Learning About Class A Amplifiers - Links & Resources Provided

[imsmooth]

I have been reading various online tutorials on the subject. Does anyone have some good links they will share. I am trying to build my understanding of the class A amp so I understand what I am doing if I chose to build a Nelson Pass Amplifier.

I have an understanding of transistors and basic topologies already.

Thanks.

 

[JimmyAlz]

You might want to read what Rod Elliot has to say on the subject of Nelson Pass's ZEN Amplifier: (Is this what you are looking into?)
http://sound.westhost.com/project36.htm

 

[jsgruszynski]

I have been reading various online tutorials on the subject. Does anyone have some good links they will share. I am trying to build my understanding of the class A amp so I understand what I am doing if I chose to build a Nelson Pass Amplifier.

I have an understanding of transistors and basic topologies already.

Thanks.

Class A refers to using single transistors (or tubes) in the single path to cover the entire dynamic range (positive and negative) of the signal for power amplification. See wikipedia:

http://en.wikipedia.org/wiki/Electronic_amplifier#Power_amplifier_classes

From what I've seen of his (Nelson Pass) design docs (below), it's basically a pretty standard Op Amp design implemented in discrete devices.

http://www.passdiy.com/pdf/classa_amp.pdf

It's using a differential pair to cover the + and - phases but the rest is a cascade of pretty traditional single-ended class A stages until you get to the Q9-Q16 which are actually Class B or AB drivers.

Again, pretty typical of how Op Amps are design and how most IC-based power amplifiers are designed. Note that the date of first Nelson Pass Class A is 1977, which is when IC Op Amps were still fairly new as a standard electronic part. Compare his design to a familiar Op Amp circuit like a 741:

http://www.national.com/ds/LM/LM741.pdf

This picture may be helpful in visualizing what I'm describing above:

http://upload.wikimedia.org/wikiped...0050626193617!Opamptransistorlevelcolored.png

Dark Blue - input differential pair CC-CB cascode
Red - current mirrors for biasing
Magenta - darlington gain cell with feedback
Green - CB amplifier/level shifting (cascode with gain cell)
Light Blue - class AB output driver

 

[Studiot]

This site is a good place to study the Pass and other (class A) amps.

http://www.diyaudio.com/