Class AB operates classA for the first 200mW or so, then operates classB as required by the load, thus the term Class AB, but unlike class B the transistors are never fully off
to give you an idea, a ClassA amp can have an idling power consumption of say 50 watts for a ten watt output, while a class AB 5 watts for a 250 watt output!!!
class A amps uses huge heatsinks...mark levinson had a class A amp bigger and hevier than this 250 watt models..
to guide you, if you see an amp that says it is classA, the first thing to do is to lift it, if is lightweight, then chances are it is not, also the power consumption rating, a true ss Class A will put the dynaco st70 in shame with regards to power consumption....
hope this enlightens you....
