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In Reply to: RE: Class A to B switching nonsense posted by morricab on April 04, 2014 at 12:44:06
That is, to eliminate so-called notch distortion. With adequate transistors this occurs only at very low output levels and transitioning to class B adds no distortion at higher output levels.With good design the class A is only "insurance" against minute residual notch distortion. Enough bias, however, should ensure no abrupt, significant increase in distortion.
At the levels I listen, my Pass X150.5 is probably operating mostly in class A. In any case I don't hear any "klunk" or "florescent bulb" effect as RGA would have it.
BTW, according to Pass the X series amps operate single-end class A at very low levels, i.e. below one watt.
I love the music of ... ... Gustav Mahler
Edits: 04/04/14Follow Ups:
What you say about Class B is not true there is distortion beyond just the crossover notch distortion. Boyk and Sussman demonstrate that clearly with the math. With or without feedback it is FAR FAR worse than Class A...otherwise no one would have ever bothered with a Class A amp in the first place!
Read Cheever about the level dependence of harmonic generation and also the sliding scale of perception...it's a complicated story but worth telling. Your Pass is probably 10-15 watts Class A so for most listening it will stay Class A. Whether you hear the switch to Class B will depend on speaker sensitivity and impedance, meaning it depends on how loud and what power draw is occurring as to the audibility. This means the amp speaker interaction but not for the reasons people usually think.
"...otherwise no one would have ever bothered with a Class A amp in the first place!"
I'm only semi-ancient but when young had lot's of experience with gear that was old even then. So... from that perspective, i.e. historical, you have it wrong. Single ended amplifiers have the advantage of the lowest possible parts count (and cost). Tubes used to cost a bundle and had wide production spreads so needing twice as many and matching just didn't cut it.
Historical Rick
I was referring specifically to PUsh/pull amps since Single ended can only be Class A anyway. When the first decided to make PP amps they probably took one listen to their latest Class B creation (early transistor era just for HR) and said OH F#$K what a piece of S$*T!! :-)
Morricab: Triodes v Bipolar Anything to say re the Humble contribution ?
Edits: 04/05/14 04/05/14 04/05/14
What are you looking for exactly? There is not much to say except that triodes (not tubes in general) are much more intrinsically linear that a bipolar transistor?? It is a simple fact. Transistors were never really designed for audio amplification (computer sure, audio not really). They were designed as digital switches not linear amplification devices. THey have way too much gain for audio purposes NECESSITATING the use of lots of negative feedback to work at all. This is where MOSFETS have an advantage that they can be run without feedback in Class A and work very well indeed.
The only significant drawback to triodes is that they need either a VERY good output transformer to match well with speakers OR you need a bunch in parallel to get the output impedance low enough. Otherwise, they are in just about every way superior to the BJT.
Morricab The earliest transistors from the late fifties were in three groups,
Red spot for AUDIO,Green spot etc for RF, and Yellow spot also for RF for the intermediate filter stage in radios all germanium then, and today Companies like Profusion have a catalogue of Bipolar and Mosfets specially designed for Audio.People should check their references, which includes how transistors actually work , and that constantly repeating a statement without defining the limits simply confuses the situation. I think I have more than adequately answered the question of triodes and their distortion characteristics and the conditions under which it is evident, and why it is evident. I have also emphasized that transistors are as different to valves as Blancmonge is to cheesecake, but that transistors can be configured in so many ways that valves cannot and thus in the area of distortion or linearity makes arguments moot since the Transistor circuit beats the valve hands down and that is a fact. Any transistor can be used as an amplifying device, the term "Switch" simply means that the transistor has been designed for a very low input capacitance and thus it is capable of working well at much higher frequencies without transit delays, but will work perfectly well at audio frequencies... this comes from my friend Humble
Edits: 04/07/14
Yes but unfortunately Nelson Pass doesn't design most amplifiers on the market - nor are most of them anywhere near as good.
Somewhere in this thread I noted that I liked certain SS amplifiers more than tubes - umm Pass Labs/First Watt is one of them - Sugden is another.
I am looking to build a SS system because I think it's helpful for reviewers to have both things that people like (ie; tubes and solid state) and Nelson's stuff tops my list). There are others but they usually cost ridiculously more money.
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